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J. Biol. Chem., Vol. 278, Issue 34, 32313-32316, August 22, 2003
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From the
Department of Cell and Molecular Biology,
Section for Molecular Pathogenesis, Lund University, S-221 84 Lund, Sweden and
¶Biovitrum AB, SE-112 76 Stockholm and the
Department of Medical Chemistry, Uppsala University, SE-751 23 Uppsala,
Sweden
Received for publication, May 6, 2003 , and in revised form, June 2, 2003.
| ABSTRACT |
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| INTRODUCTION |
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This study was undertaken to investigate the occurrence of proteins containing CSMs in bacteria and viruses through an in silico approach. We found that CSMs are encoded by a minority of bacteria and bacteriophages and that these CSMs differ from human collagens in several important aspects. A novel mechanism for the stabilization of bacterial CSMs is presented. We also propose that horizontal gene transfer has contributed to the evolution of CSMs.
| EXPERIMENTAL PROCEDURES |
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Homology models of proline-rich CSMs were constructed on a trimeric
((GPP)10)3 template from a 1.3-Å x-ray structure
(11). The models were built by
global energy minimization using the template as constraint. After the model
structures had been minimized, conformations of all threonine side chains were
sampled through an exhaustive systematic search of the
-angles of the
side chains. All molecular mechanics calculations were carried out using the
ICM software (12). All
statistical analyses for differences were performed using Fisher's exact
test.
| RESULTS AND DISCUSSION |
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Characteristics of Bacterial and Viral CSMsThe CSMs were generally dissimilar in primary structure, except for the periodicity of the glycines and a relatively high proline content. The length of the CSM varied from 7 (the detection limit of the pattern) to 745 continuous GXY repeats, with a mean number of 76 GXY repeats. In 18 of the 103 CSM-containing proteins identified, more than one CSM was identified. Both COOH- and NH2-terminal to the CSM, other regions of varying length and sequence were found in all proteins. A position-specific analysis of the amino acid content of the identified CSMs revealed several differences compared with human collagens (Fig. 1B). Because no statistically significant differences were noted between bacteriophage CSMs and CSMs from other viruses, these were pooled for the purpose of subsequent analyses. Perhaps most striking is the difference in proline content and the preference for proline in position X in the CSMs from bacteria and viruses. The proline content is significantly lower in these CSMs as compared with human collagens (p = 3.7 x 10147 for the bacterial CSMs and p = 2.8 x 1016 for the viral CSMs). In bacterial and viral CSMs the fraction of prolines found in the Y position was significantly lower as compared with this fraction in human collagens (p = 3.2 x 1077 for bacterial and p = 1.2 x 1059 for the viral CSMs). Prolines in position Y are generally hydroxylated in human collagen; thus, the relative absence of prolines in this position among CSMs from bacteria and viruses probably reflects their inability to synthesize hydroxyproline. In this context it should be mentioned that bacterial homologues of eukaryotic prolyl hydroxylases could not be identified despite extensive similarity searches.
The CSMs identified contained a significantly higher proportion of threonine (p = 7.6 x 10206 for bacterial CSMs and p = 1.9 x 1036 for viral CSMs) and glutamine (p = 2.1 x 1012 for bacteria and p = 4.1 x 1014 for viruses) in the Y position as compared with human collagen. A minority of the bacterial CSMs (17 of 53) had more than 50% of Y positions occupied by threonine (mean 94%). In these threonine-rich CSMs, the X position is typically occupied by prolines, alanines, or serines, whereas charged residues are less common than in the other bacterial CSMs and in human collagen. The proteins containing threonine-rich CSMs are clustered in five bacterial species: the spore-forming human pathogens Clostridium difficile, Bacillus anthracis, and Bacillus cereus, the nitrogen-fixating Metorhizobium loti, and the sulfur-metabolizing Desulfitobacter hafniense. To determine whether threonine can influence stability of a collagen-like triple-helix, we built homology models of six representative proline-rich CSMs on a trimeric ((GPP)10)3 template constructed from a 1.3-Å x-ray structure (11). The resulting models show that threonine in the Y position is able to form direct interchain hydrogen bonds to backbone carbonyls in its energetically most favored conformation (Fig. 2). When the amino acids in the X and Y positions of one threonine-rich CSM were switched, the threonines were not able to form interchain hydrogen bonds (data not shown). Interestingly, there are indications that threonine in the Y position can also stabilize a triple-helical structure through indirect hydrogen bonding or through glycosylations (8, 13, 14).
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Most of the remaining 83 CSMs could instead be classified as rich in prolines (>30% in X and Y position) or charged (>45% charged residues in X and Y). Eight of the identified CSMs did not meet any of the criteria, whereas five CSMs fell into more than one class (Fig. 3A). Not only the prolines, but also the charged residues, were found to be unevenly distributed between the X and Y positions (Fig. 3B). Most often, negatively charged residues are found in position X, whereas positively charged residues (arginine and lysine) are found in the Y position. This may be related to triple-helix stability, because especially arginine in the Y position stabilizes trimers (9). Negative charges in the Y position are generally destabilizing, with the exception of GR/KD triplets (15). Among the charged CSMs with a net negative charge in the Y position, aspartate is more common than glutamate and the aspartate is in 50% of the cases preceded by a positively charged residue. In addition to stabilization, charges in collagen are important for the interactions between collagen and other macromolecules (16).
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Function of the CSMThe function of CSMs in bacteria and viruses is not well understood. The few CSMs studied have been shown to mediate trimerization or elongation (2, 10, 17, 18) of proteins found at bacterial or bacteriophage surfaces (2, 1825). Furthermore, the CSM of one protein from Streptococcus sanguis has been shown to mediate aggregation of platelets (26), although it is less clear whether the CSMs from two Streptococcus pyogenes proteins contribute to the adhesive properties of these proteins (21, 24). The viral proteins containing CSMs identified in this study were mainly encoded by bacteriophages and had to a large extent been annotated or could easily be annotated by similarity searches. The majority of these proteins were tail-fiber or host-specificity proteins, involved in the interactions between bacteriophages and bacteria (see e.g. Ref. 27). Only a minority of the bacterial proteins containing a CSM could be annotated. Three proteins had previously been described as surface or spore-associated (2025); from pattern and similarity searches, an additional 13 proteins could be classified as cell wall-attached (28) or transmembrane proteins. The non-collagen-related parts of the proteins showed surprisingly little sequence similarity to other proteins, making a functional classification of the bacterial proteins difficult. The presence of CSMs in proteins with so little sequence similarity indicates that the CSM is, indeed, a structural motif that elongates or trimerizes a variety of different, mainly extracellular, proteins. The possibility remains, however, that some CSMs may mediate binding of bacterial or bacteriophage proteins to other macromolecules.
Evolutionary AspectsBecause so few bacterial and unicellular eucaryotic genomes encode CSMs and because archaebacteria completely lack such sequences, it seems unlikely that CSMs have evolved before the diversification of archaea, bacteria, and eukaryotes. It appears more plausible that genes encoding CSMs have moved horizontally or arisen on several occasions during evolution rather than selective gene loss among archaea, bacteria, and lower eukaryotes. Horizontal gene transfer between bacteria and vertebrae has been proposed to be relatively common (29, 30), although this view has also been questioned (31). If horizontal gene transfers of CSMs have occurred, we believe that the direction of transfer has been from eukaryotes to bacteria. After horizontal transfer of sequences encoding a CSM, bacteriophages may have promoted further horizontal transfer within the bacterial kingdom leading to the "patchy" distribution seen for CSMs among bacteria. We have no evidence for a role of viruses or bacteriophages in the putative horizontal transfer between eukaryotes and bacteria, but this cannot be excluded. After the putative horizontal transfer, extensive rearrangements seem to have occurred in the bacterial CSMs, because there are limited sequence similarities except for the spacing of glycines in the bacterial CSMs. The limited sequence similarities also made phylogenetic analyses meaningless. Interestingly, the threonine-rich CSMs are very dissimilar in amino acid composition from human collagens and from the other bacterial CSMs. Instead, the threoninerich CSMs have their closest homologues in hypothermal vent worm cuticle collagens (14). This suggests that several events of horizontal gene transfer of the genetic material encoding CSMs could have occurred during evolution. An alternative evolutionary explanation to the threonine-rich CSMs is that these have arisen de novo in bacteria, followed by horizontal spread to a few other bacterial species. The threonine-rich CSMs are highly repetitive and could have evolved from duplications of a short DNA segment.
The presence of CSMs in bacteria and viruses underlines the importance of collagen as a structural motif in nature. This work also suggests that alternative means for triple-helix stabilization probably operate in bacteria and viruses, and future elucidation of the structure and function of bacterial and viral CSMs represents an interesting scientific task.
| FOOTNOTES |
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To whom correspondence should be addressed: Section for Molecular
Pathogenesis, Dept. of Cell and Molecular Biology, Lund University, BMC, B14,
Tornavägen 10, S-221 84 Lund, Sweden. Tel.: 46-46-2224489; Fax:
46-46-157756; E-mail:
Magnus.Rasmussen{at}medkem.lu.se.
1 The abbreviation used is: CSM, collagen-related structural motif. ![]()
| ACKNOWLEDGMENTS |
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| REFERENCES |
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