THE 37/40-KILODALTON AUTOANTIGEN IN INSULIN-DEPENDENT DIABETES-MELLITUS IS THE PUTATIVE TYROSINE PHOSPHATASE IA-2

Major targets for autoantibodies associated with the development of insulin-dependent diabetes mellitus (IDDM) include tryptic fragments with a molecular mass of 37 kDa and/or 40 kDa of a pancreatic islet cell antigen of unknown identity, The assay identifying autoantibodies against the 37/40-kDa antigen in human sera is based on the immunoprecipitation of S-35-labeled rat insulinoma cell proteins with sera from IDDM patients, followed by limited trypsin digestion of the immunoprecipitated material, To identify cDNA clones coding for the 37/40-kDa antigen, we have screened a cDNA expression library from rat insulinoma cells with a serum from an IDDM patient that precipitated the 37/40-kDa antigen in our assay, Among the cDNA products that reacted with the IDDM serum, we identified one cDNA clone whose open reading frame encodes a protein with a predicted mass of 105 kDa that we termed ''ICA105'' for 105-kDa islet cell antibody, The deduced amino acid sequence has high homology to a recently cloned putative tyrosine phosphatase IA-2 from human and mouse cDNA libraries, Translation of the cDNA in vitro results in a polypeptide with the expected molecular mass of 105 kDa, The evidence that ICA105 is indeed the precursor of the 37/40-kDa tryptic fragments is based on the following three results: (i) Sera from IDDM patients containing autoantibodies to the 37/40-kDa antigen precipitate the in vitro translated polypeptide, whereas sera from healthy subjects as well as sera from IDDM patients not reactive with the 37/40-kDa antigen do not precipitate the cDNA product, (ii) Immunoprecipitation of the in vitro translated protein with sera containing autoantibodies to the 37/40-kDa antigen followed by limited trypsin digestion of the precipitated proteins results in a 40-kDa polypeptide. (iii) The protein derived from our cDNA but not from an unrelated control cDNA clone fan block immunoprecipitation of the 37/40-kDa antigen from a labeled rat insulinoma cell extract, The availability of the cloned 37/40-kDa antigen should facilitate the identification of individuals at risk of IDDM with increased accuracy, Furthermore, the identification of the 37/40-kDa antigen as the putative tyrosine phosphatase IA-2 is of relevance in elucidating the role of this antigen in the development of IDDM.