Calmodulin serves an essential role in eukaryotic cells as a Ca2+ sensor protein. Ca2+ binding leads to conformation changes in calmodulin that enable engagement of a repertoire of enzymes and the regulation of their catalytic activities. Classically, Ca2+-Calmodulin binds to a linear sequence C-terminal to the kinase domain in members of the Ca2+-Calmodulin-dependent protein kinase (CAMK) family and restores activity by sequestering an auto-inhibitory domain that formerly obstructed the active site.
For some time, researchers in the field have suspected Ca2+-sensor proteins may functionally interact to decode Ca2+-signals in unrealised ways. Supporting this idea, we have discovered a new mode of recognition meditated by the cell-cycle regulator Checkpoint kinase-2 (Chk2). The Chk2 kinase functions as a regulator of the cell cycle, principally to arrest cells at the mitotic boundary in response to DNA damage. Given Ca2+ levels systemically fluctuate throughout the cell cycle and Chk2 is a CAMK family member, we investigated whether Ca2+-Calmodulin can bind and modulate the affinity of Chk2.
Using an integrated structural, biochemical and proteomic approach we found Ca2+-Calmodulin bound lysine residues exclusively within the kinase domain and allosterically inhibited Chk2 kinase activity. We validated our findings in vivo by mutating the conserved binding residues in Cds1, the Chk2 homolog in fission yeast, where under induced genotoxic stress the mutant cells were unable to grow. Our findings are significant, as despite decades of research on the cell cycle, we have discovered an effector protein that directly links Ca2+ signalling with the cell cycle. Further, our discovery challenges the dogma of Calmodulin-binding mechanisms, as it proposes that Ca2+-Calmodulin can bind a 3D surface, rather than a linear sequence.