c-Abl stabilizes p73 by a phosphorylation-augmented interaction.

The proapoptotic function of c-Abl is in part mediated by its functional interaction with p73, a p53 homologue. Although it has been shown that c-Abl-mediated p73 activation in response to genotoxic stress is associated with an increase of p73 protein levels, the underlying mechanism remains unclear. We show here that c-Abl increases the cellular p73 abundance through a mode of posttranslational regulation. Analogous to its functional activation of p73, the kinase activity is essential for c-Abl to up-regulate p73 protein levels. Analysis of phosphorylation-resistant mutants of p73 reveals that the effect of c-Abl is mediated by its direct phosphorylation on the p73 protein. Consequence to the phosphorylation is a marked increase of the association between c-Abl and p73 via the binding of tyrosine-phosphorylated p73 to the c-Abl Src homology 2 (SH2) domain. Of functional importance of this phosphorylation-induced interaction in p73 stabilization is the demonstration that expression of a c-Abl SH2 domain peptide, which impedes phosphorylation-dependent association, results in an almost complete abrogation of c-Abl-dependent p73 accumulation. Importantly, expression of the c-Abl SH2 domain peptide also leads to an efficient inhibition of cisplatin-induced accumulation of endogenous p73, highlighting the biological significance. In keeping with its retained phosphorylation sites, the NH(2)-terminal truncated (Delta N) isoforms of p73, which are antiapoptotic, are also phosphorylated and stabilized by c-Abl, suggesting a possibility that c-Abl contributes to either pro- or antiapoptotic process depending on the expression profile of p73 isoforms.