SeM protein (SeM)

The SeM protein is a 58-kDa M-like surface protein encoded by the seM gene in Streptococcus equi subsp. equi, the causative agent of strangles, a highly contagious upper respiratory infection in horses characterized by purulent lymphadenitis and pharyngitis[1][3]. SeM functions as a major virulence factor by conferring antiphagocytic properties, protecting the bacterium from host phagocytosis and enabling systemic spread[1][3]. It is structurally an alpha-helical fibrillar molecule with a signal sequence and membrane anchor, showing no close homology to other streptococcal M proteins beyond these features, and binds equine fibrinogen[3]. Unlike related proteins in S. equi subsp. zooepidemicus, SeM is opsonogenic specifically for S. equi, eliciting protective immunity that requires SeM-specific antibodies[3]. Genetic variation in seM alleles correlates with differing virulence levels in experimental models, from mild to severe disease[2]. SeM is exclusive to S. equi subsp. equi and used as a PCR marker for its detection, distinguishing it from the sodA gene in the related subsp. zooepidemicus[1]. Its acquisition is postulated as key to S. equi's clonal success as a host-restricted pathogen derived from S. zooepidemicus[3].

Other names

M-like proteinM protein