MCM proteins and checkpoint kinases get together at the fork.

Abstract

An article by Cortez et al . (1) in a recent issue of PNAS makes several important connections between minichromosome maintenance (MCM) proteins and the DNA damage response. The authors establish that two MCM subunits, MCM2 and MCM3, are substrates for ATM- and Rad3-related (ATR) and ataxia-telangiectasia-mutated (ATM) checkpoint kinases, respectively. They also identify one of the phosphorylation sites on each MCM subunit. In addition, Cortez et al . show that decreasing the level of MCM7 protein, an ATR-interacting protein (ATRIP)-interacting subunit, causes an S-phase checkpoint defect as seen by radio-resistant DNA synthesis. DNA replication starts at discrete origins marked by the origin recognition complex (ORC)-dependent assembly of a pre-replicative complex (pre-RC) (2, 3). Subsequent to ORC binding, Cdc6, Cdt1, and the MCM proteins assemble. Eukaryotes contain six related MCM proteins (MCM2–MCM7). MCM proteins are members of the AAA+ family (4) and display ATPase activity. Only the MCM467 subcomplex displays in vitro DNA helicase activity (5). MCM proteins and their associated ATPase activities are all essential for DNA replication (6) and are required for replication fork progression (7). Although it has not yet been conclusively established that MCM proteins are the replicative helicase unwinding DNA at the replication fork, they are the best candidates for this activity (8). The MCM-containing pre-RC is converted into an initiation complex after the concerted activity of S-phase kinases Cdk2/CycE and Cdc7/Dbf4 (3). Although the essential targets of Cdk2 and Cdc7 protein kinases on the pre-RC have not been identified, one consequence of their action is loading of Cdc45, an essential protein involved in DNA polymerase loading (9). Several signal transduction pathways prevent duplication of damaged DNA, thus preserving genome integrity throughout S-phase (10). These cell-cycle checkpoint pathways can be activated in a replication-dependent or independent manner. They are triggered by a …