Stability of proICA512/IA-2 and Its Targeting to Insulin Secretory Granules Require beta 4-Sheet-Mediated Dimerization of Its Ectodomain in the Endoplasmic Reticulum

作者：Torkko Juha M; Primo M Evangelina; Dirkx Ronald; Friedrich Anne; Viehrig Antje; Vergari Elisa; Borgonovo Barbara; Soenmez Anke; Wegbrod Carolin; Lachnit Martina; Muenster Carla; Sica Mauricio P; Ermacora Mario R; Solimena Michele^*

来源：Molecular and Cellular Biology, 2015, 35(6): 914-927.

DOI：10.1128/MCB.00994-14

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 beta 2- or beta 4-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 beta 2-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 beta 4-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-Delta NTF G553D mutant to exit the endoplasmic reticulum, and ICA512-Delta NTF 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 beta 2-strand fosters proICA512 dimerization until NTF prevents N506 glycosylation. Removal of this constraint allows for proICA512 beta 4-strand-induced dimerization, exit from the endoplasmic reticulum, O-glycosylation, and RESP18-HD-mediated targeting to granules.

• 出版日期2015-3