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 G2/M arrest was previously shown to be nuclear-limited. Cells lacking ATR-like Mecl exhibit no DSBinduced cell cycle delay; however, cells lacking Mecl's downstream protein kinase targets, Rad53 or Chki, still have substantial G2/M delay, as do cells lacking securin, Pds1. This delay is eliminated only in the triple mutant chk1A rad53 Delta pds1 Delta, 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 rad53A and chk1A 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 semiclominant cdc5-ad mutation. We suggest that Mecl-dependent checkpoint signaling through Rad53 and Chk1 includes the repression of nuclear movements that are normally associated with the execution of anaphase.