CTF18-RFC structure reveals how it binds PCNA

What the study found: The human CTF18–RFC clamp loader has distinctive structural features, including a flexible regulatory module and an autoinhibited RFC module bound to PCNA. The study also identifies a novel β-hairpin in the large subunit that helps connect the complex to PCNA and the RFC5 subunit.
Why the authors say this matters: The authors conclude that these structural features provide insight into how CTF18–RFC loads PCNA and how it may stimulate leading-strand synthesis by Pol ε, a leading-strand DNA polymerase.
What the researchers tested: The researchers used cryo-electron microscopy (cryo-EM) to characterize the human CTF18–RFC complex and its interaction with PCNA. They examined the structure of the complex and tested the effect of deleting a β-hairpin on complex stability, clamp loading, and primer synthesis.
What worked and what didn't: Cryo-EM data supported that the Ctf8 and Dcc1 subunits are flexibly tethered to the RFC module. A 2.9 Å cryo-EM structure showed the RFC module bound to PCNA in an autoinhibited conformation similar to canonical RFC, and deletion of the β-hairpin impaired complex stability, slowed clamp loading, and decreased the rate of primer synthesis by Pol ε.
What to keep in mind: The abstract does not describe broader biological limits beyond the structural and functional observations reported here. It also does not provide details on experiments outside the CTF18–RFC/PCNA system.

Key points

• CTF18–RFC was found to have a flexible regulatory module made of Ctf8 and Dcc1.
• A 2.9 Å cryo-EM structure showed the RFC module bound to PCNA in an autoinhibited state.
• The large subunit of CTF18–RFC uses an atypical low-affinity PIP box and a novel β-hairpin to engage PCNA and RFC5.
• Deleting the β-hairpin weakened the CTF18–RFC–PCNA complex and slowed clamp loading.
• β-hairpin deletion also reduced the rate of primer synthesis by Pol ε.