Localization of immunoglobulin A-binding sites within M or M-like proteins of group A streptococci.

Many strains of group A streptococci are capable of binding human immunoglobulin A (IgA) by a nonimmune mechanism. M or M-like proteins constitute a family of structurally diverse molecules which form surface fibrillae, and some of the M or M-like protein forms are responsible for the IgA-binding activity. In this report, the binding site for IgA is localized within two structurally distinct M or M-like proteins, ML2.2 and Arp4. Apart from those structural domains which are common to all M and M-like proteins, ML2.2 and Arp4 lack significant levels of amino acid sequence homology, with the exception of a short segment (ALXGENXDLR) located at residues 21 to 30 of the mature ML2.2 protein. Recombinant fusion polypeptides containing portions of the ML2.2 and Arp4 proteins were expressed in Escherichia coli and tested for binding of human myeloma IgA. A 58-residue polypeptide containing residues 14 to 71 of ML2.2 bound human IgA. The IgA-binding site of Arp4 could be localized to a 53-residue polypeptide containing residues 43 to 95, which encompasses the ALXGENXDLR consensus sequence of Arp4 positioned at residues 50 to 59. Site-specific mutagenesis at three codons within the ALXGENXDLR coding sequence of both the ML2.2 and Arp4 recombinant polypeptides leads to a loss in IgA-binding activity. Thus, the ALXGENXDLR consensus sequence is essential for the nonimmune binding of IgA by both ML2.2 and Arp4. However, the failure to bind IgA by polypeptides which partially overlap the 58- and 53-residue IgA-binding polypeptides of ML2.2 and Arp4, yet contain the ALXGENXDLR consensus sequence, strongly suggests that flanking regions are also critical for IgA binding. In summary, the results indicate that common functional domains bearing significant sequence homology are distributed within regions of M or M-like molecules that are otherwise highly divergent.