Most strains of express IgG-binding activity and this binding has been considered to be solely mediated by protein A. However, the existence of a second gene in strain 8325-4 encoding an IgG-binding polypeptide was recently reported. This novel IgG-binding polypeptide was found after panning a shotgun phage display library, made from chromosomal DNA, against immobilized human IgG. The complete gene () encoding this novel IgG-binding protein, denoted protein Sbi, has now been cloned and sequenced. Analysis of other strains showed that this gene is not unique for strain 8325-4. The protein consists of 436 amino acids and exhibits an immunoglobulin-binding specificity similar to protein A. Furthermore, it is shown that Sbi is highly expressed in strain Newman 4, which shows that IgG-binding activity in can be mediated by proteins other than protein A.


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