ICA512 RESP18 homology domain is protein condensing factor and insulin fibrillation inhibitor

Type 1 diabetes islet cell autoantigen ICA512)/IA-2/PTPRN is a catalytically inactive receptor tyrosine phosphatase involved in the biogenesis and turnover of insulin secretory granules (SGs) in the β-cells of the pancreatic islets. Whereas the ICA512 proximal luminal ectodomain has been functionally and structurally characterized, the role of its preceding N- terminal luminal segment, termed regulated endocrine-specific protein 18-homology domain (RESP18HD) encompassing amino acids 35 – 131 remains largely unknown. Here we show that ICA512 RESP18HD C-terminal motif including residues 91 – 131 contains an intrinsically disordered region (IDR), which is critically required for RESP18HD function as a pH and Zn2+ dependent condensation and reversible aggregation factor for insulin and other β-cell proteins. At variance with other SG cargoes with aggregating properties, the condensing activity of ICA512 RESP18HD is displayed at pH values close to neutral, as found in the early secretory pathway, while being resolved at acidic pH and in the presence of high Zn2+ concentrations, as found in mature insulin SGs. Moreover, we show that the ICA512 RESP18HD portion including amino acids 35 – 90, i.e. the region preceding the IDR, inhibits insulin fibrillation in vitro. Finally, we show that glucose-stimulated release of ICA512 RESP18HD from the SGs is associated with proteolysis of its IDR, conceivably to prevent its aggregation upon exposure to neutral pH in the extracellular milieu. Taken together, these findings point to ICA512 RESP18HD being a condensing factor for SG protein sorting and granulogenesis early in the secretory pathway and for prevention of insulin fibrillation.