Protein G is an immunoglobulin-binding protein expressed in group C and G streptococcal bacteria much like protein A but with differing binding specificities. It is a ~60-kDA (65 kDA for strain G148 and 58 kDa for strain C40)[1] cell surface protein that has found application in purifying antibodies through its binding to the Fab and Fc region. The native molecule also binds albumin, but because serum albumin is a major contaminant of antibody sources, the albumin binding site has been removed from recombinant forms of protein G. This recombinant protein G, either labeled with a fluorophore or a single-stranded DNA strand, was used as a replacement for secondary antibodies in immunofluorescence and super-resolution imaging.[2]
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