Wild-type, but not mutant, human p53 proteins inhibit the replication activities of simian virus 40 large tumor antigen

Murine p53 blocks many of the replication activities of simian virus 40 (SV40) large tumor antigen (T antigen) in vitro. As murine cells do not replicate SV40 DNA, it was of interest to determine how p53 from permissive human cells functions. Recombinant baculoviruses encoding either the wild-type form of human p53 or a mutant p53 cloned from a human tumor cell line were constructed, and p53 proteins were purified from infected insect cells. Surprisingly, we found that wild-type human p53 was as inhibitory to the ability of T antigen to mediate replication of an SV40 origin-containing (ori DNA) plasmid in vitro as was murine p53. Wild-type human p53 also blocked the DNA unwinding activity of T antigen, as did its murine counterpart. In contrast to murine and wild-type human p53, the mutant human p53 did not block ori DNA replication or DNA unwinding. Murine p53 formed a complex with mutant human p53 in vivo. Furthermore, mutant human p53 reduced the inhibition of SV40 ori DNA replication by murine p53 in vitro. These results provide a model for the way in which mutant p53 proteins can affect normal functions of p53.