MOLECULAR-CLONING OF PHOGRIN, A PROTEIN-TYROSINE-PHOSPHATASE HOMOLOG LOCALIZED TO INSULIN SECRETORY GRANULE MEMBRANES

An insulin granule membrane protein-tyrosine phosphatase (PTP) homolog ue, phogrin, was cloned by expression screening of a rat insulinoma cD NA library, The 3723-base pair cDNA encoded a transmembrane glycoprote in of 1004 amino acids (M(r) 111876) that underwent post-translational proteolysis to 60-64-kDa products after a 30-min delay. The kinetics of proteolytic conversion (t(1/2) = 45 min) and turnover (t(1/2) = 12 h) were consistent with sorting and conversion in a late compartment o f the secretory pathway. Studies on the native beta-cell protein sugge sted that the COOH-terminal PTP domain was on the cytosolic face of th e secretory granule, The lumenal segment was comprised of a pro tease- resistant globular domain of around 25 kDa, Its localization and topol ogy is thus consistent with a transmembrane receptor function related to granule biogenesis, exocytosis, or subsequent membrane recovery, an d it should prove to be a useful cell biological marker for the granul e membrane. High expression of the mRNA (5.4 kilobases) and protein wa s evident in islets, pancreatic alpha- and beta-cell tumor lines, brai n cells, and other cells of neuroendocrine lineage. It is closely rela ted to the diabetic autoantigen ICA512 (IA-2) (42% identity overall; 8 0% in the 260-amino acid PTP domain) and thus a potential target of au toimmunity in diabetes mellitus.