Keeping It Together and Taking It Apart: Structural Investigations of Replisome Complexes Involved in DNA Replication Fork Protection and Termination

Abstract

DNA replication is an essential cellular process whose dysregulation is implicated in severe human disease, including cancer. Broadly, DNA replication involves the unwinding of the DNA double-helix, allowing DNA polymerases to use each unwound strand as a template for nascent DNA synthesis; the complex molecular machinery responsible for DNA replication is known as the replisome. However, during unwinding and DNA synthesis, the replisome may encounter various obstacles such as DNA damage and tightly-bound proteins, necessitating specific pathways tailored to tolerating and overcoming such hinderances. Upon completion of DNA replication, the replisome is disassembled in a regulated manner.

An understanding of DNA replication requires structural knowledge of how the various replisome components assemble to form the replicative machinery. Although significant advances have been made in recent years, facilitated by the development of electron cryo-microscopy (cryo-EM), our understanding of replisome structure remains in its infancy.

Here I present high-resolution cryo-EM structures of the most complete replisome complexes to date, which are involved in multiple aspects of DNA replication and replication-coupled processes. First, I describe how four replisome components – the fork protection complex (Csm3-Tof1-Mrc1) plus Ctf4 – associate with the replicative helicase CMG. The fork protection complex is involved in achieving maximal rates of DNA replication, as well as performing roles in protecting replication forks from varied forms of replication stress. Ctf4 acts as a structural hub recruiting factors required for cohesion establishment and epigenetic inheritance. Second, I discuss structural insights into the regulation of replisome disassembly as the final stage of DNA replication, presenting the first structure of a replisome complex which has translocated onto double-stranded DNA and is bound by the termination-specific E3 ubiquitin ligase, SCFDia2.