Chk2 kinase regulation by Nbs1 and search for Chk2 interacting proteins

The mammalian Chk2 protien kinase has a key role in the activation of DNA damaged induced cell cycle checkpoints. After low doses of IR Chk2, activated in an ataxia telangiectasia mutataed (ATM)- dependent manner, can phosphorylate several substartes causing arrest in G1, S and G2-M. Recently we found that the activation of Chk2 by gamma-radiation requires, besides ATM, also Nbs1, the gene product invovled in the Nijmegen Breakage Syndrome (NBS), a disorder that shares with AT a variety of phenotypic defects. In NBS cells, the defective Chk2 phosphoryaltion and kinase activation can be complemented by reintroduction of wild type Nbs1, but neither by a C-terminal deletion mutant of Nbs1 at aa 59, that cannot form a complex with Mre11 and Rad50, nor by a Nbs1 mutated at Ser343, the ATM phosphorylation site. Because in NBS cells Chk2 maintains its nuclear localization, we can exclude a mislocalization as a the cause of its failed activation. Importantly, the failure of NBS cells to stop entry into mitosis after IR be corrected by the reintroduction of a wild type Nbs1, but not by a Ser343 mutant, suggesting that this checkpoint defect may result from the inability to activate Chk2.To find out Chk2 interacting proteins we have undertaken yeast two hybrid screens using the full lenght Chk2 as a bait and two different cDNA libraries. About 250 clones were analyzed, with 5 clones strongly interacting and 12 weakly interacting with Chk2. Some of these encode for cytoplasmic proteins while others for nuclear proteins involved on checkpoint controls, nucleotide metabolism, transcription, translation and degradation. Studies are in progress to determine the in vivo interaction of these cDNA products with Chk2.