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J Biol Chem, Vol. 275, Issue 19, 14095-14101, May 12, 2000

cDNA Cloning of Phosphoethanolamine N-Methyltransferase from Spinach by Complementation in Schizosaccharomyces pombe and Characterization of the Recombinant Enzyme^*

Michael L. Nuccio, Michael J. Ziemak, Susan A. Henry^§, Elizabeth A. Weretilnyk^¶, and Andrew D. Hanson

From the  Horticultural Sciences Department, University of Florida, Gainesville, Florida 32611, the ^§ Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, and the ^¶ Department of Biology, McMaster University, Hamilton, Ontario L8S 4K1, Canada

	ABSTRACT

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

The N-methylation of phosphoethanolamine is the committing step in choline biogenesis in plants and is catalyzed by S-adenosyl-L-methionine:phosphoethanolamine N-methyltransferase (PEAMT, EC 2.1.1.103). A spinach PEAMT cDNA was isolated by functional complementation of a Schizosaccharomyces pombe cho2 mutant and was shown to encode a protein with PEAMT activity and without ethanolamine- or phosphatidylethanolamine N-methyltransferase activity. The PEAMT cDNA specifies a 494-residue polypeptide comprising two similar, tandem methyltransferase domains, implying that PEAMT arose by gene duplication and fusion. Data base searches suggested that PEAMTs with the same tandem structure are widespread among flowering plants. Size exclusion chromatography of the recombinant enzyme indicates that it exists as a monomer. PEAMT catalyzes not only the first N-methylation of phosphoethanolamine but also the two subsequent N-methylations, yielding phosphocholine. Monomethyl- and dimethylphosphoethanolamine are detected as reaction intermediates. A truncated PEAMT lacking the C-terminal methyltransferase domain catalyzes only the first methylation. Phosphocholine inhibits both the wild type and the truncated enzyme, although the latter is less sensitive. Salinization of spinach plants increases PEAMT mRNA abundance and enzyme activity in leaves by about 10-fold, consistent with the high demand in stressed plants for choline to support glycine betaine synthesis.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

Flowering plants are unusual in how they synthesize choline (Cho)¹ moieties. In leaves and other vegetative tissues, the first and committing step is N-methylation of phosphoethanolamine (P-EA) to give phosphomonomethylethanolamine (P-MME), and the subsequent N-methylations occur at the phosphobase level, the phosphatidyl base level, or both, depending on the species (1, 2) (see Fig. 1A). For example, in spinach and sugar beet leaves essentially all flux through the last two methylations is at the phosphobase level (3, 4), whereas in soybean cells it is at the Ptd base level (5). In contrast, Cho synthesis in the bacterium Rhodobacter sphaeroides, in fungi, and in mammalian liver proceeds solely via the sequential methylation of phosphatidylethanolamine (Ptd-EA) (6-8). Nerve tissues have a phosphobase methylation route as well as a phosphatidylbase route, but the first methylation appears not to be restricted to the phosphobase level (9, 10).

The initial methylation of P-EA in plants is catalyzed by S-adenosyl-L-methionine:phosphoethanolamine N-methyltransferase (PEAMT), which has been detected in all species tested (1, 4, 11). PEAMT was recently purified 5,400-fold from spinach leaves, giving a preparation containing several polypeptides (12). This preparation catalyzed methylation of P-MME and phosphodimethylethanolamine (P-DME) as well as P-EA, and these activities co-purified in a constant ratio through the three last steps in the procedure (12). These data suggest that PEAMT could be trifunctional but do not rule out a duo or trio of similar N-methyltransferases that act on different phosphobases. Pathways involving one, two, or three N-methyltransferases all have precedents. In R. sphaeroides and liver a single Ptd-EA N-methyltransferase mediates all three methylations (6, 8), whereas in Saccharomyces cerevisiae and Schizosaccharomyces pombe there are two enzymes, one mediating the first methylation of Ptd-EA and another mediating primarily the last two (7). The phosphobase pathway in nerve tissues has three separate N-methyltransferases (9).

Certain plants (e.g. spinach, sugar beet) use large amounts of Cho to produce the osmoprotectant glycine betaine (GlyBet) via the pathway Cho  betaine aldehyde  GlyBet (2). Because GlyBet accumulation contributes to resistance to salinity and drought stress, there has been much interest in engineering GlyBet synthesis in plants that do not naturally produce it (13). However, when enzymes for Cho oxidation to GlyBet are expressed in such plants (e.g. tobacco, canola) they accumulate little GlyBet, apparently in part because their endogenous Cho supply is inadequate (11, 14). This has focused attention on the pathway of Cho biosynthesis and its regulation (13).

Biochemical and physiological evidence shows that the PEAMT-mediated step is a control point in the biosynthesis of Cho moieties and that at least two mechanisms are involved. One is feedback inhibition; PEAMT activity in crude Lemna extracts (15) and purified spinach preparations (12) is inhibited by P-Cho, and ¹⁴C tracer data for sugar beet leaf tissue indicate that this occurs in vivo (3). Another mechanism may be regulation of PEAMT gene expression. The de novo synthesis of Cho in Lemna, soybean, and carrot cells is suppressed by exogenous Cho, and this suppression is accompanied by a decrease in extractable PEAMT activity (15, 16). Conversely, salinization in spinach, which increases the consumption of Cho in GlyBet synthesis, causes an increase in PEAMT activity (17). Direct evidence that flux through the PEAMT step limits the synthesis of Cho moieties in vivo comes from tobacco engineered to convert Cho to GlyBet; supplying MME or DME increases the flux to Cho and GlyBet but supplying EA does not (11).

Despite the importance of PEAMT as the committing step in the biogenesis of Cho moieties, it has not been cloned or unambiguously characterized with respect to the reaction(s) it catalyzes. Nor has any other plant N-methyltransferase participating in Cho biogenesis been cloned. In this study, we identified a spinach PEAMT by complementing a S. pombe cho2 mutant, which lacks the first methyltransferase required for the synthesis of Cho moieties (see Fig. 1B). We found that PEAMT contains two methyltransferase domains and can convert P-EA to P-Cho.

	EXPERIMENTAL PROCEDURES

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

Chemicals-- [³²P]dCTP (3000 mCi mmol¹) and [methyl-¹⁴C]AdoMet (59 mCi mmol¹; in 10 mM H₂SO₄:ethanol, 9:1) were purchased from NEN Life Science Products; the specific activity of AdoMet was adjusted to the desired value with unlabeled compound (Sigma). Chiral HPLC (18) showed that [methyl-¹⁴C]AdoMet was 99% in the S,S (biologically active) form. Unlabeled AdoMet was 85% in the S,S form and 15% in the R, S (inactive) form; specific activities were calculated using the S,S-AdoMet content. [³³P]P-MME and [¹⁴C]P-Cho were made as described (19). Restriction and modification enzymes and oligonucleotides were from Life Technologies, Inc., New England Biolabs, or Roche Molecular Biochemicals. P-Cho (Na₂ salt) was from TCI America (Portland, OR). P-EA and all other biochemicals were from Sigma. AG-50 (H⁺) ion exchange resin was from Bio-Rad. Silica gel G (0.25-mm) TLC plates were from Merck.

Plant Materials-- Spinacia oleracea L. plants (cv. Savoy hybrid 612, Harris Moran seeds) were grown in coarse vermiculite with an 8-h day at 24 °C (photosynthetic photon flux density 300 µmol m² s¹) and 16-h night at 19 °C. Plants were irrigated daily with 0.5× Hoagland's solution. Prior to harvest, plants were held in nighttime conditions for 40 h and then placed under day conditions for 8 h; salinized plants were irrigated with 200 mM NaCl at the onset of the 8-h day period (12). Harvested leaves were frozen in liquid N₂ and stored at 80 °C.

Yeast Strains and Culture Media-- The S. pombe strains used were 972 (h^s) and Bx22 (h^s cho2-20 ade6-M210 leu1-32) (20). Both were maintained on YEA medium (21) supplemented with 40 µM myo-inositol. SD medium was as described (21) except that it contained 110 µM myo-inositol, 1 g liter¹ of CSM-leu (Bio101), 5 mg liter¹ of thiamine, and 1 mM Cho. SSD medium contained EMM (Bio101) plus 250 mg liter¹ each of lysine, adenine, uracil, and histidine, 1 mM EA and, for plates only, 20 mg liter¹ phloxine B. All liquid media also contained 0.5 g liter¹ asparagine.

cDNA Library Construction-- A salinized spinach cDNA expression library was constructed in the S. pombe vector pREP3, which contains a thiamine-repressible expression cassette driven by the nmt promoter (22). To prepare pREP3 for library construction, it was digested with MscI and SalI, dephosphorylated with shrimp alkaline phosphatase, ligated to an adapter made by hybridizing the oligonucleotides 5'-PCTCGAGATCTG-3' and 5'-PTCGACAGATCTCGAG-3', digested with XhoI, gel purified, and religated. This replaced the MscI site with XhoI and BglII sites in the polylinker, creating pREP3A. A 2.2-kb BglII/SalI fragment from pJD301 (23) was ligated to pREP3A, creating pREP3B. The cDNA library was directionally cloned into pREP3B. Total RNA was isolated from salinized spinach leaves as described (24). Poly(A)⁺ RNA was isolated on poly(U) Sephadex (25) and used to construct cDNA with the Stratagene ZAP-cDNA synthesis kit, replacing the kit's EcoRI adapter with a BglII adapter made by hybridizing the oligonucleotides 5'-PCTCGTGCCA-3' and 5'-GATCTGGCACGAG-3. Size-selected cDNAs (average length, 1.2 kb) were ligated to pREP3B BglII/SalI under optimized conditions and transformed into XL1-Blue MRF' cells by electroporation. The expression library (3.2 × 10⁶ colony forming units) was amplified as described (26).

Complementation of a cho2 Mutant-- Plasmid DNA from the spinach expression library was prepared and transformed into Bx22 cells by standard procedures (21). Transformants were selected on SD medium and then replica plated onto SSD medium. Complementing plasmids were rescued and transformed into Escherichia coli DH10B cells for analysis. Sequencing, homology searches and alignments were performed as described previously (27).

Construction of PEAMT-- The plasmid encoding PEAMT, designated pREP3-PEAMT, was digested with BamHI and end-polished with T4 DNA polymerase. The 3' sequence of PEAMT was removed by digestion with MscI; the remaining sequence plus vector was gel purified and religated to give PEAMT. In PEAMT the native coding sequence terminates at Gly-286, and the pREP3 polylinker adds the sequence Ile-Pro-Gly to the C terminus.

RNA Gel Blot Analysis-- Total RNA was prepared from unstressed and salinized spinach leaves using the RNAeasy plant mini kit (Qiagen). 10-µg samples of RNA were separated in formaldehyde/1.5% agarose gels and transferred to supported nitrocellulose membrane (Micron Separations, Inc.), hybridized, and washed according to the manufacturer's protocols. The PEAMT probe template was a 1.2-kb BamHI/BglII fragment of pREP3-PEAMT. The rRNA probe template was a 0.9-kb SmaI fragment of a Zamia pumila rRNA clone (28). Probes were labeled with [³²P]dCTP by the random primer method. Hybridization was detected by autoradiography.

Enzyme Isolation and Molecular Mass Determination-- Cells were grown to an A₆₀₀ of 1 in 50 ml of medium at 30 °C, shaking at 250 rpm. The medium was SD containing 250 mg liter¹ leucine for Bx22 cells and SSD for wild type cells and Bx22 cells harboring pREP3-PEAMT or pREP3-PEAMT. Subsequent operations were at 0-4 °C. Cells were harvested by centrifugation (5000 × g, 10 min), washed twice with water and once with 100 mM Hepes-KOH, pH 7.8, resuspended in 0.5 ml of lysis buffer (100 mM Hepes-KOH, pH 7.8, 2 mM Na₂EDTA, 5 mM dithiothreitol, 10% glycerol), and transferred to 2-ml microfuge tubes containing 1.5 g of acid-washed glass beads (425-600 µm). Cells were broken by vortexing at maximum speed for 2 min. The homogenate was centrifuged (16,000 × g, 2 min), the supernatant was removed, and the beads were washed three times with 1 ml of lysis buffer. The combined supernatants were clarified by centrifugation (16,000 × g, 15 min) and desalted on a PD-10 column (Amersham Pharmacia Biotech) equilibrated with storage buffer (10 mM Hepes-KOH, pH 7.8, 2 mM Na₂EDTA, 5 mM dithiothreitol, 10% glycerol). The extract was frozen in aliquots in liquid N₂ and stored at 80 °C. Spinach leaf extracts were prepared as described (4). Protein was determined using the Bio-Rad dye reagent. Native molecular mass was estimated using a Waters 626 HPLC system equipped with a Superdex 200 HR 10/30 column (Amersham Pharmacia Biotech) as described (27).

Enzyme Assays-- PEAMT activity was measured by a modification of published methods (1, 4), under conditions in which product formation was proportional to enzyme concentration and time. To compensate for H₂SO₄ present in the [methyl-¹⁴C]AdoMet solution, the assay buffer was titrated to a pH such that, after adding [methyl-¹⁴C]AdoMet and other assay components, the mixture had the desired pH. Unless otherwise indicated, assays (final volume, 100 µl) contained 10 µl of 10× assay buffer (1 M Hepes-KOH, 20 mM Na₂EDTA, pH 8.6), 10% glycerol, 100 nCi of [methyl-¹⁴C]AdoMet, P-EA, and enzyme extract as specified in the text and were incubated at 30 °C for 7.5 or 15 min. The final pH in the assays was 8.1. Reactions were stopped by adding 1 ml of ice-cold water; each was applied to a 1-ml AG-50 (H⁺) column held at 4 °C, and the assay tube was rinsed with 1 ml of water, which was also applied to the column. Products were eluted with 10 ml of 0.1 N HCl, and 2 ml of the eluate was mixed with 3 ml of scintillation fluid (Beckman Ready Gel) and counted. For assay blanks, enzyme was omitted during incubation and added at the ion exchange step. Product recovery was determined to be 82% by spiking unlabeled reaction mixtures with [¹⁴C]P-Cho, and experimental data were corrected accordingly. Reaction products were separated by TLC on silica gel G developed in methanol:acetone:concentrated HCl (90:10:4, v/v/v) (TLC system 1) and detected by autoradiography. To confirm that product hydrolysis did not interfere significantly with PEAMT assays, phosphobase phosphatase activity was measured in the conditions used for PEAMT assays, with [¹⁴C]P-Cho (10 nmol, 50 nCi) as substrate; the labeled Cho product was isolated as described (19). The activity in extracts of complemented S. pombe was 0.16 pkat mg¹ protein or  0.02% of their PEAMT activity.

AdoMet:EA N-methyltransferase activity was assayed as above except that reactions contained 200 nmol of EA and 170 nmol (40 nCi) of [methyl-¹⁴C]AdoMet. The [methyl-¹⁴C]AdoMet was removed by treating with activated charcoal and centrifuging (29), and ¹⁴C incorporation into free base products was measured by counting a sample of the supernatant. TLC tests confirmed that the charcoal did not bind free bases. For assay blanks, EA was omitted. AdoMet:Ptd-EA N-methyltransferase activity was measured essentially as described (1). Assays (final volume, 100 µl) contained 9 µl of 10× assay buffer, 0.8 nmol (47 nCi) of [methyl-¹⁴C]AdoMet, 10 µl of Ptd-EA emulsion in 1× assay buffer (10 mg ml¹, sonicated three times for 30 s each time), and enzyme extract; incubation was for 30 min at 30 °C. The phospholipid fraction was isolated for ¹⁴C quantification as described (1) except that the methanol:chloroform:water proportions were 12:5:1 (v/v/v). Data were corrected for the ¹⁴C incorporation in control assays containing extract from Bx22 cells.

	RESULTS

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

PEAMT Cloning by Complementation of a S. pombe cho2 Mutation-- Cho moieties are synthesized de novo in S. pombe via three sequential methylations of Ptd-EA. The cho2 gene product mediates the first of these, Ptd-EA  Ptd-MME (Fig. 1B) and cho2 mutants require MME, DME, or Cho for growth (7). S. pombe incorporates preformed Cho or other free bases into phospholipids via the CDP base or Kennedy pathway, i.e. base  P-base  CDP-base  Ptd-base (Fig. 1B) (7). These features of Cho metabolism in S. pombe suggested that PEAMT could be cloned by complementation of a cho2 mutant, because expression of PEAMT would restore Cho prototrophy by installing a bypass to the blocked Ptd-EA methylation step (Fig. 1B).

View larger version (21K): [in this window] [in a new window]   Fig. 1.   The biogenesis of choline moieties in plants and in S. pombe and the complementation strategy used to identify PEAMT. A, the pathways found in leaves and other vegetative tissues of higher plants. B, the native pathway in S. pombe (light arrows) and the bypass route (heavy arrows) that allows PEAMT to substitute for the defective cho2 gene product.

A cDNA expression library was therefore constructed in the S. pombe expression vector pREP3 using mRNA from salinized spinach leaves, salinization being known to increase PEAMT activity (17). The pREP3 plasmid contains the leu2 gene for selection and a thiamine-repressible expression cassette (22). The amplified library was transformed into S. pombe strain Bx22, a leu1 cho2 mutant (20). Transformants were selected first for leucine prototrophy on medium containing thiamine and Cho and then replica plated onto medium without thiamine and with EA in place of Cho. Screening 30,000 transformants in this way identified 24 complemented colonies (Fig. 2A) that harbored plasmids with the same 2.2-kb insert, as judged by restriction analysis and sequencing. No complementation was obtained with the vector alone (Fig. 2A), and retransformation of Bx22 with plasmid rescued from complemented colonies conferred Cho prototrophy (not shown), establishing that the complementation is due to the encoded plant protein. The complemented strains lacked detectable activity with Ptd-EA or EA as substrates (<1 pkat mg¹ protein) but had high PEAMT activity (Fig. 2B). TLC analysis of the PEAMT reaction mixtures confirmed that P-MME was formed (Fig. 2B, inset). The specific activity of PEAMT in extracts of complemented cells was 80-fold greater than that of a spinach leaf extract assayed under the same conditions (Fig. 2B). We identified only one other complemented colony harboring a plasmid that, upon retransformation, conferred Cho prototrophy. The 1.6-kb insert in this plasmid specified a polypeptide with homology to ubiquitin C-terminal hydrolases and was not investigated further in this study.

View larger version (49K): [in this window] [in a new window]   Fig. 2.   Complementation of a S. pombe cho2 mutant by spinach cDNAs and PEAMT activities in complemented cells. A, cells of 972 (hs) (wild type) (segment 1), the cho2 mutant Bx22 (segment 2), and Bx22 transformed with the pREP3 vector alone (segment 3) or containing PEAMT (segment 4) or PEAMT (segment 5) were plated on rich medium (YEA) containing 1 mM Cho or minimal medium (SSD) containing 1 mM EA. B, PEAMT activity in desalted extracts from unsalinized spinach leaves (sp) and strains 972 (hs) (wt), Bx22 (cho2), and Bx22 expressing PEAMT (P), or PEAMT (P). Assays contained 50 nmol of P-EA, 44 nmol of [methyl-14C]AdoMet, and 15-710 µg of protein. Data are the means ± S.E. (n = 2 or 3). P-MME is the principal product (>90%) formed in these assays, as shown in the inset, which is an autoradiograph of a TLC separation of a [33P]P-MME standard(s) and the reaction product from representative assay mixtures. C, schematic representation of the complementing cDNAs. PEAMT is the full-length cDNA, containing two sets of the methyltransferase motifs I, post-I (p-I), II, and III. PEAMT is truncated after Gly-286 (see "Experimental Procedures").

Because sequence analysis showed that PEAMT has two methyltransferase domains (see below), we used a convenient MscI site to remove almost all of the C-terminal domain (Fig. 2C). The truncated construct (PEAMT) remained able to complement the cho2 mutation and specified a protein with PEAMT activity (Fig. 2, A and B).

Analysis of the PEAMT cDNA Sequence-- The PEAMT cDNA comprises a 494-residue open reading frame (Fig. 3A) flanked by long 5'- and 3'-untranslated regions (253 and 496 base pairs, respectively). The deduced polypeptide (56.4 kDa) can be divided in the center (Fig. 3A, arrowhead) into two sequences that share significant homology (21% identity, 53% similarity), each of which contains the consensus sequences (30, 31) for methyltransferase motifs I, post-I, II, and III (Figs. 2C and 3A). PEAMT thus appears to contain two distinct but related methyltransferase domains. This is consistent with its size, which is roughly double that typical of small molecule methyltransferases (32). The deduced PEAMT sequence has no recognizable N-terminal signal sequence, consistent with biochemical evidence that the spinach enzyme is cytosolic (17).

View larger version (71K): [in this window] [in a new window]   Fig. 3.   Alignment of the deduced amino acid sequence of spinach PEAMT with homologous sequences from other plants. PEAMT homologs were identified in GenBankTM by BLAST searches. Identical residues are shaded in black, similar residues are shaded in gray. Bars mark conserved methyltransferase motifs I, post-I, II, and III (30, 31). Arrowheads indicate the approximate position of the junction between the domains. A, alignment of spinach PEAMT (So) with an Arabidopsis homolog (At) deduced from nucleotides 65132-67885 of P1 clone MEB5 (accession number AB019230). The Arabidopsis genomic DNA has 13 exons and 12 introns; the identity of the region including amino acids 369-494 is authenticated by alignment with an expressed sequence tag (N65610). The asterisk marks the residue (Gly-286) at which PEAMT is truncated. B, alignment of the central section (residues 154-297) of spinach PEAMT (So) with the deduced amino acid sequences of expressed sequence tags from cotton (Gh, AI731819), barley (Hv, AJ234432), rice (Os, C72466), and canola (Br, L33626). The expressed sequence tags are numbered according to the nucleotide sequence.

The novel, bipartite structure of PEAMT prompted us to search for homologous sequences in GenBank^TM, to determine whether similar proteins occur in other plants. PEAMT homologs were found in species representing three other families: Malvaceae (cotton), Brassicaceae (Arabidopsis, canola), and Gramineae (barley, rice). The Arabidopsis protein was predicted from genomic DNA sequence and was complete (Fig. 3A); those for other species were fragments deduced from expressed sequence tags spanning the junction between the domains (Fig. 3B).

Properties of Recombinant PEAMT-- The native molecular mass was estimated by size exclusion chromatography to be 57 kDa, indicating that the enzyme exists as a monomer. PEAMT activity showed a broad pH optimum in the region 7.8-8.5; the K_m values for P-EA and AdoMet were 96 and 132 µM, respectively. In view of the bipartite structure of PEAMT and the finding that highly purified (but not homogeneous) PEAMT preparations from spinach catalyze the methylation of P-MME and P-DME as well as of P-EA (12), it was of interest to determine whether the recombinant enzyme mediates more than the first methylation of P-EA. We therefore analyzed the products formed when PEAMT is incubated for long periods with a small amount of P-EA and a 3.5-fold excess of [methyl-¹⁴C]AdoMet. In these conditions, ¹⁴C appears successively in P-MME, P-DME, and P-Cho, as shown qualitatively in Fig. 4A. In a separate experiment, these products were quantified to demonstrate that, as incubation proceeds, the molar amounts of P-MME and P-DME peak and then decline, whereas P-Cho continues to accumulate (Fig. 4B). These results establish that PEAMT catalyzes all three methylations required to convert P-EA to P-Cho and also show that the intermediates P-MME and P-DME do not invariably remain bound to the enzyme because both accumulate transiently during the reaction.

View larger version (52K): [in this window] [in a new window]   Fig. 4.   Evidence that PEAMT has three methyltransferase activities. The data in A and B are from separate experiments. A, autoradiograph of a TLC separation of PEAMT reaction products. Extract (130 µg of protein) from Bx22 expressing PEAMT was incubated with 2 nmol of P-EA and 7 nmol (400 nCi) of [methyl-14C]AdoMet; samples were removed at intervals, stopped with acetone, fractionated by ion exchange, and separated in TLC system 1. The positions of P-MME, P-DME, and P-Cho zones and the origin (ori) are indicated. B, progress curve of the PEAMT reaction. Extract (15 µg of protein) from Bx22 cells expressing PEAMT was incubated with 5 nmol of P-EA and 24 nmol (400 nCi) of [methyl-14C]AdoMet. Samples were removed at various times, stopped by freezing, fractionated, and separated in TLC system 1. The P-MME, P-DME, and P-Cho zones were located by autoradiography, scraped, and counted. Radioactivity in P-DME and P-Cho zones was corrected for spillover from the P-MME zone; all data were adjusted for recovery from the TLC plate. , P-MME; , P-DME; ×, P-Cho.

Products of the Reaction Catalyzed by the N-terminal PEAMT Domain-- Deleting the C-terminal domain of PEAMT did not abolish complementation or enzyme activity (Fig. 2, A and B). The N-terminal domain thus catalyzes the first methylation of P-EA, but not necessarily the others. The reaction products of the truncated (PEAMT) and wild type enzymes were therefore compared. After a 3-h incubation with a small quantity of P-EA and a 5-fold excess of [methyl-¹⁴C]AdoMet, PEAMT yields only [¹⁴C]P-MME, whereas the wild type enzyme yields, as expected, almost solely [¹⁴C]P-Cho (Fig. 5A). This result demonstrates that the N-terminal methyltransferase domain mediates only the first N-methylation of P-EA and strongly implies that the C-terminal domain mediates the other two. In this connection it is noteworthy that the kinetics of product formation for the wild type enzyme (Fig. 4B) show that P-MME accumulates to a higher level than P-DME. This is consistent with the second and third methylations occurring at the same catalytic site, from which the intermediate P-DME does not exit very freely.

View larger version (39K): [in this window] [in a new window]   Fig. 5.   Evidence that PEAMT and PEAMT have distinct methyltransferase activities and are inhibited by P-Cho. A, autoradiograph of a TLC separation of the reaction products of wild type PEAMT (P) and the truncated enzyme PEAMT (P). Extracts (200 µg of protein) from Bx22 cells expressing PEAMT or PEAMT were incubated with 5 nmol P-EA and 23 nmol (100 nCi) of [methyl-14C]AdoMet for 3 h. The positions of P-MME and P-Cho zones and the origin (ori) are indicated. B, extracts from Bx22 cells expressing PEAMT (25 µg of protein) or PEAMT (50 µg of protein) were incubated for 10 min with 50 nmol of P-EA, 170 nmol of [methyl-14C]AdoMet, and various concentrations of the disodium salt of P-Cho. Activities in the absence of P-Cho were 737 ± 18 and 200 ± 10 pkat mg1 protein for PEAMT and PEAMT, respectively. Data are the means of duplicates. , PEAMT; , PEAMT.

Sensitivity to P-Cho and Other Metabolites-- Recombinant wild type PEAMT was strongly inhibited by its product P-Cho (Fig. 5B), as reported for the activity extracted from spinach or Lemna plants (12, 15). The IC₅₀ value was 0.49 mM; activity was reduced by 95% at 10 mM P-Cho, which is approximately the P-Cho concentration in the cytosol of spinach leaves (33, 34). The inhibition was mixed competitive with respect to P-EA and appeared to be specific inasmuch as Cho, GlyBet, MME, and DME were not inhibitory at 2 mM (not shown). The truncated PEAMT enzyme was also sensitive to P-Cho, although significantly less than the wild type enzyme. The IC₅₀ value for PEAMT was 1 mM, and at a P-Cho concentration of 10 mM it retained twice as much of its initial activity as did wild type PEAMT (Fig. 5B).

Effect of Salinization on PEAMT mRNA Level-- RNA gel blot analyses revealed a 2.5-kb PEAMT mRNA in spinach leaves (Fig. 6A), which is consistent with the size of the cDNAs that were isolated. Upon salinization, there was an approximately 10-fold increase in PEAMT mRNA abundance (Fig. 6A), and this was accompanied by an 8-fold rise in enzyme activity (Fig. 6B). This indicates that the salt induction of PEAMT activity reported previously (4, 17) results principally if not solely from increased gene expression.

View larger version (60K): [in this window] [in a new window]   Fig. 6.   Effect of salinization on PEAMT expression in spinach leaves. A, RNA gel blot showing the PEAMT transcript; each lane contains 10 µg of total RNA. B, the RNA gel blot was stripped and then hybridized to an 18 S rRNA probe as a loading control. C, PEAMT activity; assays contained 50 nmol of P-EA, 170 nmol of [methyl-14C]AdoMet, and 100 µg of protein, and were incubated for 30 min. Data are the means ± S.E. (n = 3).

	DISCUSSION

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

The complementation strategy used to clone PEAMT was not narrowly specific because in principle the cho2 mutation (in Ptd-EA N-methyltransferase) could have been complemented by plant genes encoding N-methyltransferases acting on Ptd-EA or free EA as well as on P-EA. It is therefore significant that of the 25 complemented S. pombe strains recovered, 24 harbored a plasmid encoding PEAMT, for this implies that N-methyltransferases acting on Ptd-EA or EA are either absent or expressed at far lower levels. This evidence reinforces the conclusion from in vivo radiolabeling studies (2-5) and enzyme assays (1, 4, 17) that in plant leaves the first methylation in the synthesis of Cho moieties takes place exclusively at the phosphobase level.

The large size and tandem domain structure of PEAMT indicate that it is the product of a fusion between two related methyltransferase genes. That this fusion is an ancient one that predates the radiation of the Angiosperms is shown by the occurrence of PEAMT homologs in species from four diverse families (Chenopodiaceae, Malvaceae, Brassicaceae, and Gramineae) whose progenitors diverged >125 million years ago (35). In view of the probable evolutionary age of the fused enzyme, it is particularly interesting that at least one of its domains retains what is presumably its ancestral activity (conversion of P-EA to P-MME) when expressed separately.

Spinach PEAMT is a novel protein, there being no precedent for the synthesis of Cho moieties at the phosphobase level via an enzyme composed of two fused N-methyltransferases. Three separate enzymes are required to convert P-EA to P-Cho in nerve tissues (9), and two separate enzymes carry out the Ptd-EA  Ptd-MME and Ptd-MME Ptd-Cho steps in fungi (7). Fungal Ptd-EA methyltransferases contain regions (100 residues) of internal duplication, but the functional significance of this is not known, and these enzymes mediate only one methylation (7). R. sphaeroides and liver have enzymes that mediate all three methylations in Ptd-Cho synthesis, but these are small (22-23 kDa) proteins that appear to have only one methyltransferase domain (6, 8). It is, however, noteworthy that the genome of the nematode Caenorhabditis elegans includes a hypothetical protein (GenBank^TM AAB04824, 437 residues) that shares homology throughout its length with PEAMT and has two methyltransferase domains. Other organisms may therefore have two-domain phosphobase N-methyltransferases that remain to be discovered. It is also noteworthy that soybean cells and cell extracts carry out only the first methylation of P-EA (1, 5) and that spinach leaves have P-MME and P-DME N-methyltransferase activities that appear to be independent of PEAMT activity (12, 17). Plants may therefore have other phosphobase N-methyltransferases besides PEAMT.

Our data show for the first time that PEAMT is regulated at the gene level, because the mRNA is strongly induced in leaves by salinization. They also confirm that PEAMT is regulated at the enzyme level, because the activity is sensitive to the reaction product P-Cho. The activity was 95% inhibited at a P-Cho concentration of 10 mM, the physiological level in spinach leaf cytosol (33). Because this inhibition was measured in the presence of a saturating P-EA concentration, the degree of inhibition in vivo is probably greater. Together, these findings support the view that PEAMT is the committing step in the synthesis of Cho moieties in plants and exerts major control over the flux to P-Cho and its metabolites (5, 15, 16). Specifically in spinach, the salt induction of PEAMT fits with the increased demand for Cho to support GlyBet accumulation, for which the two biosynthetic enzymes are also induced at the gene level by salinity (2, 24).

Our cloning and characterization of PEAMT was driven by the need to understand the pathway and regulation of Cho synthesis in plants in order to engineer an enhanced Cho supply to support GlyBet synthesis (11, 13). In this context, it is convenient that a single gene encodes all three N-methyltransferase activities needed to produce P-Cho. However, the strong feedback inhibition of native PEAMT by P-Cho could render raising the level of PEAMT relatively ineffective in increasing the flux to P-Cho. The fact that the N-terminal domain expressed separately is less sensitive to feedback control may therefore be valuable in engineering. Because most plants have the capacity to carry out the second and third methylations of Cho synthesis at the phosphatidylbase level (1, 2) and because there may also be phosphobase N-methyltransferases specific for these steps (12, 17), a partially desensitized enzyme able to catalyze just the first methylation could prove more effective than native PEAMT in enhancing Cho biogenesis.

	ACKNOWLEDGEMENTS

We thank Dr. Peter Summers for spinach leaf material, Dr. Kinsey Maundrell for the pREP3 vector, Dr. Scott McNeil for assistance with molecular mass determinations, Dr. Douglas Gage for carrying out chiral HPLC of AdoMet, and Dr. Margaret Kanipes and Hak Chang for advice and help with yeast transformation.

	FOOTNOTES

^* This work was supported in part by United States Department of Agriculture-National Research Initiative-Competitive Grants Program Grant 98-35100-6149 (to A. D. H.), by National Institutes of Health Grant GM19629 (to S. A. H.), by an endowment from the C. V. Griffin, Sr. Foundation, and by the Florida Agricultural Experiment Station (Journal Series R-07329).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.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EMBL Data Bank with accession number(s) AF237633.

To whom correspondence should be addressed: Horticultural Sciences Dept., University of Florida, P.O. Box 110690, Gainesville, FL 32611. Tel.: 352-392-1928; Fax: 352-392-6479; E-mail: adha@gnv.ifas.ufl.edu.

	ABBREVIATIONS

The abbreviations used are: Cho, choline; PEAMT, S-adenosyl-L-methionine:phosphoethanolamine N-methyltransferase; P-, phosphoryl-; Ptd-, phosphatidyl; EA, ethanolamine; MME, monomethylethanolamine; DME, dimethylethanolamine; AdoMet, S-adenosy-L-methionine; GlyBet, glycine betaine; kb, kilobase(s).

	REFERENCES

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

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