# Structural mechanisms of sliding clamp loader ATPases

> **NIH NIH R01** · UNIV OF MASSACHUSETTS MED SCH WORCESTER · 2021 · $351,750

## Abstract

All life forms require a ring-shaped sliding clamp to coordinate replication of their genome. These 
sliding clamps act as master regulators of DNA replication, coordinating replisome action with 
other cellular processes. These master regulators are themselves regulated by large ATPase machines 
called clamp loaders that either install or remove sliding clamps from DNA. This project seeks to 
gain an atomic-level understanding of clamp loader mechanism. These protein remodeling machines 
open the sliding clamp ring as a key step in their action. We have found that in the key 
intermediate complex—consisting of an open clamp, an ATP-bound clamp loader and the target DNA—the 
protein components form an open spiral that matches the helical symmetry of DNA. This symmetric 
spiral activates ATP hydrolysis leading to clamp closure and release of the loaded clamp. In Aim 1, 
we now turn our attention to the critical first two steps of the reaction: the opening of the clamp 
and the binding of DNA to the inner chamber of the complex. We will identify the conformational 
changes in the clamp loader complex that allow for opening the clamp ring, as well as how the 
assembly can rapidly bind a specific DNA structure in the tight confines of the complex's interior. 
In Aim 2, we investigate how the single subunit change in the clamp loader complex (Rfc1 replaced 
with Elg1) converts a dedicated clamp loader into a dedicated unloader. This work will not only 
reveal the mechanism and structure of a key protein involved in cancer development, but will also 
provide a blueprint for how an ATPase machine can be reprogrammed to perform a reverse reaction.
Finally, in Aim 3 we explore how replacement of the Rfc1 subunit with the Ctf18 protein leads to an 
assembly that is bifunctional as both a loader and unloader, and that connects DNA replication to 
the process of sister chromatid cohesion. Our structures and analysis of this complex will reveal 
how an ATPase machine can be mechanistically flexible to catalyze both forward and reverse 
reactions. In addition, this work will provide insight into how this mysterious complex can link 
the seemingly disparate processes of DNA replication and sister chromatid cohesion. Because clamp 
loaders and sliding clamps are fundamental to all life, the structural insights that we obtain from 
completing our aims will be used for developing novel antimicrobial or chemotherapeutic drugs.

## Key facts

- **NIH application ID:** 10092193
- **Project number:** 5R01GM127776-03
- **Recipient organization:** UNIV OF MASSACHUSETTS MED SCH WORCESTER
- **Principal Investigator:** Brian Anthony Kelch
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2021
- **Award amount:** $351,750
- **Award type:** 5
- **Project period:** 2019-02-01 → 2024-01-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10092193

## Citation

> US National Institutes of Health, RePORTER application 10092193, Structural mechanisms of sliding clamp loader ATPases (5R01GM127776-03). Retrieved via AI Analytics 2026-05-22 from https://api.ai-analytics.org/grant/nih/10092193. Licensed CC0.

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