The retinoblastoma family proteins bind to and activate diacylglycerol kinase β

The retinoblastoma protein (pRB) is a tumor suppressor and key regulator of the cell cycle. We have previously shown that pRB interacts with phosphatidylinositol-4-phosphate 5-kinases, lipid kinases that can regulate phosphatidylinositol 4,5-bisphosphate levels in the nucleus. Here, we investigated pRB binding to another lipid kinase in the phosphoinositide cycle, diacylglycerol kinase (DGK) that phosphorylates the second messenger diacylglycerol to yield phosphatidic acid. We found that DGK xi, but not DGK alpha or DGK beta, interacts with pRB in vitro and in vivo. Binding of DGK xi to pRB is dependent on the phosphorylation status of pRB, since only hypophosphorylated pRB interacts with DGK xi. DGK xi also binds to the pRB-related pocket proteins p107 and p130 in vitro and in cells. Although DGK xi did not affect the ability of pRB to regulate E2F-mediated transcription, we found that pRB, p107, and p130 potently stimulate DGK xi activity in vitro. Finally, overexpression of DGK xi in pRB-null fibroblasts reconstitutes a cell cycle arrest induced by gamma-irradiation. These results suggest that DGK xi may act in vivo as a downstream effector of pRB to regulate nuclear levels of diacylglycerol and phosphatidic acid.