Regulated expression and dynamic changes in subnuclear localization of mammalian Rad18 under normal and genotoxic conditions.

Rad18 plays a crucial role in postreplication repair in both lower eukaryotes and higher eukaryotes. However, regulation of the Rad18 expression in higher eukaryotes is largely unknown. We found that the RAD18 transcript is expressed ubiquitously in various tissues and very highly in the testis in mammals. Although human RAD18 (hRAD18) transcription levels fluctuate during the cell cycle, being maximal in the late S and minimal in the early G1, the protein levels remain constant throughout the cell cycle. Following UV-irradiation, hRAD18 transcription levels decrease significantly, but Rad18 protein levels change little. The protein levels are maintained at least in part by enhanced translation rates. hRad18 localizes in the nucleus in two forms: a diffused form and a condensed form forming nuclear dots. These nuclear dots disperse rapidly in the nucleoplasm after treatments with various genotoxic agents, resulting in an enhancement of the intranuclear Rad18 concentration of the diffused form. No de novo protein synthesis is required for this process. These results suggest that in higher eukaryotes, the maintenance and dynamic translocation of Rad18 protein is important for postreplication repair.