A NOVEL LINK BETWEEN REC2, A DNA RECOMBINASE, THE RETINOBLASTOMA PROTEIN, AND APOPTOSIS

The REC2 recombinase is essential for recombinational repair following DNA damage as well as for successful meiosis and gene targeting in th e corn smut Ustilago maydis, Here we report that overexpression of REC 2 induced apoptotic cell death in human HuH-7, Hep G2, and Hep 3B hepa toma cells. Apoptosis was related to recombinase activity and was sign ificantly increased by inhibition of retinoblastoma (Rb) expression wi th transforming growth factor-beta 1. REC2-induced apoptosis was assoc iated with a significantly reduced percentage of cells in the G(1) pha se of the cell cycle and a significant reduction in G(2)/M only in the Rb((-)/(-)) Hep 3B cells. Overexpression of REC2 resulted in increase d abundance of the hyperphosphorylated form of Rb. However, by immunop recipitation REC2 was associated primarily with hypophosphorylated Rb, suggesting that REC2 may be involved in modulating the phosphorylatio n state of Rb. The A and B pocket domains with the spacer amino acid s equence and the carboxyl-terminal region of Rb were required for maxim al binding to REC2. Overexpression of Rb significantly inhibited REC2- induced apoptosis even in the presence of transforming growth factor-b eta 1. Taken together, these data suggest a novel interaction of Rb wi th the recombinase REC2 and a role for this complex in bridging DNA re combination and apoptosis.