Mdt1, a Novel Rad53 FHA1 Domain-Interacting Protein, Modulates DNA Damage Tolerance and G₂/M Cell Cycle Progression in Saccharomycescerevisiae

Abstract

The Rad53 kinase plays a central role in yeast DNA damage checkpoints. Rad53 contains two FHA phosphothreonine-binding domains that are required for Rad53 activation and possibly downstream signaling. Here we show that the N-terminal Rad53 FHA1 domain interacts with the RNA recognition motif, coiled-coil, and SQ/TQ cluster domain-containing protein Mdt1 (YBl051C). The interaction of Rad53 and Mdt1 depends on the structural integrity of the FHA1 phosphothreonine-binding site as well as threonine-305 of Mdt1. Mdt1 is constitutively threonine phosphorylated and hyperphosphorylated in response to DNA damage in vivo. DNA damage-dependent Mdt1 hyperphosphorylation depends on the Mec1 and Tel1 checkpoint kinases, and Mec1 can directly phosphorylate a recombinant Mdt1 SQ/TQ domain fragment. MDT1 overexpression is synthetically lethal with a rad53 deletion, whereas mdt1 deletion partially suppresses the DNA damage hypersensitivity of checkpoint-compromised strains and generally improves DNA damage tolerance. In the absence of DNA damage, mdt1 deletion leads to delayed anaphase completion, with an elongated cell morphology reminiscent of that of G₂/M cell cycle mutants. mdt1-dependent and DNA damage-dependent cell cycle delays are not additive, suggesting that they act in the same pathway. The data indicate that Mdt1 is involved in normal G₂/M cell cycle progression and is a novel target of checkpoint-dependent cell cycle arrest pathways.

We thank Nora Tenis for help with recombinant protein expression; Catherine Dale for help in generating some of the yeast strains; Tony Tiganis for help with FACS analyses; Lena Burri for help with tetrad dissections; Rodney Rothstein, Stephen Jackson, Matthew O'Connell, and Andy Poumbourios for strains and antibodies; and members of our laboratories, Steve Dalton, and Anton Gartner for discussions and comments on the manuscript.

This work was supported by grants from the National Health and Medical Research Council of Australia (NHMRC) and the Cancer Council of Victoria (to J.H.), National Institutes of Health grant CA87031 (to M.-D.T.), an Australian postgraduate award (to B.L.P.), and an NHMRC senior research fellowship (to J.H.).