Streptococcal protein MAG--a protein with broad albumin binding specificity

Protein MAG is a cell surface protein from Streptococcus dysgalactiae which binds alpha 2-macroglobulin (alpha 2M), serum albumin and immunoglobulin G (IgG). In this work protein MAG was expressed in Escherichia coli, purified and analysed for its albumin-binding specificity. The binding of protein MAG to serum albumins of different species origin was studied in a dot-blot assay and compared with the binding of streptococcal protein G, so far the best studied bacterial albumin receptor. The albumin-binding of protein MAG was also characterized using real-time biospecific interaction analysis (BIA), and the ka, kd and the Kaff values for different albumins were determined. Amino acid sequence alignment revealed homology between the albumin-binding domain of protein MAG and earlier described streptococcal albumin receptors including protein G. However, the MAG protein was reactive with serum albumin from bovine, dog, goat, horse, human, mouse, pig, rat and sheep origin and therefore displays a broader albumin-binding profile than protein G concerning the albumins tested in this work. Comparison of the albumin-binding spectrum of protein MAG with the earlier described albumin receptors of various S. dysgalactiae strains and other streptococci, suggests that protein MAG is a new type of albumin receptor.