Sequence and structural analysis of surface protein antigen I/II (SpaA) of Streptococcus sobrinus

Abstract

Streptococcal antigen I/IT or the surface protein antigen A (SpaA) of Streptococcus sobrinus is an adhesin which mediates binding of the organism to tooth surfaces. The complete sequence of the gene which encodes SpaA has been determined. The gene consists of 4,584 bp and encodes a protein of 1,528 amino acid residues. The deduced amino acid sequence shows extensive homology with those of the cell surface adhesins from Streptococcus mutans serotypes c and f and from Streptococcus sanguis. Structural analysis of the N-terminal region (residues 50 to 550), which is rich in alanine and includes four tandem repeats of an 82-residue sequence, suggests that it adopts an or-helical coiled-coil conformation. Cell surface hydrophobicity may be associated with this region. The C-terminal region is more conserved and includes two tandem repeats of a 39-residue proline-rich sequence. A further proline-rich sequence in this region is predicted to span the cell wall. Although a hydrophobic sequence is present in the C-terminal region, it appears to be too short to span the cell membrane. Anchoring of SpaA in the cell membrane may therefore require some form of posttranslational modification or association with another membrane protein.

Cell surface protein receptors in oral streptococci

“…Expression of the antigen I/II polypeptide PAc by S. rnutans is necessary for binding of cells to salivary glycoprotein pellicle [11]. The genes encoding antigen I/II polypeptides from S. mutans [11][12][13], S. sobrinus [14,15] and S. gordonii [16] 135 have been isolated and sequenced (see Table 1). The structural features of SpaP, SpaA and SSP5 based on their inferred amino acid sequences are shown in Fig.…”

“…The similarities in amino acid sequence be- Table 1 Two families of cell-surface polypeptide receptors in oral streptococci tween these various polypeptides and some functional implications have been discussed elsewhere [15]. In essence, the A region repeats are moderately well conserved across species, the sequence between A and PR1 is highly variable, and a sequence of roughly 200 amino acids C-terminal to PR1 is highly conserved [17,18].…”

“…3). Since the a region of the LraI family is predicted from several algorithms to have high surface probability, and the A region segments of antigen I/II polypeptides are likely to be surface-exposed [ 15,18], it is possible that ScaA and SSP5 present common epitopes. Any functional significance in this structural similarity must await experimental analysis.…”

Localization on the cell surface of streptococcal protein antigen A

“…Proteins antigenicaUy similar to the SpaA protein have been shown to be produced by all of the serotypes of the mutans streptococci with the exception of serotype b [2,7,8] and also by Streptococcus sanguis [9]. Sequence analysis of the spaA gene indicates that the mature SpaA protein has a M r of 160500 [10]. In contrast, the SpaA protein exhibits a Mr of 210000 when determined by SDS-PAGE [3].…”

“…The deduced amino acid sequence of SpaA indicates that the amino terminal region has a signal sequence, is alanine-rich and has three tandem repeats of a sequence based on 82 amino acid residues. The carboxyl terminal re-gion of SpaA is abundant in proline residues, contains two tandem repeats of a 39-residue proline-rich sequence and a hexapeptide sequence thought to be involved in membrane attachment [10,11].…”