Hus1p, a conserved fission yeast checkpoint protein, interacts with Rad1p and is phosphorylated in response to DNA damage

The hus1(+) gene is one of six fission yeast genes, termed the checkpoint pad genes, which are essential for both the S-M and DNA damage checkpoints. Classical genetics suggests that these genes are required for activation of the PI-3 kinase-related (PIK-R) protein, Rad3p. Using a dominant negative allele of hus1(+), we have demonstrated a genetic interaction between hus1(+) and another checkpoint rod gene, rad1(+). Hus1p and Rad1p form a stable complex in wild-type fission yeast, and the formation of this complex is dependent on a third checkpoint rad gene, rad9(+), suggesting that these three proteins may exist in a discrete complex in the absence of checkpoint activation. Hus1p is phosphorylated in response to DNA damage, and this requires rad3(+) and each of the other checkpoint rad genes. Although there is no gene related to hus1(+) in the Saccharomyces cerevisiae genome, we have identified closely related mouse and human genes, suggesting that aspects of the checkpoint control mechanism are conserved between fission yeast and higher eukaryotes.