HRad17 colocalizes with NHP2L1 in the nucleolus and redistributes after UVirradiation

The rad17 gene of Schizosaccharomyces pombe plays an important role as a ch eckpoint protein following DNA damage and during DNA replication. The human homologue of S, pombe rad17, Hrad17, was recently identified, but its func tion has not yet been established. Using the yeast two-hybrid system, we de termined that HRad17 can interact with a nucleolar protein, NHP2L1. This in teraction was also demonstrated biochemically, in human cells. Immunofluore scence studies revealed that HRad17 and NHP2L1 colocalize to the nucleolus, and immunogold labeling further resolved the location of NHP2L1 to the den se fibrillar component of the nucleolus. Interestingly, the localization of HRad17 in the nucleolus was altered in response to UV irradiation. These r esults provide some insight into the DNA damage and replication checkpoint mechanisms of HRad17.