AN ISLET-CELL PROTEIN-TYROSINE-PHOSPHATASE IS A LIKELY PRECURSOR TO THE 37-KDA AUTOANTIGEN IN TYPE-1 DIABETES - HUMAN AND MACAQUE SEQUENCES, TISSUE DISTRIBUTION, UNIQUE AND SHARED EPITOPES, AND PREDICTIVE AUTOANTIBODIES

Background: We sought to identify novel islet-cell autoantigens to bet ter understand the pathogenesis, prediction, and immunotherapy of type 1 diabetes. Materials and Methods: Macaque and human islet cDNA libra ries expressed in mammalian cells were screened with human diabetes se ra. A positive clone was sequenced directly and after 5' rapid amplifi cation of cDNA ends (RACE). Northern blotting and in situ hybridizatio n revealed the tissue distribution of the corresponding protein. Antig en, expressed by in vitro translation, and tryptic peptides were analy zed by SDS-PAGE. For the immunoprecipitations, 183 diabetic, 60 predia betic, and 91 control sera were used. Truncated antigens were used in immunoprecipitations for epitope mapping. Recombinant antigen expresse d in transfected fibroblasts was used in competition assays. Results: Sequencing yielded an 111-kDa, 1,013 amino acid, transmembrane protein (M1851) containing consensus protein tyrosine phosphatase (PTPase) se quence. M1851 was 77% identical in the intracellular domain, but only 31% identical extracellularly, to the islet-cell autoantigen ICA512. m RNA localized to brain, prostate, pancreatic islets, and adrenal medul la. After limited trypsinization, the in vitro translated antigen was 37 kDa. M1851 was recognized by 47% of prediabetes sera, 31% of new di abetes sera, but only 1% of healthy controls. Only 1/73 sera binding M 1851 failed to bind ICA512, whereas 42/114 binding ICA512 did not bind M1851. M1851 reactivity was not fully displaced by ICA512 in 24/49 se ra. Removing the C-terminal 27, 80, or 160 amino acids of M1851 decrea sed reactivity by 70%, 90%, and 100%, respectively. Conclusions: This new islet-cell PTPase is likely to be the precursor to the 37-kDa tryp tic fragment antigen. It is structurally related to ICA512 but has dis tinct diabetes autoantibody epitopes located at the C terminus.