PMID: 25561468

 

    Legend: Sugar

Title : Stability of pro ICA512 / IA-2 and its targeting to insulin secretory granules require β 4-sheet-mediated dimerization of its ectodomain in the endoplasmic reticulum

Abstract :

1. 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
2. The role of its extracellular region remains unknown
3. Structural studies indicated that β 2- or β 4-strands in the mature ectodomain (ME ICA512 ) form dimers in vitro
4. Here we show that ME ICA512 prompts pro ICA512 dimerization in the endoplasmic reticulum
5. Perturbation of ME ICA512 β 2-strand N-glycosylation upon S508A replacement allows for pro ICA512 dimerization , O-glycosylation, targeting to granules, and conversion, which are instead precluded upon G553D replacement in the ME ICA512 β 4-strand
6. S508A/G553D and N506A/G553D double mutants dimerize but remain in the endoplasmic reticulum
7. 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
8. The signal for SG sorting is located within the NTF RESP18 homology domain ( RESP18-HD), whereas soluble NTF is retained in the endoplasmic reticulum
9. Hence, we propose that the ME ICA512 β 2-strand fosters pro ICA512 dimerization until NTF prevents N506 glycosylation
10. Removal of this constraint allows for pro ICA512 β 4-strand-induced dimerization , exit from the endoplasmic reticulum, O-glycosylation, and RESP18-HD-mediated targeting to granules