Association with Cdc2 and inhibition of Cdc2/cyclin B1 kinase activity by the p53-regulated protein Gadd45

Recently Gadd45, a p53-regulated stress protein, has been implicated in the activation of a G2/M checkpoint after damage by UV radiation and alkylatin g agents, While inhibitory phosphorylation of Cdc2 and suppression of cycli n B1 levels are known to be involved in GZ delays after genotoxic stress, G add45 has non been found to directly inhibit the activity of Cdc2/Cyclin B1 complex, while it had no appreciable effect on Cdk2/ Cyclin E activity eve n at very high levels of Gadd45, In contrast, p21(Cip1/Waf1) is an universa l cdk/cyclin inhibitor and inhibited both of the cyclin complexes tested he re, Gadd45 was also able to physically interact with Cdc2, but not Cyclin B 1, Addition of Gadd45 to immunoprecipitated Cdc2/Cyclin B1 in vitro led to a dissociation of this complex, and thus may represent a new checkpoint mec hanism whereby Cdc2/Cyclin B1 can be inhibited, With the use of an antisens e approach, reduced Gadd45 expression attenuated the suppression of Cdc2/Cy clin B1 activity in UV-irradiated human cells. Taken together, these result s implicate Gadd45 in the control of G2/M cell cycle progression after cert ain stresses.