alphaIIbbeta3 biogenesis is controlled by engagement of alphaIIb in the calnexin cycle via the N15-linked glycan

Although much is known about alphaIIbbeta3 structure and function, relatively little is understood about its biogenesis. Thus, we studied the kinetics of pro-alphaIIb 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-alphaIIb (t1/2) was the same in (1) HEK293 cells transfected with (a) alphaIIb plus beta3, (b) alphaIIb alone, (c) mutant V298FalphaIIb plus beta3, or (d) I374TalphaIIb plus beta3; and (2) murine wild-type and beta3-null megakaryocytes. Inhibition of the proteasome prolonged the t1/2 values in both HEK293 cells and murine megakaryocytes. Calnexin coprecipitated with alphaIIb from HEK293 cells transfected with alphaIIb alone, alphaIIb plus beta3, and V298FalphaIIb plus beta3. 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-alphaIIb degradation in beta3-null murine megakaryocytes. alphaIIb contains a conserved glycosylation consensus sequence at N15, and an N15Q mutation prevented pro-alphaIIb maturation, complex formation, and degradation. Our findings suggest that pro-alphaIIb engages the calnexin cycle via the N15 glycan and that failure of pro-alphaIIb to complex normally with beta3 results in proteasomal degradation.