Type III IgGbinding proteins

The type III IgG-binding proteins are more frequently designated as streptococcal protein G. Protein G is expressed by the majority of human isolates of group C and group G streptococci. A number of variant forms of protein G have been described at both the gene and protein level. All forms of protein G are antigenically related and the majority also bind human serum albumin. The gene encoding protein G has been cloned and the albumin-binding and IgG-binding domains have been shown to be distinct. This has enabled nonalbumin-binding forms of protein G to be genetically engineered.

Protein G demonstrates the widest species and subclass reactivity of any bacterial IgG-binding protein (Figure 1). Protein G, like protein A, can form a number of different-sized complexes with IgG and these demonstrate a similar pattern of complement-fixing potential to that observed with protein A. Protein G has been shown to mediate mitogenic responses when added to human peripheral blood lymphocytes. This activity, however, could not be correlated with either the albumin or IgG-binding domain of the protein.

By virtue of its broad species and subclass reactivity profile, protein G is an extremely useful reagent as a tracer and for affinity purification of antibodies that are not reactive with protein A.

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