The Cdc6 protein is an essential regulator for initiation of DNA replication. Following the G1/S transition, Cdc6 is degraded through a ubiquitin-mediated proteolysis pathway. In this study, we tagged Cdc6 with green fluorescent protein (GFP) and used site-specific mutations to study the regulation of Cdc6 localization and degradation in living yeast cells. Our major findings are: (1) Cdc6-GFP distributes predominantly in the nucleus in all cell cycle stages, with a small increase in cytoplasmic localization in G2/M cells. (2) This nuclear localization is critical for Cdc6 degradation. When the N-terminal nuclear localization signal (NLS) was mutated. Cdc6-GFP no longer accumulated in the nucleus, and the mutant cdc6 was stabilized compared to wild type. (3) The putative CDK phosphorylation sites are not required for Cdc6 nuclear localization, but are important for protein stability. These observations suggest that the stability of Cdc6 protein is regulated by two factors: nuclear localization and phosphorylation by CDK1. (C) 2003 Elsevier Science (USA). All rights reserved.