GADD45 IS A NUCLEAR-CELL CYCLE-REGULATED PROTEIN WHICH INTERACTS WITHP21(CIP1)

GADD45 was originally identified as a cDNA clone induced by growth arr est and DNA damage. We show that Gadd45 is a nuclear protein, widely e xpressed in normal tissues, particularly in quiescent cellular populat ions. Using cell synchronisation methods we show that Gadd45 levels ar e highest in the G1 phase of the cell cycle, and are greatly reduced d uring S phase. Immunoprecipitation of Gadd45 from mammalian cells reve als that it is tightly associated with a protein which reacts with ant ibodies to the cyclin dependent kinase inhibitor p21(Cip1). Binding of recombinant Gadd45 protein to overlapping p21(Cip1) peptides in ELISA assays and use of the yeast two hybrid assay show that Gadd45 directl y interacts with this cell cycle inhibitor. These data suggest that Ga dd45 may act in the regulation of the cell cycle. It is postulated tha t the interactions of Gadd45 with both p21(Cip1) and PCNA are importan t for the modulation of cell cycles, and for the inhibition of DNA rep lication.