CBFβ-SMMHC must interact with CBF and inhibit CBF transactivation to attenuate p53 induction, and reduces p53 mRNA expression in Ba/F3 cells. (A) MTINV(β▵2-11) Ba/F3 cells, which express a CBFβ-SMMHC variant that cannot bind CBF, were cultured ± 100 μmol/L zinc chloride for 18 hours. Half of the cells from each of these cultures were then irradiated to 10 Gy. Cells exposed initially to zinc were continued in zinc after irradiation. Total cellular protein extracts were prepared 90 minutes later and subjected to Western blotting with antisera specific for murine p53 and CBFβ-SMMHC. Fast Green staining of the blot is shown as a control for protein loading. (B) MTCB6 and MTINV-3 cells were cultured ± zinc for 18 hours. Poly-A–containing mRNA was then prepared from each culture. Five micrograms of each sample was subjected to electrophoresis on a 1% formaldehyde-agarose gel, transferred to a nylon membrane, and probed sequentially for p53, mdm2, and actin.