Plays a role in vesicle-mediated secretory processes (PubMed:24843546). Required for normal accumulation of secretory vesicles in hippocampus, pituitary and pancreatic islets (By similarity). Required for the accumulation of normal levels of insulin-containing vesicles and preventing their degradation (PubMed:24843546). Plays a role in insulin secretion in response to glucose stimuli (PubMed:24843546). Required for normal accumulation of the neurotransmitters norepinephrine, dopamine and serotonin in the brain (By similarity). In females, but not in males, required for normal accumulation and secretion of pituitary hormones, such as luteinizing hormone (LH) and follicle-stimulating hormone (FSH) (By similarity). Required to maintain normal levels of renin expression and renin release (By similarity). Seems to lack intrinsic enzyme activity (By similarity). May regulate catalytic active protein-tyrosine phosphatases such as PTPRA through dimerization (By similarity). ICA512-TMF regulates dynamics and exocytosis of insulin secretory granules (SGs); binding of ICA512-TMF to SNTB2/beta-2-syntrophin is proposed to restrain SGs mobility and exocytosis by tethering them to the actin cytoskeleton depending on UTRN; the function is inhibited by cytoplasmic ICA512-CFF dimerizing with ICA512-TMF and displacing SNTB2. ICA512-CCF translocated to the nucleus promotes expression of insulin and other granule-related genes; the function implicates binding to and regulating activity of STAT5B probably by preventing its dephosphorylation and potentially by inducing its sumoylation by recruiting PIAS4 (PubMed:15596545, PubMed:16622421, PubMed:18178618). Enhances pancreatic beta-cell proliferation by converging with signaling by STAT5B and STAT3 (PubMed:15596545, PubMed:16622421, PubMed:18178618). ICA512-CCF located in the cytoplasm regulates dynamics and exocytosis of insulin secretory granules (SGs) by dimerizing with ICA512-TMF and displacing SNTB2 thus enhancing SGs mobility and exocytosis (PubMed:18824546, PubMed:20886068). Homodimer; shown for the unprocessed protein (proICA512) in the endoplasmic reticulum and resolved during protein maturation as ICA512-TMF seems to be predominantly monomeric in secretory granules; however, ICA512-CCF interacts with ICA512-TMF disrupting the ICA512-TMF:SNTB2 complex. The isolated lumenal RESP18 homology domain has been shown to form disulfide-linked homooligomers (PubMed:25561468, PubMed:18824546, PubMed:26836020). Interacts (via cytoplasmic domain) with phosphorylated SNTB2; this protects PTPRN against cleavage by CAPN1 to produce ICA512-CCF. Dephosphorylation of SNTB2 upon insulin stimulation disrupts the interaction and results in PTPRN cleavage (PubMed:11483505). Interacts with SNX19 (PubMed:16273344). ICA512-CCF interacts with PIAS4; in the nucleus (PubMed:15596545). Interacts with STAT5B (phosphorylated); down-regulated by ICA512-CCF sumoylation; ICA512-CCF prevents STAT5B dephosphorylation; ICA512-CCF mediates interaction of STAT5B with PIAS4 (PubMed:15596545, PubMed:16622421). Interacts (via RESP18 homology domain) with insulin and proinsulin (PubMed:26836020). Interacts with PTPRN2, PTPRA and PTPRE (By similarity). Detected on neuronal secretory vesicles, but not on synaptic vesicles. Colocalizes with insulin-containing secretory granules (PubMed:25561468). Primarily detected on secretory vesicle membranes. Transiently found at the cell membrane, when secretory vesicles fuse with the cell membrane to release their cargo. Is then endocytosed and recycled to secretory vesicles via the Golgi apparatus membranes. Expression is restricted to neuroendocrine cells. Found in pancreas, brain and pituitary. The RESP18 homology domain is sufficient for targeting proICA512 to secretory granules. N-glycosylated. O-glycosylated with core 1 or possibly core 8 glycans. Subject to proteolytic cleavage at multiple sites (PubMed:11483505). Subject to cleavage on a pair of basic residues (By similarity). On exocytosis of secretory granules in pancreatic beta-cells ICA512-TMF is transiently inserted in the plasma-membrane and cleaved by mu-type calpain CPN1 to yield ICA512-CCF (By similarity). Sumoylated at two sites including Lys-754. Sumoylation decreases interaction with STAT5. Autoantigen in insulin-dependent diabetes mellitus (IDDM). Belongs to the protein-tyrosine phosphatase family. Receptor class 8 subfamily. Does not possess catalytic activity due to replacement of highly conserved residues in tyrosine-protein phosphatase domain.