Pro-αIIb containing either of 2 Glanzmann thrombasthenia mutations is degraded at the same rate as normal pro-αIIb. Cells were transiently transfected with cDNA constructs expressing both normal αIIb and β3 (Nl) (A), αIIb alone (B), or the αIIb mutants V298F (VF; C) or I374T (IT; D) in combination with β3. At 36 hours, cells were metabolically labeled with 35S-methionine/cysteine and then harvested at the indicated time points. Radiolabeled protein was immunoprecipitated using a combination of the αIIb-specific mAbs, B1B5 and M-148; the samples were then subjected to SDS-PAGE under reducing conditions, and dried gels were exposed to film. Arrowheads indicate bands representing pro-αIIb, mature αIIb, and β3; the migration of molecular weight standards is shown on the right. (E) Band intensities representing each subunit were measured by densitometry and plotted as a percentage of maximum band density of pro-αIIb (at 2 hours for Nl and at 0 hours for VF and IT). The times to half-maximum appearance of mature αIIb and β3 were both 2 plus or minus 1 hours (n = 5).