Cyclin-dependent kinase complex

A cyclin-dependent kinase complex (CDKC, cyclin-CDK) is a protein complex formed by the association of an inactive catalytic subunit of a protein kinase, cyclin-dependent kinase (CDK), with a regulatory subunit, cyclin. Once cyclin-dependent kinases bind to cyclin, the formed complex is in an activated state. Substrate specificity of the activated complex is mainly established by the associated cyclin within the complex. Activity of CDKCs is controlled by phosphorylation of target proteins, as well as binding of inhibitory proteins. The structure of CDKs in complex with a cyclin subunits (CDKC) has long been a goal of structural and cellular biologists starting in the 1990s when the structure of unbound cyclin A was solved by Brown et al. and in the same year Jeffery et al. solved the structure of human cyclin A-CDK2 complex to 2.3 Angstrom resolution. Since this time, many CDK structures have been determined to higher resolution, including the structures of CDK2 and CDK2 bound to a variety of substrates, as seen in Figure 1. High resolution structures exist for approximately 25 CDK-cyclin complexes in total within the Protein Data Bank. Based on function, there are two general populations of CDK-cyclin complex structures, open and closed form. The difference between the forms lies within the binding of cyclin partners where closed form complexes have CDK-cyclin binding at both the C and N-termini of the activation loop of the CDK, whereas the open form partners bind only at the N-terminus. Open form structures correspond most often to those complexes involved in transcriptional regulation (CDK 8, 9, 12, and 13), while closed form CDK-cyclin complex are most often involved in cell cycle progression and regulation (CDK 1, 2, 6). These distinct roles, however, do not significantly differ with the sequence homology between the CDK components. In particular, among these known structures there appear to be four major conserved regions: a N-terminal Glycine-rich loop, a Hinge Region, an αC-helix, and a T-loop regulation site.