Stxbp4 Regulates ΔNp63 Stability by Suppression of RACK1-Dependent Degradation

p63, a member of the p53 tumor suppressor family, is essential for the development of epidermis as well as other stratified epithelia. Collective evidence indicates that Delta Np63 proteins, the N-terminally deleted versions of p63, are essential for the proliferation and survival of stratified epithelial cells and squamous cell carcinoma cells. But in response to DNA damage, Delta Np63 proteins are quickly downregulated in part through protein degradation. To elucidate the mechanisms by which Delta Np63 proteins are maintained at relatively high levels in proliferating cells but destabilized in response to stress, we sought to identify p63 interactive proteins that regulate p63 stability. We found that Stxbp4 and RACK1, two scaffold proteins, play central roles in balancing Delta Np63 protein levels. While Stxbp4 functions to stabilize Delta Np63 proteins, RACK1 targets Delta Np63 for degradation. Under normal growth conditions, Stxbp4 is indispensable for maintaining high basal levels of Delta Np63 and preventing RACK1-mediated p63 degradation. Upon genotoxic stress, however, Stxbp4 itself is downregulated, correlating with Delta Np63 destabilization mediated in part by RACK1. Taken together, we have delineated key mechanisms that regulate Delta Np63 protein stability in vivo.