Investigating sliding clamps and their contribution to genome stability

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Project Summary All cells must replicate their genome once per cell cycle. To ensure proper duplication, cells integrate hundreds of factors that copy, surveil, and repair our genetic information. Proliferating Cell Nuclear Antigen [PCNA] and Rad9-Rad1-Hus1 [9-1-1] are ring-shaped clamps that act as master “conductors” that regulate many of the factors that replicate and maintain our DNA. PCNA is a homotrimeric ring that coordinates the replisome during DNA synthesis to work in tandem with DNA repair, chromatin remodeling, and cell cycle progression. When cells experience dsDNA breaks, they use the heterotrimeric clamp 9-1-1 to coordinate specific “SOS” repair factors. The collaborative efforts of both clamps are critical for genome stability. Many cancers are linked to inappropriate clamp coordination and changes in their expression. Because sliding clamps are central to many oncogenic pathways, we must address how they regulate themselves and their client partners. This proposal aims to address the following questions about sliding clamps: 1) How do sliding clamps coordinate their various partners? 2) Does the time sliding clamps spend on DNA influence genome stability? and 3) What determines site-specific loading of sliding clamps? I propose a multidisciplinary approach to address these questions about sliding clamps by investigating two-disease causing PCNA variants [PCNA-S228I [serine to isoleucine] and PCNA-C148S [cysteine to serine]] and the loading mechanism of 9-1-1. I hypothesize that sliding clamps control genome integrity via site-specific loading, proper partner interactions, and residence-time on DNA. I further hypothesize that PCNA-S228I and PCNA-C148S disrupt genome integrity by either promoting premature DNA dissociation or disrupting partner interactions. Finally, I hypothesize that the Rad17 subunit alters the clamp loader structure to specifically load the 9-1-1 clamp at sites of DNA damage. In aims 1 and 2, I will use PCNA-S228I and PCNA-C148S to address how clamps “choose” their partners and regulate their time on DNA. I will use x-ray crystallography, unfolding experiments, and a series of functional assays to determine how each variant compromises genome stability. In aim 3, I will determine the loading mechanism of clamp 9-1-1 to address how clamps are loaded to specific sites in the genome. I will use cryo-electron microscopy to determine how Rad17-RFC binds to clamp 9-1-1. Collectively, my work will broaden our insight into the factors that cause genome instability which may augment the development of personalized chemotherapeutics.