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Post‐translational modifications of ICA512, a receptor tyrosine phosphatase‐like protein of secretory granules

The autoantigen of type I diabetes ICA512 is a receptor tyrosine phosphatase‐like protein enriched in the secretory granule membranes of neurons and peptide secreting endocrine cells. While the function of ICA512 remains unknown, it is thought to link regulated neuropeptide and peptide hormone secretion with signal transduction pathways involving tyrosine phosphorylation/dephosphorylation. To characterize further its biochemical properties, we conducted studies in the bovine pituitary, an abundant source of native ICA512, as well as in fibroblasts transfected with various human ICA512 cDNA constructs. Based on these studies we have established that the signal peptide of ICA512 encompasses residues 1–34 and that the ectodomain of ICA512 undergoes multiple post‐translation modifications, including N‐glycosylation. Newly synthesized ICA512 appears first as a pro‐protein of 110 kDa that is then converted by post‐translational modifications into a 130‐kDa species. Cleavage of pro‐ICA512 at a consensus for furin‐like convertases generates a 60–66‐kDa ICA512 transmembrane fragment (amino acids 449–979). Such processing ICA512 is not restricted to neuroendocrine cells, as it can also occur in transfected fibroblasts. Finally, the predicted N‐terminal fragment of ICA512 resulting from this cleavage (amino acids 35–448) or parts thereof are present in the neurosecretosomes of posterior pituitary, raising the possibility that they may be secreted upon exocytosis of secretory granules.