The type-III Fc receptor from Streptococcus dysgalactiae is also an alpha 2-macroglobulin receptor

We have cloned and sequenced the gene for a type-III Fc receptor from a Streptococcus dysgalactiae strain isolated from bovine mastitis and report here the complete sequence of the gene. The 1992-nucleotides-long open reading frame codes for a protein of 664 amino acids with a predicted molecular mass of 72 kDa (including the signal peptide). The C-terminal part of the protein is very similar to other streptococcal type-III receptors but contains five homologous IgG-binding domains compared to two or three in earlier reported Fc receptors of this type. Upstream of the IgG-binding domains, there is a unique sequence containing short repeated motifs. Subcloning different parts of the gene and expressing them as maltose-binding fusion proteins revealed that the upstream part is mediating binding to the plasma proteinase inhibitor alpha 2-macroglobulin after its complex formation with proteases. In contrast to earlier described type-III Fc receptors, this molecule does not bind human serum albumin.