MOLECULAR CHARACTERIZATION OF PROTEIN SIR, A STREPTOCOCCAL CELL-SURFACE PROTEIN THAT BINDS BOTH IMMUNOGLOBULIN-A AND IMMUNOGLOBULIN-G

Cell surface proteins that bind to the Fc part of immunoglobulin (Ig) A and/or IgG are expressed by many strains of the group A Streptococcu s, an important human pathogen. Two extensively characterized proteins in this group of molecules are protein Arp that preferentially binds IgA and protein H that binds IgG. In addition, recent work has shown t hat many group A strep streptococcal strains express a novel type of F c-binding protein, designated protein Sir, that binds both IgA and IgG . Protein Sir22, the molecule expressed by a strain of serotype M22, h as now been purified and characterized after expression of the cloned gene in Escherichia coli. Dot-blot analysis with a large number of pur ified monoclonal Igs showed that protein Sir22 reacted with 19 out of 20 IgA proteins and with 19 out of 24 IgG proteins. The affinity const ants for the reactions between protein Sir22 and Ig were determined to be 7.0 x 10(8) M(-1) for serum IgA, 2.4 x 10(8) M(-1) for secretory I gA, and 7.8 x 10(8) M(-1) for IgG. Inhibition experiments showed that the bindings of IgA and IgG to protein Sir22 were mutually exclusive, indicating shared or contiguous binding sites. Analysis of the sequenc e of the sir22 gene indicated a gene product with 365 amino acid resid ues, including a 41-residue signal peptide. The processed form of the protein, 324 residues, has a calculated M(r) of 37,186. Deletion analy sis of the sir22 gene showed that a 156-residue NH2-terminal fragment of protein Sir22 retained the ability to bind both IgA and IgG. The ov erall organization of protein Sir22 is similar to that of the IgA-bind ing protein Arp and the IgG-binding protein H. All three of these prot eins are members of the M protein family and have a central repeat reg ion of the C type.