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 in sulin-dependent diabetes mellitus (IDDM) include tryptic fragments wit h a molecular mass of 37 kDa and/or 40 kDa of a pancreatic islet cell antigen of unknown identity, The assay identifying autoantibodies agai nst the 37/40-kDa antigen in human sera is based on the immunoprecipit ation of S-35-labeled rat insulinoma cell proteins with sera from IDDM patients, followed by limited trypsin digestion of the immunoprecipit ated material, To identify cDNA clones coding for the 37/40-kDa antige n, we have screened a cDNA expression library from rat insulinoma cell s with a serum from an IDDM patient that precipitated the 37/40-kDa an tigen 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'' f or 105-kDa islet cell antibody, The deduced amino acid sequence has hi gh homology to a recently cloned putative tyrosine phosphatase IA-2 fr om human and mouse cDNA libraries, Translation of the cDNA in vitro re sults in a polypeptide with the expected molecular mass of 105 kDa, Th e evidence that ICA105 is indeed the precursor of the 37/40-kDa trypti c fragments is based on the following three results: (i) Sera from IDD M patients containing autoantibodies to the 37/40-kDa antigen precipit ate the in vitro translated polypeptide, whereas sera from healthy sub jects as well as sera from IDDM patients not reactive with the 37/40-k Da antigen do not precipitate the cDNA product, (ii) Immunoprecipitati on of the in vitro translated protein with sera containing autoantibod ies to the 37/40-kDa antigen followed by limited trypsin digestion of the precipitated proteins results in a 40-kDa polypeptide. (iii) The p rotein derived from our cDNA but not from an unrelated control cDNA cl one fan block immunoprecipitation of the 37/40-kDa antigen from a labe led rat insulinoma cell extract, The availability of the cloned 37/40- kDa antigen should facilitate the identification of individuals at ris k 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 o f IDDM.