Streptococcal M protein Emm55 (Emm55)

The Streptococcal M protein Emm55 is a surface-exposed, fibrillar coiled-coil dimer on Group A Streptococcus (GAS) that extends from the bacterial cell wall, serving as a major virulence factor. It inhibits phagocytosis by binding host proteins like fibrinogen, IgG, and plasminogen (including via an S-region motif shared with M1 and M64), promotes adherence to epithelial cells, and aids in overcoming innate immunity through interactions along its length. Emm55 belongs to an emm-type classification, where its N-terminal sequence defines strain typing (emm-typing by CDC) among over 275 identified types. The full protein structure is highly predictive from this N-terminal region, showing high conservation (88-100% identity) within its specific emm-type, though lengths can vary significantly across different emm patterns (e.g., A-C being longest, E shortest). Structural features include heptad repeats stabilizing the coiled coil, contributing to its functionality. In disease, Emm55 drives GAS pathogenesis in infections ranging from pharyngitis to invasive disease, with emm-types like Emm55 correlating to specific emm patterns (e.g., potentially D or E) linked to tissue tropism and epidemiology. While Emm55 is a key vaccine antigen target, its extensive sequence diversity across hundreds of emm-types poses significant challenges for developing broadly protective vaccines. No approved therapeutics directly target Emm55; rather, vaccine development focuses on leveraging M protein conservation.

Other names

M55 proteinEmm55 M protein