A SMALL DOMAIN (6.5 KDA) OF BACTERIAL PROTEIN-G INHIBITS C3 COVALENT BINDING TO THE FC REGION OF IGG IMMUNE-COMPLEXES

Attachment of the complement component C3 to antigen-antibody (Ag-Ab) complexes (immune complexes, IC) is the key molecular event responsibl e for the elimination of many Ag in the form of Ag-Ab-C3b. The CH1 dom ain and the Fc region of the Ab, which have previously been involved i n the binding of C3b, are also the targets of several bacterial IgG-bi nding proteins, particularly proteins G and A. Here we describe the ab ility of a small recombinant protein G domain (B2; 6.5 kDa) to inhibit the covalent binding of C3b to the Pc portion of IgG without affectin g the binding to the Fab part. Protein G (B2 domain) produced a remark able inhibition of covalent binding of C3b to IC formed with rabbit Ig G, but none with the F(ab')(2) fragment, indicating that B2 interferes with the C3b binding to the Fc region. A weak inhibition was observed with IC formed with mouse IgG2b which preferentially binds B2 domain on the CHI domain of the Fab. To confirm these data, recombinant singl e-chain Ab devoid of CH1 domains (scAb), and including the rabbit or h uman Pc portion (hinge-CH2-CH3), were produced and used to form IC. Pr otein G-B2 domain inhibited C3b binding to IC formed with scAb of eith er human or rabbit constant regions, supporting the view of a specific blockade of C3b binding to the Pc region. A similar inhibition of C3b binding was observed using protein A instead of protein G B2 domain a nd the same set of IC. On the CHI domain, C3b and B2 bind on opposite faces, an therefore do not interfere with each other in their binding. However, B2 domain bound to the inter-CH2-CH3 region impedes the C3b binding to the Fc. This inhibition clarifies the specificity of C3b fo r the different regions of IgG and explains how bacterial IgG-binding proteins provide the bacteria with a mechanism of evasion from the ops onizing action of complement and contribute to the virulence. This cou ld be a general mechanism of escape because protein G binds the majori ty of mammalian Ig.