Joshua Heyza, PhD

Assistant Professor, Institute of Environmental Health Sciences and the Department of Pharmacology, Wayne State University

“The PST Repeat Region of MDC1 is a Tunable Multivalent Chromatin Tethering Domain”

Abstract

The Mediator of DNA damage checkpoint1 (MDC1) is an adaptor protein that binds to γH2AX at DNA double-strand breaks (DSB). In interphase MDC1 initiates a chromatin ubiquitin signaling cascade by the E3 ubiquitin ligases RNF8 and RNF168 leading to recruitment of effectors that promote DSB repair by non-homologous end joining and homologous recombination. In mitosis where RNF8 and RNF168-mediated ubiquitination are suppressed, MDC1 forms a complex with TOPBP1 and CIP2A to tether DNA ends together preserving them for repair in G1 phase. In addition to several well-described domains, MDC1 contains a poorly characterized intrinsically-disordered region consisting of 13 repeats of 41 amino acids known as the PST repeat region which has unclear functions in DNA repair. Here we demonstrate that the PST repeat region of MDC1 is a multivalent nucleosome binding domain, sufficient to tether chromatin in multiple contexts. In mitotic cells the affinity of the PST repeats for chromatin is downregulated by phosphorylation to prevent chromosome missegregation, while still contributing to DNA break tethering by the MDC1-TOPBP1-CIP2A complex. In interphase, the PST repeat region is critical for RAD51 focus formation but not 53BP1 focus formation, consistent with a chromatin tethering function. In total, this work demonstrates that the PST repeat region is a multivalent chromatin binding domain with a tunable affinity that may contribute to DNA break tethering during HR and in mitosis.