The t1/2 values of normal pro-αIIb and VFαIIb subunits expressed with β3 are prolonged in the presence of a proteasome inhibitor. Pulse-chase analyses were performed as described in Figure 1 on HEK293 cells transiently transfected with β3 and either normal αIIb or mutant VFαIIb in the presence of either DMSO (A,C), or MG132 (B,D). (E) The density of the pro-αIIb bands of one representative experiment are plotted as a percentage of their 3-hour postchase density. The normal pro-αIIb and pro-VFαIIb subunits disappeared from the cells cultured in DMSO with t1/2 values of 5 ± 2 (n = 3) and 5 ± 3 (n = 5) hours, respectively. Disappearance of both the normal and mutant pro-αIIb subunits was delayed in the presence of MG132 to 11 plus or minus 2 hours (n = 3, P = .03), and 14 plus or minus 10 hours (n = 5, P = .05), respectively.