Dynamic methylation of Numb by Set8 regulates its binding to p53 and apoptosis.

Although Numb exhibits its tumor-suppressive function in breast cancer in part by binding to and stabilizing p53, it is unknown how the Numb-p53 interaction is regulated in cells. We found that Numb is methylated in its phosphotyrosine-binding (PTB) domain by the lysine methyltransferase Set8. Moreover, methylation uncouples Numb from p53, resulting in increased p53 ubiquitination and degradation. While Numb promotes apoptosis in a p53-dependent manner, the apoptotic function is abolished when Numb is methylated by Set8 or the Lys methylation sites in Numb are mutated. Conversely, the Numb-p53 interaction and Numb-mediated apoptosis are significantly enhanced by depletion of Set8 from cancer cells or by treating the cells with doxorubicin, a chemotherapeutic drug that causes a reduction in the mRNA and protein levels of Set8. Our work identifies the Set8-Numb-p53 signaling axis as an important regulatory pathway for apoptosis and suggests a therapeutic strategy by targeting Numb methylation.

Phosphorylation-independent regulation of metabotropic glutamate receptor signaling by G protein-coupled receptor kinase 2.

The accepted paradigm for G protein-coupled receptor kinase (GRK)-mediated desensitization of G protein-coupled receptors involves GRK-mediated receptor phosphorylation followed by the binding of arrestin proteins. Although GRKs contribute to metabotropic glutamate receptor 1 (mGluR1) inactivation, beta-arrestins do not appear to be required for mGluR1 G protein uncoupling. Therefore, we investigated whether the phosphorylation of serine and threonine residues localized within the C terminus of mGluR1a is sufficient to allow GRK2-mediated attenuation of mGluR1a signaling. We find that the truncation of the mGluR1a C-terminal tail prevents mGluR1a phosphorylation and that GRK2 does not contribute to the phosphorylation of an mGluR1 splice variant (mGluR1b). However, mGluR1a-866Delta- and mGluR1b-stimulated inositol phosphate formation is attenuated following GRK2 expression. The expression of the GRK2 C-terminal domain to block membrane translocation of endogenous GRK2 increases mGluR1a-866Delta- and mGluR1b-stimulated inositol phosphate formation, presumably by blocking membrane translocation of GRK2. In contrast, expression of the kinase-deficient GRK2-K220R mutant inhibits inositol phosphate formation by these unphosphorylated receptors. Expression of the GRK2 N-terminal domain (residues 45-185) also attenuates both constitutive and agonist-stimulated mGluR1a, mGluR1a-866Delta, and mGluR1b signaling, and the GRK2 N terminus co-precipitates with mGluR1a. Taken together, our observations indicate that attenuation of mGluR1 signaling by GRK2 is phosphorylation-independent and that the interaction of the N-terminal domain of GRK2 with mGluR1 contributes to the regulation of mGluR1 G protein coupling.