Identification of an immunoglobulin A binding motif located in the beta-antigen of the c protein complex of group B streptococci

Abstract

The ␤-antigen of the c protein complex of group B streptococci contains two immunoglobulin A (IgA)binding domains called A and B. A 73-amino-acid segment in domain A is responsible for most of the IgA-binding activity. To identify the IgA binding motif, the 73-amino-acid domain was divided into 60 14amino-acid overlapping peptides spot synthesized onto a cellulose membrane. A 20-residue putative antigenic epitope was identified and expressed as a fusion protein. The fusion protein was purified by fast protein liquid chromatography and used to raise rabbit antiserum. By use of a membrane with spot-synthesized peptide amino acids of decreasing length (from 14 to 6 amino acids), the major antigenic epitope recognized by the anti-fusion protein antibodies was mapped to motif MLKKIE. Anti-fusion protein antibodies inhibited the binding of IgA to group B streptococci. This inhibition could be blocked by the peptide containing the motif MLKKIE. These results indicate that the motif MLKKIE is located in the IgA-binding site. The IgA-binding domain of ␤-antigen from three group B streptococcal strains reacted with the anti-fusion protein antibodies, and their coding sequences gave positive signals in Southern hybridization. The sequences of ␤-antigen from these strains were amplified by PCR, and sequence analysis showed them to be identical. The results indicate that the motif MLKKIE is required for IgA binding and is present in different group B streptococcal strains.

Unencapsulated Streptococcus pneumoniae from conjunctivitis encode variant traits and belong to a distinct phylogenetic cluster

“…4 ). The region of canonical CbpA that mediates binding to the eukaryotic polymeric Ig receptor (pIgR) 35 , has been replaced by S. agalactiae domains that bind the Fc portion of IgA directly 33 , 36 . The different variations in the divergence of CbpAC1 and CbpAC2 from CbpA suggest that variations of the hybrid CbpA locus evolved independently.…”

“…Interestingly, these no longer possess the key domain that mediates binding to host polymeric immunoglobulin receptor (pIgR), which S. pneumoniae use to facilitate transcytosis from nasopharyngeal epithelia into the blood stream 35 . Instead, both CbpAC1 and CbpAC2 appear to have independently swapped that domain for one that mediates direct binding to secretory IgA (sIgA) 33 , 36 , 43 . The implication is that ECC strains bind sIgA in a subtly, but importantly different way, possibly coating themselves with IgA, in a manner analogous to that mediated by protein A of S. aureus 44 .…”

Progress toward a group B streptococcal vaccine

“…4B). 116,117 Additionally, it was also shown to bind to human factor H (FH) to protect GBS from opsonophagocytosis. 118 Rib protein shares several biochemical features with AlphaC protein, but no immunological cross-reaction with either AlphaC or BetaC proteins has been found.…”

Surface Proteins of Gram-Positive Bacteria and Mechanisms of Their Targeting to the Cell Wall Envelope

“…Analysis of recombinant C␤ expressed in E. coli indicated that the C␤ protein contains two or more separate IgA binding domains within the N-terminal half of the molecule, designated A and B (382). A later study mapped the binding site of the A domain to a region centered around a hexapeptide motif (MLKKIE), but it was not conclusively determined that this motif actually participated in the binding of IgA (383). Neither IgA binding domain on C␤ bears any significant homology to the IgA binding domains found in M proteins (50,391).…”