The half-lives of murine pro-αIIb are prolonged in the presence of a proteasome inhibitor in both wild-type and β3-null murine megakaryocytes. Megakaryocytes generated from the bone marrow of WT (A-B) and β3-null mice (C-D) were cultured in the presence of DMSO or MG132, and the cells were lysed at the times indicated. Samples were analyzed as described in Figure 1. (E) The densities of the pro-αIIb bands from a representative experiment are plotted as a percentage of the density at 0 hours. The disappearance of pro-αIIb subunits was delayed in WT megakaryocytes and significantly delayed in β3-null megakaryocytes in the presence of MG132 4 plus or minus 2 hours versus 9 plus or minus 5 hours (n = 3), and 3 plus or minus 2 hours versus 7 plus or minus 2 hours (n = 6; P = .01), respectively. This indicates that pro-αIIb is degraded by a proteasomal mechanism in WT and β3-null murine megakaryocytes.