How a phosphorylation circuit promotes the DNA Damage Checkpoint

What is it about?

DNA damaging events can alter the genetic information encoded by DNA; however life has evolved a defence system to detect, signal, and repair DNA damage. DNA double strand breaks, a lethal form of DNA damage, activates a cell cycle checkpoint that halts cell division to allow time for repair. At the centre of this DNA damage checkpoint is a giant protein kinase called Mec1 (known as Ataxia-Telangiectasia Mutated and Rad3 related, in humans), that signals DNA damage and arrests the cell cycle through phosphorylating key proteins in DNA repair and cell cycle control. Here we have uncovered specific phosphorylation events that drive the recruitment of Mec1 to DNA damage sites that are important for its checkpoint function.

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Photo by Ankush Minda on Unsplash

Photo by Ankush Minda on Unsplash

Why is it important?

How Mec1 (and its human ortholog ATR) is efficiently recruited to the damaged site is not well understood. In this study, we show that phosphorylation events regulate the efficient recruitment by promoting clustering of Mec1. Mec1 is recruited to the damage site via interactions between its integral binding partner Ddc2 and Replication Protein A (RPA), a protein that coats single stranded DNA (ssDNA), which is prevalent during replication stress or during DNA double strand break repair. We show that phosphorylation of RPA and Ddc2 strengthen Mec1 recruitment to RPA on ssDNA, which is important for the DNA damage checkpoint. Importantly, our study uncovers higher-order clustering of Mec1-Ddc2 complexes bound to Replication Protein A upon phosphorylation, which is aided by zinc ions. Our findings suggest a rapid dynamic assembly and disassembly process regulated by post translational modifications and provide new insight into the mechanism of DNA damage signalling.