IDENTIFICATION OF A FAMILY OF STREPTOCOCCAL SURFACE-PROTEINS WITH EXTREMELY REPETITIVE STRUCTURE

The group B Streptococcus (GBS) causes the majority of life threatenin g bacterial infections in newborn children. Most GBS strains isolated from such infections express a surface protein, designated Rib, that c onfers protective immunity and therefore is of interest for analysis o f 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 seq uence 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 com mon among GBS strains lacking Rib, However, highly purified preparatio ns of Rib and alpha did not crossreact immunologically, although the t wo proteins show extensive amino acid residue identity (47% in the rep eat region). When analyzed in Western blots, Rib and alpha give rise t o a regularly spaced ladder pattern, apparently due to hydrolysis of a cid-labile Asp Pro bonds in the repeats. We conclude that Rib and alph a are members of a novel family of streptococcal surface proteins with unusual repetitive structure.