Type I PIPkinases interact with and are regulated by the retinoblastoma susceptibility gene product-pRB.

Inositide signaling at the plasma membrane has been implicated in the regulation of numerous cellular processes including cytoskeletal dynamics, vesicle trafficking, and gene transcription. Studies have also shown that a distinct inositide pathway exists in nuclei, where it may regulate nuclear processes such as mRNA export, cell cycle progression, gene transcription, and DNA repair. We previously demonstrated that nuclear PtdIns(4,5)P(2) synthesis is stimulated during progression from G1 through S phase, although mechanistic details of how cell cycle progression impinges on the regulation of nuclear inositides is unknown. In this study, we demonstrate that pRB, which regulates progression of cells from G1 through S phase interacts both in vitro and in vivo with Type I PIPkinases, the enzymes responsible for nuclear PtdIns(4,5)P(2) synthesis. Moreover, this interaction stimulates the activity of Type Ialpha PIPkinase in an in vitro assay. Using murine erythroleukamia (MEL) cells expressing a temperature-sensitive mutant of large T antigen (LTA), we demonstrate changes in vivo in nuclear PtdIns(4,5)P(2) levels that are consistent with the ability of LTA to disrupt pRB/Type I interactions. This study, for the first time, provides a potential mechanism for how cell cycle progression could regulate the levels of nuclear inositides.