Identification of a family of streptococcal surface proteins with extremely repetitive structure

The group B Streptococcus (GBS) causes the majority of life-threatening bacterial infections in newborn children. Most GBS strains isolated from such infections express a surface protein, designated Rib, that confers protective immunity and therefore is of interest for analysis of pathogenetic mechanisms. Sequence analysis demonstrated that Rib has an exceptionally long signal peptide (55 amino acid residues) and 12 repeats (79 amino acid residues each) that account for >80% of the sequence of the mature protein. The repeats are identical even at the DNA level, indicating that an efficient mechanism operates to maintain a highly repetitive structure in Rib. The structure of Rib is similar to that of alpha, a previously characterized surface protein that is common among GBS strains lacking Rib. However, highly purified preparations of Rib and alpha did not cross-react immunologically, although the two proteins show extensive amino acid residue identity (47% in the repeat region). When analyzed in Western blots, Rib and alpha give rise to a regularly spaced ladder pattern, apparently due to hydrolysis of acid-labile Asp-Pro bonds in the repeats. We conclude that Rib and alpha are members of a novel family of streptococcal surface proteins with unusual repetitive structure.