Expression of cell cycle–associated proteins during terminal erythroid differentiation.

Total cellular protein was isolated from FVA erythroblasts cultured for 0, 24, or 48 hours with EPO (lanes 1-3), p21 null erythroblasts (lane 4), or p21+/+ erythroblasts exposed to the DNA-damaging agent ActD for 6 hours (lane 5). Proteins were electrophoretically separated in 12% polyacrylamide gels and probed with antibodies to p21, p27, cdk2, cdk4, cdk6, cyclin D3, or cyclin E. gp55 protein expression is shown as a control for protein loading.