Identification and Characterization of a Novel Secreted Immunoglobulin Binding Protein from Group A Streptococcus

Immunoglobulin binding proteins are one of several pathogenicity factors which have been associated with invasive disease caused by group A streptococci. The surface-bound M and M-like proteins of Streptococcus pyogenes are the most characterized of these immunoglobulin binding proteins, and in most cases they bind only a single antibody class. Here we report the identification of a novel non-M-type secreted protein, designated SibA (for secreted immunoglobulin binding protein from group A streptococcus), which binds all immunoglobulin G (IgG) subclasses, the Fc and Fab fragments, and also IgA and IgM. SibA has no significant sequence homology to any M-related proteins, is not found in the vir regulon, and contains none of the characteristic M-protein regions, such as the A or C repeats. Like M proteins, however, SibA does have relatively high levels of alanine, lysine, glutamic acid, leucine, and glycine. SibA and M proteins also share an alpha-helical N-terminal secondary structure which has been previously implicated in immunoglobulin binding in M proteins. Evidence presented here indicates that this is also the case for SibA. SibA also has regions of local similarity with other coiled-coil proteins such as Listeria monocytogenes P45 autolysin, human myosin heavy chain, macrogolgin, and Schistoma mansoni paramyosin, some of which are of potential significance since cross-reactive antibodies between myosin proteins and M proteins have been implicated in the development of the autoimmune sequelae of streptococcal disease.