PHOSPHORYLATION OF THE ANTIONCOGENE PRODUCTS AND CONTROL OF THE CELL-CYCLE

To investigate the function of the RB protein, we have studied cellula r RB binding proteins and protein kinases responsible for phosphorylat ion of the RB protein. (1) We purified a cellular RB-associated protei n p56 which competes with SV40 large T antigen for binding to the RB p rotein. (2) ln another experiment, we screened expression libraries of U937 monocytic leukemia cell line by West-Western method and obtained two cDNA clones that encode RB binding proteins. (3) The RB protein w as found to be phosphorylated by cdk2 and MAP kinase in vitro. Most of the sites phosphorylated in vitro are the same as those phosphorylate d in vivo and the time course of activation of cdk2 in the cell cycle were similar to that of phosphorylation of the RB protein.