Relationship of the 37,000- and 40,000-Mr tryptic fragments of islet antigens in insulin-dependent diabetes to the protein tyrosine phosphatase-like molecule IA-2 (ICA512)

Abstract

Sera from patients with insulin-dependent diabetes immunoprecipitate 64,000-Mr proteins, distinct from glutamate decarboxylase, that are cleaved to 37,000- and 40,000-Mr fragments by trypsin. We investigated possible relationships between 37,000- or 40,000-Mr. fragments of antigen and the tyrosine phosphatase-like protein, IA-2 (ICA512). Antibodies from nondiabetic relatives bound differentially to 37,000- and 40,000-Mr fragments indicating presence of distinct epitopes. Precursors of these fragments could be separated on immobilized lectins, suggesting different carbohydrate content. Levels of antibodies to 40,000-Mr fragments were strongly associated with those to the intracellular domain of IA-2. Recombinant intracellular domain of IA-2 blocked binding of antibodies to 40,000-Mr fragments expressed by insulinoma cells and partially blocked binding to 37,000-A/r fragments. Furthermore, trypsinization of recombinant intracellular domain of IA-2 generated proteolytic fragments of identical Mr to the 40,000-Mr fragments of insulinoma antigen; 37,000-M2 fragments were not generated. Thus, 40,000-Mr fragments of islet autoantigen are derived from a protein similar or identical to the tyrosine phosphatase-like molecule, IA-2. The 37,000-Mr fragments are derived from a different, although related, protein.