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

TORKKO, J. M.; PRIMO, M E; DIRKX, R.; FRIEDRICH, A; VIEHRIG, A.; VERGARI, E.; BORGONOVO, B.; SONMEZ, A.; WEGBROD, C.; LACHNIT, M.; MUNSTER, C.; SICA, M P; ERMÁCORA, M. R.; SOLIMENA, M.

MOLECULAR AND CELLULAR BIOLOGY

AMER SOC MICROBIOLOGY

Lugar: Washington; Año: 2015 vol. 35 p. 914 - 927

0270-7306

The type-1 diabetes autoantigen ICA512/IA-2/RPTPN is a receptor protein tyrosine phosphatase of the insulin secretory granules, which regulates the size of granule stores, possibly via cleavage/signaling of its cytosolic tail. The role of its extracellular region, instead, remains unknown. Structural studies indicated that 2- or 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 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 4-strand. S508A/G553D or 38 N506A/G553D double mutants dimerize, but remain in the endoplasmic reticulum. Removal of the N-terminal fragment (ICA512-NTF) preceding ME ICA512 allows instead an ICA512-NTF G553D mutant to exit the endoplasmic reticulum and ICA512- 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 2-strand fosters proICA512 dimerization until NTF prevents N506 glycosylation. Removal of this constraint allows for proICA512 4-strand induced dimerization, exit from the endoplasmic reticulum, O-glycosylation and RESP18-HD-mediated targeting to