Table 3.

Variable organization of the emm gene superfamily of M-related proteinsa

Chromosomal emmpatternSubfamily
First gene: mrp(fcrA), mrpCentral gene: emm,emmL, sir, arpLast gene:emmL, enn, sph
  • a Similarities of genes in theemm gene superfamily include the amino-terminal leader sequence and carboxy-terminal anchor; a peptidoglycan-spanning region rich in proline, glycine, serine, and threonine residues; tandemly arranged amino acid sequence repeats; and a seven-residue periodicity in amino acid placement (238). The fcrA gene encodes a type IIa immunoglobulin-binding protein, mrpencodes an M-related protein that binds to IgG and fibrinogen,emmL encodes a protein that binds IgG, sir andarp encode IgA-binding proteins, and that encoded bysir can also bind to IgG; enn encodes a protein that binds either IgG3 or IgA, and sph encodes protein H, which binds IgG. The central gene contains emm gene sequence at the 5′ end. The class I epitope is found in the C repeat region of central SF1 emm genes (44, 50, 52, 56, 238, 239). Members of the emm gene family in group A streptococci encode antiphagocytic cell surface proteins and/or immunoglobulin-binding proteins (239). IgG binding is observed for almost all of the genes listed above in at least one or more strains, but not all are antiphagocytic. The emm gene immunoglobulin-binding product varies in human subclass specificity. For example, SF4 usually binds human IgG1, -2, and -4 but not IgG3. Some emm gene products only bind IgG3, while others bind all four human subclasses. The nomenclature scheme devised by Boyle includes characterization by immunoglobulin binding to human as well as other immunoglobulin species (408). Boyle classification is types IIo, II′o, IIa, IIb, and IIc based on the subclasses of immunoglobulins bound from different species (408).

  • b IgA binding is restricted to pattern E in either the SF2 or SF3 gene, depending on the strain. The emmgene immunoglobulin-binding product varies in human subclass specificity.