Abstract
A single HO endonuclease-induced double-strand break (DSB) is sufficient to activate the DNA damage checkpoint and cause
Saccharomyces
cells to arrest at G
2
/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 G
2
/M delay, as do cells lacking securin, Pds1. This delay is eliminated only in the triple mutant
chk1
Δ
rad53
Δ
pds1
Δ, suggesting that Rad53 and Chk1 control targets other than the stability of securin in enforcing checkpoint-mediated cell cycle arrest. The G
2
/M arrest in
rad53
Δ and
chk1
Δ 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.