The site of cleavage in human alpha chains of IgA1 and IgA2:A2m(1) allotype paraproteins by the clostridial IGA protease.

Fc fragments of human immunoglobulin A(IgA) of IgA1 subclass and IgA2 subclass of A2m(1) allotype were prepared from IgA paraproteins by digestion with a protease from Clostridium sp. (M.O.-6). The N-terminal tetrapeptide of Val-Pro-Ser-Thr- for the Fc of IgA1 subclass, and that of Val-Pro-Pro-Pro- for the Fc of IgA2:A2m(1) allotype, were identified by sequence analysis. The site of cleavage by the protease was defined to be at the Pro-Val peptide bond, which is a common peptide bond present at 221-222 in both alpha chains. IgA of IgA2 subclass of A2m(2) allotype is resistant to the protease due to the different, Arg-Val, peptide bond at the same position.