The type‐III Fe receptor from Streptococcus dysgalactiae is also an α2‐macroglobulin receptor

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Abstract
We have cloned and sequenced the gene for a type‐III Fe 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 Fe receptors of this type. Upstream of the IgG‐binding domains, there is a unique sequence containing short repeated motifs. Subeloning 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 α2‐macroglobulin after its complex formation with proteases. In contrast to earlier described type‐III Fe receptors, this molecule does not bind human serum albumin.