Trans-activation of the DNA-damage signalling protein kinase Chk2 by T-loop exchange

Abstract

The protein kinase Chk2 (checkpoint kinase 2) is a major effector of the replication checkpoint. Chk2 activation is initiated by phosphorylation of Thr68, in the serine–glutamine/threonine–glutamine cluster domain (SCD), by ATM. The phosphorylated SCD‐segment binds to the FHA domain of a second Chk2 molecule, promoting dimerisation of the protein and triggering phosphorylation of the activation segment/T‐loop in the kinase domain. We have now determined the structure of the kinase domain of human Chk2 in complexes with ADP and a small‐molecule inhibitor debromohymenialdisine. The structure reveals a remarkable dimeric arrangement in which T‐loops are exchanged between protomers, to form an active kinase conformation in trans . Biochemical data suggest that this dimer is the biologically active state promoted by ATM‐phosphorylation, and also suggests a mechanism for dimerisation‐driven activation of Chk2 by trans ‐phosphorylation.