The yeast DNA damage checkpoint proteins control a cytoplasmic response to DNA damage

A single HO endonuclease-induced double-strand break (DSB) is sufficient to activate the DNA damage checkpoint and cause Saccharomyces cells to arrest at G₂/M for 12-14 h, after which cells adapt to the presence of the DSB and resume cell cycle progression. The checkpoint signal leading to G ₂/M arrest was previously shown to be nuclear-limited. Cells lacking ATR-like Mec1 exhibit no DSB-induced cell cycle delay; however, cells lacking Mec1's downstream protein kinase targets, Rad53 or Chk1, still have substantial G₂/M delay, as do cells lacking securin, Pds1. This delay is eliminated only in the triple mutant chk1Δ rad53Δ pds1Δ, suggesting that Rad53 and Chk1 control targets other than the stability of securin in enforcing checkpoint-mediated cell cycle arrest. The G₂/M arrest in rad53Δ and chk1Δ revealed a unique cytoplasmic phenotype in which there are frequent dynein-dependent excursions of the nucleus through the bud neck, without entering anaphase. Such excursions are infrequent in wild-type arrested cells, but have been observed in cells defective in mitotic exit, including the semidominant cdc5-ad mutation. We suggest that Mec1-dependent checkpoint signaling through Rad53 and Chk1 includes the repression of nuclear movements that are normally associated with the execution of anaphase.