Figure 4.

Induction of p21^Cip1 by ARF or p53 retroviral vectors. Cells infected with vector alone (lanes 1–3), a retrovirus encoding HA-tagged p19^ARF (lanes 4 and 5), or a vector encoding wild-type p53 (lanes 6–8) were lysed 48 hr after infection. Proteins separated on gels were immunoblotted for p53 (Top), p21^Cip1 (Middle), and p19^ARF (Bottom) as indicated at the left. Infected cells included p53-null early passage MEFs (lanes 1 and 6), wild-type MEFs (lanes 3, 5. and 8), or early passage ARF-null MEFs (lanes 2, 4, and 7). Endogenous p19^ARF, elevated in p53-null cells (lane 1), is repressed after infection with p53 virus (lane 6). HA-tagged ARF (indicated by asterisks) migrates slower than the endogenous protein. Growth arrest was assayed at 48 hr by incorporation of [³H]-thymidine in replica plates.