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Microbiology (2002), 148, 2055-2064.
© 2002 Society for General Microbiology

Research Paper

Bovine immunoglobulin A (IgA)-binding activities of the surface-expressed Mig protein of Streptococcus dysgalactiae

Xin-Ming Song1, Jose Perez-Casal1, Michael C. Fontaine1 and Andrew A. Potter1,2

Veterinary Infectious Disease Organization (VIDO), Saskatoon, Saskatchewan, Canada S7N 5E31
Canadian Bacterial Diseases Network, Calgary, Alberta, Canada2

Author for correspondence: Andrew A. Potter. Tel: +1 306 966 7484. Fax: +1 306 966 7478. e-mail: Potter{at}sask.usask.edu

The Mig protein of Streptococcus dysgalactiae is a type III immunoglobulin G (IgG)-binding protein, expressing IgG- and {alpha}2-macroglobulin ({alpha}2-M)-binding receptors. This study showed that the Mig protein also displays binding activities to bovine immunoglobulin A (B-IgA). Biotin-labelled bovine serum IgA bound immobilized recombinant Mig and {alpha}2-M receptors derived from Mig, as well as the native Mig extracted from the surface of S. dysgalactiae strain SDG8 and the {alpha}2-M receptor released from the isogenic mig mutant strain Mig8-Mt, as determined by Western blotting and ELISA. There was no B-IgA binding activity to the immobilized IgG receptor derived from Mig or the proteins in the culture supernatant from the mig mutant strain Mig7-Mt, in which expression of Mig or Mig-related peptides on the cell surface was completely abolished. In a reciprocal experiment, biotin-labelled Mig was found to bind immobilized bovine serum IgA but not human IgA (H-IgA). The binding of Mig to bovine serum IgA was competitively inhibited by unlabelled Mig, intact and truncated {alpha}2-M receptors, and bovine serum IgA, but not by the Mig-IgG receptor, H-IgA or B-IgG. The binding of Mig and partially purified bovine secretory IgA (B-sIgA) was also characterized by Western blotting. Membrane-immobilized B-sIgA did not react with the biotin-labelled Mig, whereas soluble B-sIgA showed binding activity to the immobilized {alpha}2-M receptor of Mig. It is therefore concluded that the 11 kDa N-terminal region of the {alpha}2-M receptor of the S. dysgalactiae Mig protein specifically binds soluble and immobilized bovine serum IgA, as well as soluble B-sIgA. This is believed to be the first report of a B-IgA-binding protein in S. dysgalactiae.

Keywords: IgG, {alpha}2-macroglobulin, receptor, bovine mastitis

Abbreviations: {alpha}2-M, {alpha}2-macroglobulin; AP, alkaline phosphatase; B-, bovine (immunoglobulins); H-, human (immunoglobulins); sIgA, secretory immunoglobulin A






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Copyright © 2002 Society for General Microbiology.