Characterization of protein G expressed by human group C and G streptococci

Binding of native alpha 2-macroglobulin to human group G streptococci

Binding of human alpha 2-macroglobulin (alpha 2M) to group G streptococci and to their immunoglobulin G (IgG)-binding proteins (protein G) was investigated. Native alpha 2M bound specifically to strain G-148 with an apparent dissociation constant of (2.2 +/- 1.5) x 10(-9) M. Proteinase-complexed alpha 2M did not compete for the binding sites, and 125I-labelled proteinase-complexed alpha 2M did not bind to the bacteria. Binding of native alpha 2M to the cells was not affected by IgG or protein G consisting of only IgG-binding domains. 125I-labelled recombinant protein G did not bind to native or proteinase-complexed alpha 2M. However, a lysate of G-148 cells inhibited binding of alpha 2M to the bacteria, and immobilized wild-type protein G bound alpha 2M directly from fresh human plasma. In 13 group G streptococcal isolates, IgG-binding proteins were immunologically identified as protein G. In 11 isolates, these molecules reacted also with alpha 2M and human serum albumin (HSA). Western blots (immunoblots) of two wild-type protein G variants revealed identical bands reactive with goat IgG, HSA, and native alpha 2M. Digestion of wild-type protein G with clostripain destroyed in both variants the binding sites for alpha 2M but not for albumin and IgG. N-terminal fragments of protein G (lacking the IgG-binding region) bound both alpha 2M and HSA, whereas a similar HSA-binding peptide lacking the first 80 amino acids did not react with alpha 2M. Our findings are consistent with a specific binding site for native alpha 2M in the N-terminal region of protein G and suggest that binding of alpha 2M via IgG-binding proteins may be a general feature of human group G streptococci.

Characterization of a gene coding for a type IIo bacterial IgG-binding protein

Two antigenic classes of non-immune IgG-binding proteins can be expressed by group A streptococci. One antigenic group of proteins is recognized by an antibody prepared against the product of a cloned fcrA gene (anti-FcRA). In this study, the immunogen used to prepare the antibody that defines the second antigenic class was shown to be the product of the emm-like (emmL) gene of M serotype 55 group A isolate, A928. The emmL55 gene expressed in E. coli produced an M(r) approximately 58,000 molecule which bound human IgG1, IgG2, IgG3 and IgG4, as well as horse, rabbit and pig IgG in a non-immune fashion. These properties are characteristic of the previously described type IIo IgG-binding protein isolated from this strain. In addition, the recombinant protein was reactive with human serum albumin and fibrinogen. The emmL 55 gene sequence was analysed and found to have the organization and sequence characteristics of a typical class I emm-like gene.

Functional and serological analysis of type II immunoglobulin G-binding proteins expressed by pathogenic group A streptococci

Bacterial immunoglobulin-binding proteins expressed on the surface of group A streptococci represent a heterogeneous family of functionally related proteins. In this report, we describe efficient methods for extracting immunoglobulin-binding proteins and classifying them functionally and antigenically. A common characteristic of immunoglobulin-binding proteins expressed by group A streptococci appears to be the absence of internal methionine residues in the binding protein. This has enabled development of a rapid, efficient, cyanogen bromide-based extraction procedure for solubilizing these molecules from intact bacteria. Studies carried out with a series of monospecific polyclonal antibodies prepared in chickens have identified two major antigenic classes of immunoglobulin-binding proteins. The methods described in this report facilitate a rapid functional and serological screening of immunoglobulin-binding proteins that should now enable detailed epidemiological studies of the importance of these molecules in group A streptococcal infections and their relationship to other surface proteins, in particular, the antiphagocytic M protein.