The Mcm2-7 Replicative Helicase: A Promising Chemotherapeutic Target
Figure 1
Mcm2-7 is a key regulatory component of cell cycle progression. (a) Homology model of the human Mcm2-7 complex. No high resolution structure yet exists for the eukaryotic Mcm2-7 complex. However, the archaea have homohexameric Mcm helicases, and a crystal structure of the S. solfataricus Mcm complex has been solved [58]. To generate a homology model, the human Mcm protein sequences were uploaded into the Phyre 2 server (http://www.sbg.bio.ic.ac.uk/phyre2/) that assigns secondary structure based upon alignment against homologous proteins with solved structures [59]. The resulting Mcm structure predictions were then threaded into an existing hexameric archaeal Mcm structure (PDB ID 2VL6) using PYMOL (http://www.pymol.org) and the previously determined arrangement of adjoining Mcm subunits [18, 19]. As shown, the Mcm2-7 complex generates a toroidal structure resembling the SV-40 large T antigen, a related AAA+ helicase [60]. (b) The Mcm complex is functionally asymmetric. Numerous lines of biochemical and structural evidence demonstrate that the six active sites formed by the six subunits in trans are functionally distinct (reviewed in [11]). The Mcm2/5 site has low ATP turnover, suggesting it is regulatory in nature and forms a reversible discontinuity that must be closed in order to activate helicase activity. (c) Mcm2-7 is the key component of S-phase activation (reviewed in [11, 31, 61]). In early G1 phase, Mcm hexamers are recruited to the origin recognition complex (ORC), and bound to origins of replication by the loading factors Cdc6 and Cdt1. The Mcm toroid is bound around dsDNA [35, 62], presumably requiring the ring to be opened at the Mcm 2/5 active site [38]. Along with ORC and Cdc6, head-to-head Mcm2-7 dimers remain in a biochemically inactive state as part of the prereplication complex until their irreversible activation by the regulatory kinases DDK (Dbf4 dependent kinase) and CDK (cyclin dependent kinase). CDC45 and GINS are targeted to the Mcm2-7 complex by the activity of additional recruitment factors such as Sld2, Sld3, and Dbp11, and the Mcm complex shifts from dsDNA bound state to a ssDNA bound state. DNA unwinding commences to provide a ssDNA template for the rest of the DNA replication machinery. Concurrently, Cdc6 and Cdt1 are removed from the nucleus to prevent reloading of the helicase and deleterious rereplication of the genome.
