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The IgA‐binding β antigen of the c protein complex of Group B streptococci: sequence determination of its gene and detection of two binding regions

The β antigen of the Ibc protein complex of Group B streptococci is a cell‐surface receptor which binds the Fc region of human immunoglobulin A(IgA). Determination of the nucleotide sequence of the β antigen gene shows that it encodes a preprotein having a molecular weight of 130963 daltons and a polypeptide of 1164 amino acid residues that is typical of other Gram‐positive cell‐wall proteins. There is a long signal sequence of 37 amino acids at the N‐terminus. Four of the five C‐terminal amino acid residues are basic and are preceded by a hydrophobic stretch that appears to anchor the C‐terminus in the cell membrane. To the N‐terminal side of this hydrophobic stretch is a putative cell‐wall‐spanning region containing proline‐rich repeated sequences. An unusual feature of these repeated sequences is a three‐residue periodicity, whereby every first residue is a proline, the second residue is alternating positively or negatively charged, and the third residue is uncharged. The IgA‐binding activity was approximately localized by expressing subfragments of the β antigen as fusion proteins. Two distinct but adjacent DNA segments specified pep‐tides that bound IgA, which indicates that the IgA‐binding activity is located in two distinct regions of the protein.