PROTEIN RIB - A NOVEL GROUP-B STREPTOCOCCAL CELL-SURFACE PROTEIN THATCONFERS PROTECTIVE IMMUNITY AND IS EXPRESSED BY MOST STRAINS CAUSING INVASIVE INFECTIONS

The group B Streptococcus, an important cause of invasive infections i n the neonate, is classified into four major serotypes (Ia, Ib, II, an d III) based on the structure of the polysaccharide capsule. Since the capsule is a known virulence factor, it has been extensively studied, in particular in type III strains, which cause the majority of invasi ve infections. Two cell surface proteins, alpha and beta, have also be en studied in detail since they confer protective immunity, but these proteins are usually not expressed by type III strains. We describe he re a cell surface protein, designated protein Rib (resistance to prote ases, immunity, group B), that confers protective immunity and is expr essed by most strains of type III. Protein Rib was first identified as a distinct 95-kD protein in extracts of a type III strain, and was pu rified to homogeneity from that strain. Rabbit antiserum to protein Ri b was used to demonstrate that it is expressed on the cell surface of 31 out of 33 type III strains, but only on 1 out of 25 strains represe nting the other three serotypes. Mouse protection tests showed that an tiserum to protein Rib protects against lethal infection with three di fferent strains expressing this antigen, including a strain representi ng a recently identified high virulence type III clone. Protein Rib is immunologically unrelated to the alpha and beta proteins, but shares several features with the alpha protein. Most importantly, the NH2-ter minal amino acid sequences of the Rib and alpha proteins are identical at 6 out of 12 positions. In addition, both protein Rib and the alpha protein are relatively resistant to trypsin (and Rib is also resistan t to pepsin) and both proteins vary greatly in size between different clinical isolates. Finally, both protein Rib and the alpha protein exh ibit a regular ladderlike pattern in immunoblotting experiments, which may reflect a repetitive structure. Taken together, these data sugges t that the Rib and alpha proteins are members of a family of proteins with related structure and function. Since protein Rib confers protect ive immunity, it may be valuable for the development of a protein vacc ine against the group B Streptococcus, an encapsulated bacterium.