Gadd45 is a nuclear cell cycle regulated protein which interacts with p21Cip1.

GADD45 was originally identified as a cDNA clone induced by growth arrest and DNA damage. We show that Gadd45 is a nuclear protein, widely expressed in normal tissues, particularly in quiescent cellular populations. Using cell synchronisation methods we show that Gadd45 levels are highest in the G1 phase of the cell cycle, and are greatly reduced during S phase. Immunoprecipitation of Gadd45 from mammalian cells reveals that it is tightly associated with a protein which reacts with antibodies to the cyclin dependent kinase inhibitor p21Cip1. Binding of recombinant Gadd45 protein to overlapping p21Cip1 peptides in ELISA assays and use of the yeast two hybrid assay show that Gadd45 directly interacts with this cell cycle inhibitor. These data suggest that Gadd45 may act in the regulation of the cell cycle. It is postulated that the interactions of Gadd45 with both p21Cip1 and PCNA are important for the modulation of cell cycles, and for the inhibition of DNA replication.