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J Biol Chem, Vol. 274, Issue 42, 29763-29771, October 15, 1999

The Procyclin Repertoire of Trypanosoma brucei
IDENTIFICATION AND STRUCTURAL CHARACTERIZATION OF THE GLU-PRO-RICH POLYPEPTIDES^*

Alvaro Acosta-Serrano^§, Robert N. Cole^¶, Angela Mehlert, Mary G.-S. Lee^**, Michael A. J. Ferguson, and Paul T. Englund

From the  Department of Biological Chemistry, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, the  Department of Biochemistry, University of Dundee, Dundee DD1 5EH, Scotland, United Kingdom, and the ^** Department of Pathology, New York University School of Medicine, New York, New York 10016

	ABSTRACT

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

The surface of the insect stages of the protozoan parasite Trypanosoma brucei is covered by abundant glycosyl phosphatidylinositol (GPI)-anchored glycoproteins known as procyclins. One type of procyclin, the EP isoform, is predicted to have 22-30 Glu-Pro (EP) repeats in its C-terminal domain and is encoded by multiple genes. Because of the similarity of the EP isoform sequences and the heterogeneity of their GPI anchors, it has been impossible to separate and characterize these polypeptides by standard protein fractionation techniques. To facilitate their structural and functional characterization, we used a combination of matrix-assisted laser desorption ionization and electrospray mass spectrometry to analyze the entire procyclin repertoire expressed on the trypanosome cell. This analysis, which required removal of the GPI anchors by aqueous hydrofluoric acid treatment and cleavage at aspartate-proline bonds by mild acid hydrolysis, provided precise information about the glycosylation state and the number of Glu-Pro repeats in these proteins. Using this methodology we detected in a T. brucei clone the glycosylated products of the EP3 gene and two different products of the EP1 gene (EP1-1 and EP1-2). Furthermore, only low amounts of the nonglycosylated products of the GPEET and EP2 genes were detected. Because all procyclin genes are transcribed polycistronically, the latter finding indicates that the expression of the GPEET and EP2 genes is post-transcriptionaly regulated. This is the first time that the whole procyclin repertoire from procyclic trypanosomes has been characterized at the protein level.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

African trypanosomes are protozoan parasites responsible for sleeping sickness in humans and the disease nagana in livestock. Trypanosomes alternate between the invertebrate vector (Glossina, or tsetse fly) and the mammalian host, and the different life cycle stages are uniquely adapted to survive in each host. In the mammalian bloodstream form of the parasite, 10⁷ identical variant surface glycoprotein (VSG)¹ molecules are expressed on the plasma membrane, forming a dense coat. The parasite can survive the immune attack of its host because it undergoes antigenic variation, a process in which its surface coat is replaced by another composed of an antigenically different VSG molecule (1, 2). When the trypanosome is ingested by the tsetse fly and differentiates into a procylic form, the VSG coat is totally replaced by one composed of an array of different proteins, known as procyclins (3-5).²

The Trypanosoma brucei procyclins, present in about 2.2 × 10⁶ copies/cell, have very unusual structures, with the C-terminal domain consisting of amino acid repeats. One set of proteins, the EP isoforms, is predicted to contain between 22-30 Glu-Pro (EP) repeats, whereas the GPEET form, in contrast, has six Gly-Pro-Glu-Glu-Thr (GPEET) repeats followed by three EP repeats (see Fig. 1A for a schematic diagram of procyclin structures and Fig. 1B for their amino acid sequences). Unlike VSG, procyclin is encoded by a small number of different genes, and therefore it has only a limited potential for variation. In the T. brucei 427 strain, the EP isoforms are encoded by the EP1 (4, 6-8), EP2 (10), and EP3 (5, 9) genes, whereas GPEET-procyclin is the product of the GPEET gene (11). All procyclin genes are contained in four expression sites (per diploid genome), and each site contains two procyclin genes in a tandem array (see map in Fig. 1C). Transcription of these genes is polycistronic and can occur simultaneously from two or more expression sites (reviewed in Ref. 12).

View larger version (35K): [in this window] [in a new window]   Fig. 1.   Procyclin structure and gene organization. A, schematic representation of both EP- and GPEET-procyclin structures. EP-procyclin polypeptides contain a variable number of Glu-Pro repeats in the C-terminal domain. EP-procyclin products of the EP1 and EP3 genes are glycosylated at Asn29 and contain an Asp-Pro-Asp-Pro sequence that is sensitive to cleavage by mild acid (28). The EP2 product contains neither a glycosylation site nor Asp-Pro bonds (10). GPEET-procyclin, the product of the GPEET gene, has no site for N-glycosylation and contains six GPEET repeats (11). This molecule is extensively phosphorylated at threonines in the repeats (18, 19). B, predicted amino acid sequences of the products of the EP-procyclin genes (EP1-1, EP1-2, EP3, and EP2) (5-10) and the GPEET-procyclin gene (GPEET) (11). Another EP1 gene product predicted to contain 29 EP repeats and Gly at position 24 has been reported from another strain (4), but it was not detected in this study. For simplicity, only the sequence of mature proteins (without signal peptides and GPI addition signals) are shown; sequence numbering starts at the N terminus of the mature protein (i.e. Ala1). Underlined letters represent sites for N-glycosylation (Asn29). 10 and 4 with brackets indicate the N-terminal sequences missing in some of the species identified by MALDI-TOF-MS (Figs. 2A and 6). C, simplified diagram of procyclin loci in T. brucei 427 strain. Loci names are at left. All genes are allelic, and the products of the EP1-1 and EP1-2 genes differ in sequence (see text). This model is not drawn to scale and shows only the pair of procyclin genes present in each locus. The EP1-1, EP1-2, EP2, EP3, and GPEET genes were formerly called B2, B1, B, A, and A respectively (see new nomenclature in Ref. 37). A more detailed map of procyclin expression sites is shown elsewhere (12, 37).

There are other important structural features of the procyclin proteins (Fig. 1A). Some EP forms can be N-glycosylated with the products of the EP1 and EP3 genes, but not that of the EP2 gene, containing a site for N-glycan addition (i.e. Asn²⁹; Fig. 1B). GPEET-procyclin also has no N-glycosylation site. The N-glycan is Man₅GlcNAc₂, and it is unusual in that it does not exhibit microheterogeneity (13, 14). In addition, all of the procyclin proteins have GPI anchors of unusual structure, characterized by a very large (average of 30 sugar residues), heterogeneous branched poly-N-acetyllactosamine side chain (13, 15). This side chain serves as the sialic acid acceptor for the cell surface trans-sialidase (16, 17). The anchors on EP- and GPEET-procyclin are similar or identical in structure (13). Additionally, GPEET-procyclin is modified by phosphorylation on six out of seven threonine residues in the repeat sequence (18-20), but this modification has not been reported for the EP forms.

The function of any of the procyclins is still unclear, although recently it was demonstrated that mutant parasites that express no EP isoforms cannot establish heavy infections in tsetse fly midguts (21). Furthermore, the wild type phenotype was partially rescued after overexpression of either nonglycosylated or glycosylated EP isoforms (21). Interestingly, both EP- and GPEET-procyclin are co-expressed on the parasite surface, although their ratio varies in different clones. When some of these clones are maintained in culture for several months, there is a shift in expression, from EP- to GPEET-procyclin, ending with cells containing low levels of EP proteins (13, 18).

In elucidating the function of procyclin molecules, it is of great interest to determine whether all of the different EP-procyclin genes are expressed as proteins and to determine the relative levels of expression. It is also important to establish whether this expression varies with time or under different experimental conditions. Such variation could have considerable biological relevance. For example, a programmed variation of procyclin expression could control the behavior of the parasite in its insect vector. Although the EP and GPEET forms can be detected by monoclonal antibodies (20), to date it has not been possible to analyze or resolve the protein products of the different EP-procyclin genes, because of their very similar amino acid sequences (Fig. 1B) in addition to the extreme heterogeneity in their GPI anchor.

In this paper, we report a complete characterization of the EP-procyclin repertoire by mass spectrometry, revealing most of the species predicted from gene sequences. To apply this technique, it was necessary to remove the GPI anchor by treatment with aqueous hydrofluoric acid (aq.HF), a well characterized method that has a minimal effect on the polypeptide chain and glycosidic bonds. This method was previously used in a mass spectrometric analysis of the phosphorylation of purified GPEET-procyclin (19). However, to fully characterize these molecules it was also essential to develop a new method involving mild acid hydrolysis that selectively cleaves the EP1 and EP3 (but not EP2) gene products at Asp-Pro sequences. Using this methodology, we were able to identify by mass spectrometry all of the procyclin polypeptides present in a T. brucei 427 strain as well as to obtain new information on procyclin expression. Furthermore, for each of the EP isoforms, we determined an accurate molecular mass, the extent of glycosylation, and the exact number of EP repeats.

	EXPERIMENTAL PROCEDURES

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

Parasites-- The wild type procyclic T. brucei brucei 427 clone had been stored at New York University in the laboratory of M. G.-S. Lee. This cell line was originally obtained in 1988 by in vitro differentiation of T. b.brucei 118 bloodstream clone 1 (6). The parasites had been cultured as procyclic cells for a total of about 6 weeks (since differentiation) before procyclin was isolated for the experiments described in Figs. 2-5. Ten cloned lines from the original 1988 clone were obtained at New York University by stably transforming the 1988 clone with the same DNA construct (H23H-B7) and using hygromycin as a selectable marker (22). The H23-H-B7 construct contains the hsp70 intergenic region promoter followed by the hph gene, the -tubulin intergenic region, and a targeting sequence derived from the VSG 118 expression site (22). The transformed parasites were cloned by limiting dilution in the presence of trypanosomes not resistant to the drug. Another cloned line (clone 6) was obtained at Johns Hopkins University from the same T. brucei 1988 strain by limiting dilution in conditioned medium. Parasites were grown at 27 °C in SDM-79 medium (23), supplemented with 10% heat-inactivated fetal bovine serum (Life Technologies, Inc.).

Purification of Procyclins-- For the experiments shown in Figs. 2-5, the procyclins (including GPEET- and EP forms) were purified from 10¹¹ freeze dried trypanosomes by organic solvent extraction followed by octyl-Sepharose chromatography (Amersham Pharmacia Biotech) (13, 15, 19). As judged by SDS-polyacrylamide gel electrophoresis and silver staining, the procyclin-containing fractions from the octyl-Sepharose column showed a major component with a highly polydisperse migration and an apparent molecular mass of ~45 kDa (not shown). Sugar analysis of a pool of these fractions by gas chromatography mass-spectrometry (24) yielded a composition of Man, Gal, GlcNAc, sialic acid, and myo-inositol of 5.2:14.8:27:4.2:1, similar to values reported previously (13, 15). Some preparations also contained a minor component that migrated near 11 kDa on SDS-polyacrylamide gel electrophoresis. This component was identified as the kinetoplastid membrane protein-11, a protein known to co-purify with procyclin preparations (25). MALDI-TOF-MS analysis revealed this component as a broad peak with an average molecular mass of 11,070 Da (not shown).

The mass spectrometry analysis of procyclins from cloned cell lines was performed directly from n-butanol extracts without further purification by octyl-Sepharose chromatography. The mass spectra of these preparations were as clean as those obtained using procyclin purified by the standard chromatographic method (see example in Fig. 6).

Removal of the Procyclin GPI Anchors by aq.HF Dephosphorylation-- Octyl-Sepharose purified procyclin (1-2 nmol, based on myo-inositol content) or n-butanol extracts from cloned cells were dephosphorylated with 50 µl (or 25 µl in the case of butanol extracts) of cold 48% aq.HF (Aldrich) for 16 h at 0 °C (19). After hydrolysis, samples were quickly frozen in dry ice/ethanol and dried in the Speed-Vac. Samples were resuspended in 5-10 µl of water and stored at 20 °C.

MALDI-TOF-MS-- Mass spectra were acquired in a PerSeptive Biosystems Voyager-DE mass spectrometer calibrated with insulin, thioredoxin, and apomyoglobin. For the analysis of native procyclin (500 pmol; non-aq.HF-treated), samples were co-crystallized with sinapinic acid. Polypeptides dephosphorylated with aq.HF (50 pmol) provided better spectra using -cyano-4-hydroxycinnamic acid as the matrix. All spectra were collected in the negative ion mode.

Analysis of N-terminal Sequences by Electrospray Ionization-Mass Spectrometry (ESI-MS) and Tandem-Mass Spectrometry-- ESI-MS was performed using a Finnigan LCQ atmospheric pressure ionization quadrupole ion trap mass spectrometer (ThermoQuest Corp.). The mass spectrometer was equipped with an X-Y-Z-positioner carriage source (Protana A/S) and a sample loop containing a C18 microtrap cartridge (Michrom BioResources). Samples were resuspended in 50 mM formic acid, loaded onto the C18 microtrap cartridge, and then sprayed into the mass spectrometer through a 50-µm (inner diameter) fused silica needle by eluting with 50% methanol/1% acetic acid at 10 µl/min. Spray voltage was set at 1.75 kV with no sheath or auxiliary gas flow and a capillary temperature of 200 °C. Other voltages were set automatically by tuning on the 881 [M+2H]²⁺ ion of a renin substrate tetradecapeptide standard (Sigma). Full scan and MS/MS fragmentation data were collected in centroid mode with two microscans during a 500-ms maximum injection time using the default automatic gain control target number of ions. Quadruply charged ions were fragmented with a 40% collision energy and an isolation width of 2.0 atomic mass units.

Cleavage of EP-procyclins by Mild Acid Hydrolysis-- aq.HF-treated procyclin (50 pmol of dephosphorylated polypeptides) was hydrolyzed with 40 mM trifluoroacetic acid (TFA) (Pierce) at 100 °C for 15 min. After hydrolysis, the samples were chilled in ice water and washed with 50 µl of water to remove residual TFA. For analysis of the EP-procyclin N-terminal sequences by ESI-MS, 4.5 nmol of native procyclin was submitted to TFA hydrolysis as indicated above. The dried protein, in 50 µl of 0.1 M ammonium acetate, 5% n-propanol (v/v), was loaded onto a mini-octyl-Sepharose column (~0.5 ml) previously equilibrated in the same buffer. Whereas the C-terminal fragments as well as the nondegraded protein bound tightly to the hydrophobic resin through their GPI anchors, the hydrophilic N-terminal fragments were collected in the flow-through of the column. The column was washed with 1 ml of 0.1 M ammonium acetate, 5% n-propanol (v/v), and the collected material was pooled, dried in the Speed-Vac, and freeze-dried twice to remove residual ammonium acetate.

Enzymatic Deglycosylation of Procyclin Polypeptides-- aq.HF-treated procyclin (200 pmol) was deglycosylated with 500 units of peptide N⁴(N-acetyl--glucosaminyl) asparagine amidase F (PNGase F) (New England Biolabs) in 10 µl of 25 mM sodium phosphate at 37 °C for 2 h. After digestion, 1 µl of the sample was directly mixed with 1 µl of -cyano-4-hydroxycinnamic acid and analyzed by MALDI-TOF-MS. For analysis of procyclin N-terminal polypeptides by ESI-MS, the polypeptides obtained after mild acid hydrolysis and octyl-Sepharose chromatography (see above) were incubated with PNGase F as described above except that the buffer was 50 mM sodium phosphate and the digestion conditions were 37 °C for 5 h. Prior to mass spectral analysis, the deglycosylated polypeptides were desalted using a ZipTip (Millipore Corp.) containing C18 silica. Samples were eluted as recommended by the manufacturer, dried in a Speed-Vac, and resuspended in 50 mM formic acid before analysis.

Analysis of Phosphoryl Groups in EP-procyclin-- Procyclins (450 pmol) were dephosphorylated (or mock treated) with 2 units of calf intestine alkaline phosphatase (Roche Molecular Biochemicals) in 10 µl of 50 mM Tris-HCl, 0.1 mM EDTA, pH 8.5, at 37 °C for 16 h. After digestion, the samples were dried in the Speed-Vac and incubated with 25 µl of 48% aq.HF at 0 °C for 6 h. An aliquot of the HF-treated sample (45 pmol) was submitted to mild acid hydrolysis as described above and then deglycosylated with 250 units of PNGase F (Biolabs) for 2 h at 37 °C. After each step, 10% of each sample was analyzed by MALDI-TOF-MS.

	RESULTS

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

Characterization of Native Procyclin by Mass Spectrometry-- For MALDI-TOF-MS analysis, we first used native procyclin purified by organic solvent extraction and octyl-Sepharose chromatography. The negative ion spectrum revealed a highly heterogeneous group of [M-H] pseudomolecular ions in the range of m/z 14,000-20,000 (average m/z, 17,400), consistent with an average mass of 17.4 kDa (not shown). A comparable spectrum was reported previously for purified GPEET-procyclin, and the extensive heterogeneity observed is mainly due to the polydisperse side chain of the GPI anchor (13, 19). To distinguish the different EP-procyclin species, it was first essential to remove the GPI anchors.

Characterization of the EP-procyclin Polypeptides by Mass Spectrometry after Removal of GPI Anchors-- We removed the anchors by treatment with cold 48% aq.HF, a reagent that cleaves the phosphodiester bond between the anchor ethanolamine and the GPI glycan core (26, 27). This treatment preserves most polypeptide chains with N-glycan moieties intact and leaves the GPI ethanolamine amide-linked to the C-terminal -carboxyl. In the case of EP-procyclin, HF also produced minor cleavages in the protein backbone, mainly at the mild acid-sensitive Asp-Pro bonds (see below).

The negative ion MALDI mass spectrum of HF-treated procyclins revealed many [M-H] pseudomolecular ions in the range of m/z 5,000-12,000 (Fig. 2A). Although the spectrum is complex, nearly all of these species were interpretable in terms of the well characterized procyclin gene sequences and N-glycosylation patterns. Table I lists the masses and assignments of the various species detected. Two ions at m/z 11,531 (ion 1) and 10,430 (ion 3) had molecular masses for two different glycosylated products of the EP1 gene, each also containing one ethanolamine. According to their cDNA sequences and results shown later in this paper, these proteins differ in both the number of EP repeats (30 for ion 1, 25 for ion 3) and in one amino acid at position 24 (4, 6-8) (Fig. 1B). We have designated these ions as the products of the EP1-1 and EP1-2 genes, respectively. Another major [M-H] ion at m/z 9,723 (ion 4) matches the calculated mass of the glycosylated product of the EP-procyclin EP3 gene, a protein with 22 EP repeats (5, 9) (Fig. 1B). Interestingly, no ion corresponding to the product of the EP-procyclin EP2 gene (expected mass = 8,571, including one ethanolamine) was observed. However, a tiny peak at m/z 8,344 (ion 10) was tentatively assigned as a product of this gene, but this polypeptide contained only 24 EP repeats instead of the 25 predicted from the published cDNA sequence (10). To distinguish both EP-procyclin species, we have designated them as EP2-1 (containing 25 EP repeats) and EP2-2 (containing 24 EP repeats). As expected, the mass of EP2-2 procyclin (ion 10) predicted that this polypeptide was not N-glycosylated (10).

View larger version (34K): [in this window] [in a new window]   Fig. 2.   Negative ion MALDI-TOF mass spectra of procyclin after removal of the GPI anchors. A, see text and Table I for assignment of ions. B, aq.HF-treated procyclin was incubated with PNGase F to remove N-glycans and analyzed by MALDI-TOF-MS. Note the new peaks that appeared at m/z 3,119 and 3,332, also shown in more detail in the inset (see text for discussion of these peaks). Asterisks indicate polypeptides missing 10 residues from the N terminus.

For each of the full-length EP-procyclin gene products observed in the negative ion spectrum, we also detected a corresponding ion that lacked ten amino acids from the N terminus (AEGPEDKGLT; Fig. 1B). Thus, the [M-H] ions at m/z 10,533 (ion 2), 9,432 (ion 5), and 8,725 (ion 8) (marked by asterisks on Fig. 2A) represent the truncated forms of the EP1-1, EP1-2, and EP3 gene products, respectively. These fragments varied in abundance in different preparations, and control experiments strongly indicated that they originate during the aq.HF treatment (see "Discussion").

Another group of lower intensity [M-H] ions in the range of m/z 7,500-9,500 (ions 6 and 11-15; Table I), mainly represent minor products of nonspecific cleavages from the N termini of the products of the EP1-1, EP1-2, and EP3 genes. Some of them are worth discussing because they support the assignment of two types of EP1 gene products. For instance, those ions at m/z 8,175 (ion 12) and 8,276 (ion 11) are consistent with a polypeptide containing Ser at position 24 as well as 25 EP repeats, which we have designated the EP1-2 polypeptide (Table I). Likewise, the ion at m/z 9,277 (ion 6) is consistent with a polypeptide containing a Gly at position 24 as well as 30 EP repeats, expected for the EP1-1 protein. As discussed below, we confirmed these proposed structures of the EP1-1 and EP1-2 proteins by tandem mass spectrometry. Finally, another group of ions (ions 17, 18, 22, 23, 27, 28, and 29) derives from cleavage at Asp-Pro sequences, the major side reaction of the HF dephosphorylation. We will discuss the significance of these ions below, in the section on mild acid hydrolysis.

Characterization of GPEET-procyclin Polypeptides-- We also detected polypeptides derived from GPEET-procyclin, the product of the GPEET gene. The ion at m/z 6,142 (ion 21) corresponded to the intact GPEET-procyclin polypeptide and that at m/z 6,222 (ion 20) to the same polypeptide containing one phosphate group. Presumably the other phosphates had been removed during the aq.HF dephosphorylation. These values are consistent with those recently reported for the same molecule (19). The ion at m/z 5,703 (ion 25) corresponds to nonphosphorylated GPEET-procyclin that has lost its N-terminal sequence VIVK (Fig. 1B).

Glycosylation of the EP-procyclin Polypeptides-- The major EP-procyclin polypeptides identified in Fig. 2A have molecular masses consistent with its single Asn being modified by a Hex₅HexNAc₂ oligosaccharide. Previous structural analyses have proven that this glycan is Man₅GlcNAc₂ (13, 14). To further confirm our polypeptide assignments and to determine the occupancy of the N-glycosylation sites, we deglycosylated a sample like that in Fig. 2A with PNGase F and then analyzed the products by MALDI-TOF-MS (Fig. 2B). The major ions corresponding to the intact polypeptide products of the EP1-1, EP1-2, and EP3 genes are missing (ions 1, 3, and 4) as are the corresponding ions that have lost the N-terminal decapeptide (ions 2, 5, and 8). In their place are new ions at m/z 10,314, 9,213, and 8,506, which have values corresponding to the deglycosylated products of the EP1-1, EP1-2, and EP3 genes, respectively, as well as ions at 9,316 m/z (EP1-1), 8,215 (EP1-2), and 7,508 (EP3), which are the deglycosylated species with truncated N termini. In all cases, the mass of the deglycosylated species is 1217 Da less than that of the native species, a mass corresponding to that of a Hex₅HexNAc₂ glycan. The corresponding deglycosylated species detected in Fig. 2B were present only in very low levels in the MALDI spectrum shown in Fig. 2A, indicating that most of the EP-procyclin polypeptides were occupied by a single N-glycan, as predicted in a previous report (13). Furthermore, we found no evidence of any heterogeneity in this glycan, also in agreement with previous reports (13, 14).

Interestingly, after PNGase F incubation (Fig. 2B and inset), two new very intense ions at m/z 3,119 and 3,331 were detected. These ions were identified as the deglycosylated N-terminal fragments Ala¹-Asp³⁴ (m/z 3,331) and Ala¹-Asp³² (m/z 3,119) of the EP1-1 and EP3³ proteins, produced by cleavage at Asp-Pro bonds. An amplification of the region between m/z 3,000-3,500 (inset, Fig. 2B) showed that there were two additional ions. These were identified as the deglycosylated N-terminal fragments Ala¹-Asp³⁴ (m/z 3,360) and Ala¹-Asp³² (m/z 3,147) of the EP1-2 protein. As shown in Fig. 1B, the EP1-2 and EP1-1 gene products differ in this region only by the presence of a Gly or a Ser at position 24. The corresponding ions in their glycosylated form (e.g. m/z 4,577 and 4,365) were never observed in the sample not treated with PNGase F (Fig. 2A), even when analyzed in the positive mode (not shown).

Characterization of the C-terminal Fragments of the EP-procyclins after Selective Mild Acid Hydrolysis of the Asp-Pro Bonds-- To confirm the number of EP repeats, we partially hydrolyzed the procyclin polypeptides (already aq.HF-treated) with 40 mM TFA at 100 °C for 15 min. Under these relatively mild conditions, Asp-Pro peptide bonds (found at residues 32-35 in EP3 and EP1 procyclins) are preferentially cleaved (28). The negative ion MALDI-TOF-MS spectrum after mild acid hydrolysis (Fig. 3) confirmed that cleavage occurs at these Asp-Pro sequences, generating distinct C-terminal fragments.⁴ Every EP-procyclin molecule was cleaved at one of these sites, because no ions in the range m/z 8,000 to 12,000 were detected (not shown). In the range of m/z 5,000 and 7,000, three major ions were observed. These ions were identified as the C-terminal fragments Pro³⁵-(Glu-Pro)₂₂-Gly⁸⁰-EtN (m/z 5,191), Pro³⁵-(Glu-Pro)₂₅-Gly⁸⁶-EtN (m/z 5,870) and Pro³⁵-(Glu-Pro)₃₀-Gly⁹⁶-EtN (m/z 7,001), which derive from the EP3, EP1-2, and EP1-1 products, respectively. Each of these fragments is also paired with an ion (m/z 5,403, 6,082, and 7,213) whose mass is larger by about 212 Da (the mass of a Pro-Asp dipeptide). These fragments derive from cleavage at the upstream Asp-Pro sequence. As mentioned above, partial cleavage at Asp-Pro bonds was also detected in Fig. 2A, as a side reaction of the aq.HF dephosphorylation used to remove GPI anchors. The ion at m/z 4,965 (also detected in Fig. 2A) corresponds to the C-terminal fragment (i.e. Pro(Glu-Pro)₂₁Gly-EtN) of an unidentified EP-procyclin. The origin of this species is unknown because its intensity is very low and the predicted full-length product (adding the mass of either an EP1 or EP3 N terminus) was either undetected or masked by other ions in Fig. 2A. However, this C-terminal fragment could derive from an isoform of the EP3 product.

View larger version (23K): [in this window] [in a new window]   Fig. 3.   Analysis of the EP-procyclin C-terminal domains by MALDI-TOF-MS. HF-treated procyclin was submitted to mild acid hydrolysis with 40 mM TFA, and the products were analyzed by MALDI-TOF-MS. The ions in the range of m/z 4,000-5,000 correspond to nonspecific fragmentation of the EP repeats.

Confirmation of Sequences of Procyclin N-terminal Fragments by Electrospray Ionization-MS-- To confirm that two different EP1 gene products were present, we conducted partial N-terminal sequencing by tandem mass spectrometry. We subjected native procyclin (not aq.HF-treated) to mild acid hydrolysis to cleave at the Asp-Pro bonds. We then separated the N-terminal fragments from the C-terminal domains (containing a GPI anchor) using octyl-Sepharose chromatography. We collected the hydrophilic N-terminal fragments in the flow-through of this column, deglycosylated them with PNGase F, and then analyzed the products by ESI-MS. Analysis in the positive mode showed several major [M+4H]⁴⁺ pseudomolecular ions (not shown). Two major ions at m/z 788.4 and 841.4 matched the predicted values of the nonglycosylated fragments Ala¹-Asp³² and Ala¹-Asp³⁴ from the EP1-2 protein. Likewise, the ions at m/z 780.8 and 833.9 matched the calculated [M+4H]⁴⁺ values for the same fragments from EP1-1 and EP3 procyclins, respectively (not shown). Consistent with the presence of these quadruply charged ions, we also detected a group of ions (m/z 1,040.6, 1,050, 1,111.8, and 1,121.5) that derive from the same species, except that they are triply charged (not shown).

Collision-induced dissociation (CID) daughter ion spectra of the [M+4H]⁴⁺ m/z 788.4 (Fig. 4A) and 780.8 (not shown) parent ions, generated, in both cases, multiply charged N-terminal (b-series) and C-terminal (y-series) daughter ions. The major daughter ions in both spectra were the quadruply charged b ions (b_31-32⁴⁺), which confirmed the sequences Ala¹-Thr³¹ and Ala¹-Asp³². However, the more informative series were the triply charged b ions (b³⁺), because they defined the region where the three polypeptides have different amino acid sequences (i.e. positions 18, 24 and 25). The results are summarized in Fig. 4B, in which the observed masses (from the spectrum in Fig. 4A) can be compared with the predicted masses of the b³⁺ series for the three EP-procyclin gene products (only N-terminal fragments). These results, together with those obtained from MALDI-TOF-MS analyses (Fig. 2A and Table I), confirm the presence of the products of the EP1-1, EP1-2, and EP3 genes.

View larger version (28K): [in this window] [in a new window]   Fig. 4.   CID-ESI-MS analysis of EP-procyclin N termini. Native procyclin (not HF-treated) was submitted to mild acid hydrolysis, and the N-terminal fragments were purified, deglycosylated with PNGase F, and analyzed by ESI-MS. A, CID daughter ion spectrum of the 788.4 parent ion (from EP1-2 procyclin). b and y represent the b (N-terminal) and y (C-terminal) series, respectively. The CID daughter ion spectrum of the 780.8 parent ion (from EP1-1 and EP3 proteins) is not shown, but it presents a similar pattern of fragmentation. B, N-terminal polypeptide sequences detected in the CID mass spectra. Only the b3+ series from each polypeptide is shown. Underlined values represent the theoretical masses (b3+ ions) that matched the [M+3H]3+ ions identified in the CID daughter ion spectra. Nonunderlined values were not detected in the spectra. Amino acid residues that differ in the three polypeptides are underlined.

Is EP-procyclin Phosphorylated?-- It is well established that the GPEET-procyclin is highly phosphorylated on the amino acid repeats (18-20), but the presence of this modification has not been determined for the EP isoforms. The EP-procyclins contain several Thr or Ser residues that could be phosphorylation sites. Consistent with phosphorylation, we found that MS analysis of procyclins treated for 2-8 h with aq.HF showed, in addition to the phosphorylated GPEET forms, new peaks (not detected in the MS in Fig. 2A) that were larger by 80 Da than each EP isoform (not shown). This finding suggested the presence of singly phosphorylated EP-procyclin species. We then studied whether this phosphate is present in the N-terminal domain or linked to the C-terminal ethanolamine, deriving from the GPI anchor. We incubated native procyclin for 16 h with calf intestine alkaline phosphatase and then treated it for 6 h with aq.HF, and finally submitted it to mild acid hydrolysis. Analysis by MALDI-TOF-MS showed that each of the major C-terminal polypeptides (m/z 5,191, 5,870, and 7,001) of the EP-procyclins is partly phosphorylated, resulting in ions of m/z 5271, 5,950, and 7,081, respectively (Fig. 5A). We also detected phosphorylated species of the less abundant C-terminal fragments that were cleaved at the upstream Pro-Asp bond (i.e. m/z 5,403, 6,082, and 7,213), as well as the fragment P(EP)₂₁G-EtN (m/z 4,965). Because these phosphate groups had resisted hydrolysis by alkaline phosphatase, these results indicate that they were originally present in the native molecules as phosphodiesters. Furthermore, subsequent deglycosylation of the same sample⁴ showed that none of the N-terminal fragments was phosphorylated (Fig. 5B). Taken together, these results provide strong evidence that in contrast to GPEET-procyclin, the EP-procyclins are not phosphorylated. The residual phosphate groups detected by mass spectrometry after short HF treatment must derive from the phosphodiester bond originally present between the C-terminal ethanolamine and the GPI glycan core.

View larger version (29K): [in this window] [in a new window]   Fig. 5.   Location of the phosphoryl group on EP-procyclin polypeptides. Native procyclin was dephosphorylated with alkaline phosphatase, then digested for 6 h with HF, and finally submitted to mild acid hydrolysis. The products were then analyzed by negative ion MALDI-TOF-MS. A, spectrum of the EP-procyclin C termini. P in a circle, indicates the phosphorylated C-terminal species. The ions between 3,500 and 4,000 m/z correspond to the ladder of EP repeats previously described in Fig. 3. B, analysis of the N termini after deglycosylation. Note the presence of minor truncated N-terminal fragments (between 2,000-3,000 m/z), which are consistent with most of the C-terminal fragments identified in Fig. 2A. Sequences are shown above each pair of peaks.

Stability of Expression of the Procyclin Repertoire-- The parasites used for this study had been stored at New York University since 1988, soon after they had been transformed from a cloned bloodstream form (strain 118). They had been cultured only about 6 weeks before the isolation of procyclins used in the experiments shown in Figs. 2-5. To determine whether the procyclin repertoire was identical in all cells in the population, we examined 10 clones from this population obtained at New York University and 1 additional clone obtained at Johns Hopkins. All 11 clones had virtually identical procyclin compositions (Fig. 6). They also resembled that of the 1988 strain (Fig. 2A), except that we could barely detect GPEET-procyclin species and the amount of EP3 procyclin was slightly more abundant (Fig. 6). These results indicate that there is little variation from cell to cell in the original population. We also studied whether or not the EP-procyclin repertoire changed with time in culture. In cells from cultures passaged for 2 years at New York University or 6 months at Johns Hopkins, we found that the repertoire was nearly identical to that in the 1988 strain. However, in both cultures the GPEET-procyclin had virtually disappeared (not shown).

View larger version (22K): [in this window] [in a new window]   Fig. 6.   Negative ion MALDI-TOF mass spectrum of procyclin from a clonal cell line. Procyclin from clone D58H-B7-11 (22) was extracted with organic solvent and not further purified by octyl-Sepharose chromatography. The GPI anchors were removed by HF treatment. See Table I for assignments of ions. Asterisks indicate polypeptides missing 10 residues from the N terminus.

	DISCUSSION

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

Since the discovery of T. brucei procyclin proteins (3, 29) and their encoding genes (4, 5, 11), there has been extensive study of the expression and regulation of the genes, their GPI anchors, and their N-glycan (13-15, 18, 19). However, there has been little characterization of the different procyclin polypeptides expressed on the cell surface. Of the various polypeptides predicted from the gene sequences, only an unfractionated mixture of the EP-procyclin species (3, 15, 29, 30) and, more recently, GPEET-procyclin (13, 14, 18-20) have been demonstrated to exist as proteins. The identification of individual EP-procyclin polypeptides has been extremely difficult because they have very similar sequences and therefore are not resolved by SDS-polyacrylamide gel electrophoresis or other fractionation techniques, even after removal of their highly heterogeneous GPI anchors. Fractionation is made even more difficult because the proteins are almost impossible to detect by conventional protein stains or by absorption at 280 nm. It is, however, possible to detect the unfractionated EP-procyclin species and GPEET-procyclin using specific monoclonal antibodies (3, 14, 20).

Using MALDI-TOF-MS we have been successful in identifying all of the procyclin species (both the EP and GPEET forms) in a clone of T. brucei strain 427. We used HF treatment to remove the GPI anchors and then, in a key reaction, used mild acid hydrolysis to cleave selectively the products of the EP3, EP1-2, and EP1-1 genes at Asp-Pro sequences. We found that the 427 clone expresses the products of three genes encoding EP forms (EP3, EP1-2, and EP1-1) as well as the product of the gene encoding GPEET-procyclin (GPEET). Surprisingly, we detected only a trace of EP-procyclin encoded by one of the EP2 genes, the EP2-2.

We detected several fragments derived from the full-length polypeptides. For all EP-procyclin species, we found fragments missing 10 residues from the N termini. These fragments varied in abundance in different preparations, and control experiments indicated that they probably formed during the aq.HF treatment by cleavage at the Thr¹⁰-Lys¹¹ bond. Consistent with this hypothesis, we detected no N-terminal fragment lacking this region in the ESI-mass spectrum, because this preparation had not been aq.HF-treated (not shown). In the case of GPEET-procyclin, we found polypeptides missing four residues from the N terminus. This fragment had been detected previously by MS (19) and also by Edman degradation (13, 14, 18). Because the protein used for Edman sequencing was intact, this fragment is not dependent on HF treatment and is probably present in the cell.

In agreement with previous results (18-20), we detected phosphorylation of GPEET-procyclin. Previous studies had revealed that six out of seven threonine residues in this protein are phosphorylated (19), but we detected only a single phosphate, presumably because of the longer duration of our aq.HF treatment (16 h compared to 8 h in the previous study). We did not detect phosphorylation of EP-procyclins in the mass spectra presented in this paper. We did detect singly phosphorylated EP-procyclin molecules if the time of aq.HF treatment was reduced to 2-8 h (not shown), but that phosphate is linked to the C-terminal ethanolamine and derives from the GPI anchor (Fig. 5). Furthermore, we detected no phosphoamino acids in the N-terminal domain of EP-procyclins by both ESI-MS (not shown) and CID-ESI-MS analysis (Fig. 4). This preparation had not been subjected to HF treatment and had been treated only with mild acid to cleave the protein at Asp-Pro sequences (conditions in which phosphomonoester groups should remain attached to proteins). Thus, we conclude that the EP-procyclins are not phosphorylated on the polypeptide chains.

The MS analysis was informative about the glycosylation state of EP-procyclin. As mentioned above, the major EP-procyclin gene products that we detected (i.e. EP3, EP1-2, and EP1-1) contain a single N-glycosylation site at Asn²⁹. Our results confirmed that this site is modified by a homogeneous Hex₅HexNAc₂ glycan, previously shown to be Man₅GlcNAc₂ (13, 14). Because only trace deglycosylated EP-procyclin species were detected in the absence of PNGase F treatment, we concluded that the occupancy of this asparagine is greater than 90%. It is interesting that this parasite expresses little (if any) of a variant of the EP-procyclin encoded by the EP2 gene, which is the only nonglycosylated EP-procyclin product that also lacks Asp-Pro bonds (10) (Fig. 1B). It is possible that this protein is not regularly expressed in parasites cultured in vitro but that instead it has a programmed expression during parasite development in the insect vector. Another related possibility is that the EP2 gene is active only under some conditions, and in fact we have found that a T. brucei mutant (ConA 4-1) selected by resistance to killing by concanavalin A in in vitro culture (14) expresses another variant of the EP2 gene product (the EP2-3 protein) as its major procyclin species.⁵

One potential limitation of the MALDI-TOF-MS method in evaluating the procyclin repertoire is that different components in the mixture may vary in sensitivity of detection. For example, we never detected by MALDI-TOF-MS the N-terminal fragments (resulting from the cleavage at the Asp-Pro bonds) of the EP3, EP1-2, and EP1-1 proteins, until they were deglycosylated with PNGase F (Fig. 2, compare A and B). It is not clear why these species were not detected, but the N-glycan may have interfered either with co-crystallization with the matrix or with their detection in the negative mode. Nevertheless, most procyclin species have such closely related structures that they are likely to be detected with similar sensitivities. Using Edman N-terminal sequencing analysis of total procyclin purified from the same clone, we estimated that this preparation contained ~21% GPEET-procyclin (14). Given the difficulty in analyzing mixtures of polypeptides by Edman sequencing, this result is in reasonable agreement with that presented in Fig. 2A.

These studies provide detailed information about the expression of procyclin genes. It is striking that one gene in each locus seems to be expressed, at the protein level, at a much higher level than the other. For example, although the GPEET and EP3 gene products are encoded in the same locus, very little of GPEET-procyclin compared with EP3 procyclin was detected (Figs. 2A and 6). Likewise, the EP2 and the EP1 genes are located in the same locus, but only barely detectable amounts of EP2 procyclin were observed. Thus, given the polycistronic nature of procyclin transcription, these findings suggest that expression of the EP2 and the GPEET genes is regulated by post-transcriptional mechanisms. Specific domains at the 3'-untranslated region of procyclin mRNA have been identified as regions that modulate RNA stability and translation in both bloodstream and procyclic forms (31-33). Alternatively, expression of these proteins could also be regulated by undefined translational or post-translational mechanisms. In any case, regulation of procyclin expression is a complex process that occurs at different levels (12, 34). Comparison of protein levels, as described in this paper, with mRNA levels should help to clarify these regulatory mechanisms.

It is likely that the two distinct EP1-procyclins, EP1-1 and EP1-2, are allelic copies of the same gene. An alternative possibility, that they derive from different cells in the population, is ruled out by the fact that this population of procyclic cells was derived from a cloned bloodstream form and therefore should be clonal themselves. In addition, our finding that 11 clones derived from this population all had identical repertoires of EP-procyclin provides further evidence that these cells express both forms of the EP1 gene. Both EP1 proteins had already been identified at the DNA sequence level but from different parasite clones (4, 6-8), and their allelic variability has been previously demonstrated by Southern blot analysis (35). It is also reasonable to conclude that the allelic copies of the EP2 genes also differ in sequence, because the EP2 gene sequence predicted 25 EP repeats (10), whereas the candidate EP2 protein that we detected (ion 10 in Table I) had 24 EP repeats. Likewise, the presence of a C-terminal fragment containing 21 EP repeats (Figs. 2A and 3) might suggest the presence of another EP3 gene product. Allelic variability has also been documented for others T. brucei genes, for example those for RNA polymerase II (36).

We were surprised to find that clones derived from the population of 1988 cells had lost almost all expression of GPEET-procyclin, although their EP-procyclin repertoire was similar to that of the parent cells. This finding was unexpected in that others had reported that over time, expression had switched from low levels of GPEET-procyclin to high levels (13, 18). In fact, in our own laboratory on a previous occasion, we had observed increasing levels of GPEET-procyclin after a few months of culture. This variability in switching of expression of different procyclin genes implies that some uncontrolled factor in culturing conditions influences procyclin expression. For example, variation in serum, present in the culture medium, could be responsible. The latter may be relevant in the expression of different procyclin proteins during development in the insect vector, because all protein components in serum should be gradually degraded during the first few days of infection of the vector. Furthermore, such a switch in the expression of procyclin genes could modulate the interaction of the parasite with the insect vector. We are conducting further studies to determine whether serum or other factors are responsible for this change in procyclin expression.

In summary, using a new methodology that combines mild acid hydrolysis and mass spectrometry, we were able distinguish, with high accuracy, the whole repertoire of procyclin polypeptides expressed by several procyclic T. brucei clones. Using this methodology we have obtained new information about procyclin expression and its post-translational modifications. Because the mass spectrometry analyses can be performed directly using n-butanol extracted molecules (without further purification by hydrophobic interaction chromatography) and from as few as 10⁵ cell equivalents,⁶ this methodology could aid in the determination of structure-function relationships of T. brucei surface glycoproteins in the insect vector.

	ACKNOWLEDGEMENTS

We thank Terry Pearson, Kim Paul, Yasu Morita, Isabel Roditi, Jim Morris, David Jiang, and Peter Butikofer for helpful discussions and critical reading of the manuscript. We also thank Isabel Roditi and Christine Clayton for helpful suggestions on the procyclin nomenclature.

	FOOTNOTES

^* This work was supported by National Institutes of Health Grants AI21334 (to P. T. E.) and AI28953 (to M. S.-G. L.) and by Wellcome Trust Program Grant 054491 (to M. A. J. F.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^§ Supported in part by a postdoctoral Fellowship from Consejo Nacional de Investigaciones Científicas y Tecnológicas (Venezuela). To whom correspondence should be addressed. Tel.: 410-955-3458; Fax: 410-955-7810; E-mail: aacostas@welchlink.welch.jhu. edu.

^¶ Supported by the American Health Assistance Foundation.

Burroughs Wellcome Fund New Investigator in Molecular Parasitology.

² These proteins have also been called procyclic acidic repetitive proteins (PARPs), but a new standard nomenclature for genes and proteins now uses the term procyclin (37).

³ The calculated mass of the N-terminal fragment of EP3 procyclin is only 2 Da smaller than that of the EP1-1 protein, so we assigned these peaks as containing fragments from both gene products.

⁴ The N-terminal fragments are not seen in the negative ion mode unless the N-glycan is removed.

⁵ A. Acosta-Serrano, R. N. Cole, A. Mehlert, J. D. Bangs, M. A. Ferguson, and P. T. Englund, manuscript in preparation.

⁶ A. Acosta-Serrano and P. T. Englund, unpublished observation.

	ABBREVIATIONS

The abbreviations used are: VSG, variant surface glycoprotein; EP-procyclin, a form of procyclin rich in Glu-Pro repeats; GPEET-procyclin, a form of procyclin rich in Gly-Pro-Glu-Glu-Thr repeats; GPI, glycosyl phophatidylinositol; aq.HF, 48% aqueous hydrofluoric acid; MS, mass spectrometry; MALDI-TOF-MS, matrix-assisted laser desorption ionization time-of-flight MS; TFA, trifluoroacetic acid; ESI-MS, electrospray ionization-mass spectrometry; EtN, ethanolamine; PNGase F, peptide N⁴(N-acetyl--glucosaminyl) asparagine amidase F; CID, collision-induced dissociation.

	REFERENCES

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

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