P53 Gene Family P51(p63)-encoded, Secondary Transactivator P51B(TAp63alpha) Occurs Without Forming an Immunoprecipitable Complex with MDM2, but Responds to Genotoxic Stress by Accumulation

p51(p63), a member of the p53 tumor suppressor gene family, generates multiple isoforms, including the potent and less potent transactivators p51A(TAp63gamma) and p51B(TAp63alpha), respectively, the latter poorly characterized for its protein features and functions. When constitutively expressed in 1-2-3 mouse erythroleukemic cells, p51B(TAp63alpha) appeared as a broad band with an approximate molecular mass of 85 kDa in Western blot. When cells were exposed to genotoxic stress by UV-C irradiation or by DNA-damaging drugs, including actinomycin D, bleomycin, and eptoposide, the protein accumulated intracellularly without an increase in its mRNA. Unlike p53 and p51A(TAp63gamma), however, p51B(TAp63alpha) did not activate p21(waf1) gene expression, nor did it induce apoptosis or hemoglobin production. While wild-type p53 was precipitated by an anti-MDM2 antibody, p51B(TAp63alpha) was not detectable in the MDM2 immunoprecipitates from the producer cells. After treatment with okadaic acid, a Ser/Thr phosphatase inhibitor, p51B(TAp63alpha) increased its apparent molecular mass and protein content. A 26S proteasome inhibitor, MG132 (N-CBZ-Leu-Leu-leu-al), also increased p51B(TAp63alpha) retention in an either transient or constitutive expression system. Without an interaction with MDM2, p51B(TAp63alpha) may be degraded by proteasome under normal cellular circumstances but stabilized under genotoxic stress by a posttranscriptional mechanism which might involve Ser/Thr phosphorylation.