Recombinant expression and characterization of yeast Mrc1, a DNA replication checkpoint mediator

In Saccharomyces cerevisiae, Mrc1 (homolog of human Claspin and mediator of replication checkpoint) is not only a part of the replication machine, but also participates in the replication stress response when DNA replication is blocked by hydroxyurea. Since Mrc1 is expressed in a small amount in cells and has many proteins interacting with it as a mediator, it is difficult to obtain Mrc1 with high concentration and purity. This article reports the purification of a stable truncation of Mrc1 and the full length Mrc1. High concentration and high purity of Mrc1 was obtained and the three-dimensional structure of Mrc1 was analyzed, which is a ring with a hole in the center. At the same time, we found that Mrc1 has an interaction with Rad24-RFC a clamp loader in the replication checkpoint, and can form a complex with it, implying that we can assemble large replication checkpoint complexes in vitro. These results initially reveal the ring structure of Mrc1 and its interaction with Rad24-RFC in replication checkpoints in S. cerevisiae.

The DNA Pol ϵ stimulatory activity of Mrc1 is modulated by phosphorylation

DNA replication checkpoint (Mec1-Mrc1-Rad53 in budding yeast) is an evolutionarily conserved surveillance system to ensure proper DNA replication and genome stability in all eukaryotes. Compared to its well-known function as a mediator of replication checkpoint, the exact role of Mrc1 as a component of normal replication forks remains relatively unclear. In this study, we provide in vitro biochemical evidence to support that yeast Mrc1 is able to enhance the activity of DNA polymerase ϵ (Pol ϵ), the major leading strand replicase. Mrc1 can selectively bind avidly to primer/template DNA bearing a single-stranded region, but not to double-stranded DNA (dsDNA). Mutations of the lysine residues within basic patch 1 (BP1) compromise both DNA binding and polymerase stimulatory activities. Interestingly, Mrc1-3D, a mutant mimicking phosphorylation by the Hog1/MAPK kinase during the osmotic stress response, retains DNA binding but not polymerase stimulation. The stimulatory effect is also abrogated in Mrc1 purified from cells treated with hydroxyurea (HU), which elicits replication checkpoint activation. Taken together with previous findings, these results imply that under unperturbed condition, Mrc1 has a DNA synthesis stimulatory activity, which can be eliminated via Mrc1 phosphorylation in response to replication and/or osmotic stresses.