COMPARISON OF THE SEQUENCES AND FUNCTIONS OF STREPTOCOCCUS-EQUI M-LIKE PROTEINS SEM AND SZPSE

Streprococcus equi (Streptococcus equi subsp, equi), a Lancefield grou p C streptococcus, causes strangles, a highly contagious purulent lymp hadenitis and pharyngitis of members of the family Equidae, The antiph agocytic 58-kDa M-like protein SeM is a major virulence factor and pro tective antigen, The amino acid sequence and structure of SeM has been determined and compared to that of a second, 40-kDa M-like protein (S zPSe) of S, equi and to those of other streptococcal proteins, Both Se M and SzPSe are mainly alpha-helical fibrillar molecules with no homol ogy other than that between their signal and membrane anchor sequences and are only distantly related to other streptococcal Ill 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. Bot h proteins bind equine fibrinogen. SeM and SzPSe proteins from tempora lly and geographically separated isolates of S, equi are identical in size. The results taken together support previous evidence that S, equ i is a clonal pathogen originating from an ancestral strain of S. zooe pidemicus. We postulate that acquisition of SeM synthesis was a key el ement in the success of the clone because of its effect in enhancing r esistance to phagocytosis and because protective immunity entails a re quirement for SeM-specific antibody.