IDENTIFICATION OF A 2ND TRANSMEMBRANE PROTEIN-TYROSINE-PHOSPHATASE, IA-2-BETA, AS AN AUTOANTIGEN IN INSULIN-DEPENDENT DIABETES-MELLITUS - PRECURSOR OF THE 37-KDA TRYPTIC FRAGMENT

A novel cDNA, IA-2 beta, was isolated from a mouse neonatal brain libr ary, The predicted protein sequence revealed an extracellular domain, a transmembrane region, and an intracellular domain. The intracellular domain is 376 amino acids long and 74% identical to the intracellular domain of IA-2, a major autoantigen in insulin-dependent diabetes mel litus (IDDM). A partial sequence of the extracellular domain of IA-2 b eta indicates that it differs substantially (only 26% identical) from that of IA-2. Both molecules are expressed in islets and brain tissue, Forty-six percent (23 of 50) of the IDDM sera but none of the sera fr om normal controls (0 of 50) immunoprecipitated the intracellular doma in of IA-2 beta. Competitive inhibition experiments showed that IDDM s era have autoantibodies that recognize both common and distinct determ inants on IA-2 and IA-2 beta. Many IDDM sera are known to immunoprecip itate 37-kDa and 40-kDa tryptic fragments from islet cells, but the id entity of the precursor protein(s) has remained elusive. The current s tudy shows that treatment of recombinant IA-2 beta and IA-2 with tryps in yields a 37-kDa fragment and a 40-kDa fragment, respectively, and t hat these fragments can be immunoprecipitated with diabetic sera, Abso rption of diabetic sera with unlabeled recombinant IA-2 or IA-2 beta, prior to incubation with radiolabeled 37-kDa and 40-kDa tryptic fragme nts derived from insulinoma or glucagonoma cells, blocks the immunopre cipitation of both of these radiolabeled tryptic fragments. We conclud e that IA-2 beta and IA-2 are the precursors of the 37-kDa and 40-kDa islet cell autoantigens, respectively, and that both IA-2 and IA-2 bet a are major autoantigens in IDDM.