ALLOSTERIC AND TEMPERATURE EFFECTS ON THE PLASMA-PROTEIN BINDING BY STREPTOCOCCAL M-PROTEIN FAMILY MEMBERS

Most group A streptococcal strains bind immunoglobulins (Ig) and fibri nogen to their cell walls. ft is shown in this paper that the Ig-bindi ng of three different strains was much weaker at 37 degrees C than at room temperature (20 degrees C), whereas the fibrinogen binding was un affected by temperature. The binding properties and molecular sizes of two purified group A streptococcal cell surface proteins from the M p rotein family were studied at various temperatures, M1 protein with af finity for IgG, fibrinogen and albumin, and protein Sir22 with affinit y for IgA and IgG. Both proteins appeared as monomers which bound all their ligands, including fibrinogen, very weakly at 37 degrees C, and as strongly binding dimers at 10 and 20 degrees C. Furthermore, the re sults demonstrated that the plasma protein binding of the bacterial pr oteins was allosterically regulated, i.e. the binding of a ligand to o ne site modulated the binding of a ligand to a second site. For exampl e, the binding of albumin or IgG to purified M1 protein at 10 and 20 d egrees C strongly enhanced the binding of fibrinogen at 37 degrees C. This indicates that the high affinity dimer form of the bacterial prot eins can be stabilized at 37 degrees C, a possible explanation for the strong fibrinogen binding of whole bacteria. Finally, the sizes and b inding properties of three M1 protein fragments were studied and the r esults indicated that the centrally located C-repeats, which are conse rved among the members of the M protein family, are important for the formation of the high-affinity dimers of the bacterial proteins.