Stability and targeting of proICA512/IA-2 to insulin secretory granules requires beta4-sheet mediated dimerization of its ectodomain in the endoplasmic reticulum

• Autores: Torkko, J.; Primo, M.; Dirkx, R.; Friedrich, A.; Viehrig, A.; Vergari, E.; Borgonovo, B.; Sonmez, A.; Wegbrod, C.; Lachnit, M.; Munster, C.; Sica, Mauricio; Emárcora, M.; Solimena, M.
• Año de publicación: 2015
• Idioma: inglés
• Tipo de recurso: artículo
• Materia: Biología Celular, Microbiología; type-1 diabetes autoantigen ICA512/IA-2/RPTPN; tyrosine 30 phosphatase; insulin secretory granules; β4- sheet mediated dimerization; ectodomain; endoplasmic reticulum
• Institución: Comisión de Investigaciones Científicas de la Provincia de Buenos Aires

The type 1 diabetes autoantigen ICA512/IA-2/RPTPN is a receptor protein tyrosine phosphatase of the insulin secretory granules (SGs) which regulates the size of granule stores, possibly via cleavage/signaling of its cytosolic tail. The role of its extracellular region remains unknown. Structural studies indicated that β2- or β4-strands in the mature ectodomain (ME ICA512) form dimers in vitro. Here we show that ME ICA512 prompts proICA512 dimerization in the endoplasmic reticulum. Perturbation of ME ICA512 β2-strand N-glycosylation upon S508A replacement allows for proICA512 dimerization, O-glycosylation, targeting to granules, and conversion, which are instead precluded upon G553D replacement in the ME ICA512 β4-strand. S508A/G553D and N506A/G553D double mutants dimerize but remain in the endoplasmic reticulum. Removal of the N-terminal fragment (ICA512-NTF) preceding ME ICA512 allows an ICA512-ΔNTF G553D mutant to exit the endoplasmic reticulum, and ICA512-ΔNTF is constitutively delivered to the cell surface. The signal for SG sorting is located within the NTF RESP18 homology domain (RESP18-HD), whereas soluble NTF is retained in the endoplasmic reticulum. Hence, we propose that the ME ICA512 β2-strand fosters proICA512 dimerization until NTF prevents N506 glycosylation. Removal of this constraint allows for proICA512 β4-strand-induced dimerization, exit from the endoplasmic reticulum, O-glycosylation, and RESP18-HD-mediated targeting to granules.