Protein kinase C-δ phosphorylates Ebp1 and prevents its proteolytic degradation, enhancing cell survival

ErbB3-binding protein (Ebp1) promotes cell survival by preventing apoptotic DNA fragmentation through a complex with active nuclear Akt. Ebp1 phosphorylation by protein kinase C (PKC)-δ mediates its binding to nuclear Akt. In this study, we show that Ebp1 itself acts as a substrate of active caspase 3 during the programmed cell death. PKC-δ phosphorylation on Ebp1 protects it from apoptotic degradation initiated in cell-free apoptotic solution. Moreover, Ebp1 is evidently cleaved in PKC-δ-deficient cells but not in wild-type cells. Ebp1 translated from first ATG is resistant to apoptotic cleavage; by contrast, Ebp1 from second and third ATG demonstrates robust degradation, and PKC phosphorylation on S360 suppresses its cleavage by active caspase 3. Ebp1 can be digested at both D53 and D196 sites, but cleavage at D196 appears to be a prerequisite for its further degradation at D53 site. Compared with wild-type Ebp1, D196A mutant markedly protects cells from apoptosis. Thus, PKC-δ antagonizes apoptosis through phosphorylating Ebp1 and protects it from apoptotic degradation.