MAG, A NOVEL PLASMA-PROTEIN RECEPTOR FROM STREPTOCOCCUS-DYSGALACTIAE

The gene encoding a plasma protein receptor from Streptococcus dysgalactine has been cloned and sequenced. The gene product, with a predicted molecular mass of approx. 44 kDa, binds alpha(2)-macroglobulin (alpha(2)M), serum albumin and immunoglobulin G (IgG). By subcloning and expressing various parts of the gene as fusion proteins, we found that the three binding activities reside in discrete domains of the protein. The single IgG-binding domain, localized in the C-terminal part of the molecule, shows high homology to streptococcal type-III Fc receptors. In the middle of the molecule, there is a stretch of 50 amino acids (aa) mediating albumin binding. This region has partial homology with the albumin-binding domains of streptococcal protein G. The alpha(2)M-binding domain is located in the N terminus of the molecule and is composed of a unique aa sequence. We call this trifunctional plasma protein receptor, MAG (binds alpha(2)M, albumin and IgG).