Co-evolution of functional motifs and H2A.X in the context of DNA damage response identifies the plant Mediator of DNA Damage Checkpoint 1

Abstract

Induction of DNA damage triggers rapid phosphorylation of the histone H2A.X ({gamma}H2A.X). In animals, mediator of DNA damage checkpoint 1 (MDC1) binds {gamma}H2A.X through a tandem BRCA1 carboxyl-terminal (tBRCT) domain and mediates recruitment of downstream effectors of DNA damage response (DDR). However, readers of this modification in plants have remained elusive. We report that from Arabidopsis BRCT domain proteome, BCP1-4 proteins with tBRCT domains are involved in DDR. Through its tBRCT domain BCP4 binds {gamma}H2A.X in vitro and localizes to DNA damage-induced foci in an H2A.X dependent manner. BCP4 also contains a domain that interacts directly with NBS1 and thus acts as a functional counterpart of MDC1. We also show that BCP1, that contains two tBRCT domains, co-localizes with {gamma}H2A.X but it does not bind {gamma}H2A.X suggesting functional similarity with human PAXIP1. A phylogenetic analysis supports that PAXIP1 and MDC1 in metazoa and their plant counterparts evolved independently from common ancestors with tBRCT domains. Collectively, our study reveals missing components and provides mechanistic and evolutionary insights into plant DDR.