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Streptococcus equi (Streptococcus equi subsp. equi), a Lancefield group C streptococcus, causes strangles, a highly contagious purulent lymphadenitis and pharyngitis of members of the family Equidae. The antiphagocytic 58-kDa M-like protein SeM is a major virulence factor and protective antigen. The amino acid sequence and structure of SeM has been determined and compared to that of a second, 40-kDa M-like protein (SzPSe) of S. equi and to those of other streptococcal proteins. Both SeM and SzPSe are mainly alpha-helical fibrillar molecules with no homology other than that between their signal and membrane anchor sequences and are only distantly related to other streptococcal M and M-like proteins. The sequence of SzPSe indicates that it is an allele of SzP that encodes the variable protective M-like and typing antigens of S. zooepidemicus (S. equi subsp. zooepidemicus). SeM is opsonogenic for S. equi but not for the closely related S. zooepidemicus, whereas SzPSe is strongly opsonogenic for S. zooepidemicus but not for S. equi. Both proteins bind equine fibrinogen. SeM and SzPSe proteins from temporally and geographically separated isolates of S. equi are identical in size. The results taken together support previous evidence that S. equi is a clonal pathogen originating from an ancestral strain of S. zooepidemicus. We postulate that acquisition of SeM synthesis was a key element in the success of the clone because of its effect in enhancing resistance to phagocytosis and because protective immunity entails a requirement for SeM-specific antibody.

Comparison of the sequences and functions of Streptococcus equi M-like proteins SeM and SzPSe