Structure of the human CTF18 clamp loader bound to PCNA

Sliding clamps like PCNA are crucial processivity factors for replicative polymerases, requiring specific clamp loaders for loading onto DNA. The clamp loader CTF18 interacts with the leading strand polymerase Pol ϵ and loads PCNA onto primer/template DNA using its RFC pentameric module. Here, we provide a structural characterization of the human CTF18 complex and its interaction with PCNA. Our cryo-EM data support that the Ctf8 and Dcc1 subunits of CTF18, which form the regulatory module interacting with Pol ϵ, are flexibly tethered to the RFC module. A 2.9 Å cryo-EM structure shows the RFC module bound to PCNA in an auto-inhibited conformation similar to the canonical RFC loader, marking the initial step of the clamp-loading reaction. The unique RFC1 large subunit of CTF18, which shows high relative mobility, is anchored to PCNA through an atypical low-affinity PIP box in the AAA+ domain and engages the RFC5 subunit using a novel β-hairpin at the disordered N-terminus. These dual interactions may allow CTF18 RFC1 to outcompete the canonical RFC1 subunit while recycling the RFC2-3-4-5 subunits. Our results suggest that CTF18 comprises two structurally independent modules that cooperate in loading PCNA onto the leading strand for replication by Pol ϵ.