HIGH-MOLECULAR-WEIGHT PROTEIN PHOSPHATASE TYPE-1 DEPHOSPHORYLATES THERETINOBLASTOMA PROTEIN

pRb controls cell proliferation by restricting inappropriate entry of cells into the cell division cycle, As dephosphorylation of pRb during mitotic exit activates its growth suppressive function, identificatio n of the protein phosphatase that dephosphorylates pRb, and characteri zation of the mechanism of its regulation, are essential to elucidatin g the mechanisms of cell growth control. By fractionating mitotic CV-1 P cell extracts, we identify the protein phosphatase which dephosphory lates pRb as a type 1 serine/threonine phosphoprotein phosphatase (PP1 ). Molecular sizing analyses indicate that the catalytic enzyme (PP1c) is present in a high molecular weight complex, with a predicted molec ular mass of 166 kDa. PP1-interacting proteins in the mitotic cell ext racts are identified. Two PP1-interacting proteins (41 and 110 kDa) ar e shown to form distinct complexes with PP1c from fractions of separat ed mitotic cell extracts containing phosphorylase phosphatase activity . However, only the 110-kDa PP1-interacting protein is present in frac tions containing pRb directed phosphatase activity, identifying this p rotein as a putative activator of PP1 function toward pRb during mitos is.