Previous studies of recent clinical isolates of serotype M1 group A streptococci indicated that they display two patterns of non-immune human IgG subclass binding reactivity associated with their M1 protein. One group reacted with all four IgG subclasses (type IIo), while the second group expressed an M1 protein reacting preferentially with human IgG 3 (type IIb). In this study, we have demonstrated that a cysteine protease, SpeB, present in culture supernatants of M1 serotype group A streptococcal isolates expressing type IIb IgG binding protein, can convert a recombinant Emm1 protein from a type IIo functional profile to a type IIb profile by removal of 24 amino acids from the N-terminus of the mature M1 protein. Furthermore, SpeB can convert bacteria expressing IgG binding proteins of the type IIo phenotype into those expressing type IIb proteins. The role of the cysteine protease as the central bacterial enzyme in this post-translational modification event was confirmed by generation of an isogenic SpeB-negative mutant.Un groupe de streptocoques du serogroupe A (serotype M1) reagit avec les quatre sous-classes d'IgG (le type IIo) alors qu'un autre groupe produit une proteine M1 qui reagit preferentiellement avec les IgG3 humaines (le type IIb). Nous demontrons qu'une cysteine-protease, la SpeB, qui est presente dans le surnageant de culture de streptocoques du type IIb peut convertir une proteine Emm1 recombinante de IIo en une proteine de profil IIb grace au deplacement de 24 acides amines N-terminaux de la proteine M1 mature. De plus, la SpeB peut convertir des streptocoques exprimant des proteines de phenotype IIo, fixant l'IgG, en streptocoques exprimant des proteines de phenotype IIb. Le role de la cysteine-protease comme enzyme streptococcique determinant dans les modifications observees a ete confirme par la generation d'un mutant isogenique speB-negatif.