The Eukaryotic Replication Machine

The cellular replicating machine, or "replisome," is composed of numerous different proteins. The core replication proteins in all cell types include a helicase, primase, DNA polymerases, sliding clamp, clamp loader, and single-strand binding (SSB) protein. The core eukaryotic replisome proteins evolved independently from those of bacteria and thus have distinct architectures and mechanisms of action. The core replisome proteins of the eukaryote include: an 11-subunit CMG helicase, DNA polymerase alpha-primase, leading strand DNA polymerase epsilon, lagging strand DNA polymerase delta, PCNA clamp, RFC clamp loader, and the RPA SSB protein. There are numerous other proteins that travel with eukaryotic replication forks, some of which are known to be involved in checkpoint regulation or nucleosome handling, but most have unknown functions and no bacterial analogue. Recent studies have revealed many structural and functional insights into replisome action. Also, the first structure of a replisome from any cell type has been elucidated for a eukaryote, consisting of 20 distinct proteins, with quite unexpected results. This review summarizes the current state of knowledge of the eukaryotic core replisome proteins, their structure, individual functions, and how they are organized at the replication fork as a machine.