αIIbβ3 biogenesis is controlled by engagement of αIIb in the calnexin cycle via the N15-linked glycan

Although much is known about αIIbβ3 structure and function, relatively little is understood about its biogenesis. Thus, we studied the kinetics of pro-αIIb production and degradation, focusing on whether proteasomal degradation or the calnexin cycle participates in these processes. In pulse-chase analyses, the time to half-disappearance of pro-αIIb (t1/2) was the same in (1) HEK293 cells transfected with (a) αIIb plus β3, (b) αIIb alone, (c) mutant V298FαIIb plus β3, or (d) I374TαIIb plus β3; and (2) murine wild-type and β3-null megakaryocytes. Inhibition of the proteasome prolonged the t1/2 values in both HEK293 cells and murine megakaryocytes. Calnexin coprecipitated with αIIb from HEK293 cells transfected with αIIb alone, αIIb plus β3, and V298FαIIb plus β3. For proteins in the calnexin cycle, removal of the terminal mannose residue of the middle branch of the core N-linked glycan results in degradation. Inhibition of the enzyme that removes this mannose residue prevented pro-αIIb degradation in β3-null murine megakaryocytes. αIIb contains a conserved glycosylation consensus sequence at N15, and an N15Q mutation prevented pro-αIIb maturation, complex formation, and degradation. Our findings suggest that pro-αIIb engages the calnexin cycle via the N15 glycan and that failure of pro-αIIb to complex normally with β3 results in proteasomal degradation.