PACT - CLONING AND CHARACTERIZATION OF A CELLULAR P53 BINDING-PROTEINTHAT INTERACTS WITH RB

Cellular functions of tumor suppressor proteins can be mediated by pro tein-protein interactions. Using p53 as a probe to screen an expressio n library, a cDNA encoding a 250 kDa protein was isolated. Recombinant forms of this protein, designated PACT, bind to wild type p53 while t wo different mutations abolish this interaction. PACT protein can also interfere with p53 specific DNA binding. PACT contains a serine/argin ine (SR) rich region and a C' terminal lysine rich domain. The 250 kDa PACT protein can be precipitated from cell lysates by a method specif ic for SR proteins. snRNPs can be co-immunoprecipitated from cells wit h anti-PACT antibodies. These antibodies stain cell nuclei in a speckl ed pattern reminiscent of the distribution of known splicing factors. Recently, RBQ1, a truncated human homologue of PACT was identified by virtue of Rb binding. We show that RBQ1 is truncated as a result of a possible mutational event. PACT can interact with both cellular Rb and p53.