Spatio-temporal regulation of the human licensing factor Cdc6 in replication and mitosis

• Published in: Cell cycle (Georgetown, Tex.) Vol. 14; no. 11; pp. 1704 - 1715
• Main Authors: Kalfalah, Faiza M, Berg, Elke, Christensen, Morten O, Linka, René M, Dirks, Wilhelm G, Boege, Fritz, Mielke, Christian
• Format: Journal Article
• Language: English
• Published: United States Taylor & Francis 01.01.2015
• Subjects: Bacterial Proteins - metabolism; Blotting, Western; Cell Cycle Proteins - metabolism; Centrosome - metabolism; Chromatin - metabolism; DNA Replication - physiology; Flow Cytometry; Genomic Instability - physiology; Green Fluorescent Proteins - metabolism; Humans; Immunohistochemistry; Luminescent Proteins - metabolism; Microscopy, Fluorescence; Mitosis - physiology; Nuclear Proteins - metabolism; Proliferating Cell Nuclear Antigen - metabolism; DNA replication; STR, small tandem repeat; preRC, pre-replicative complex; ORC, origin recognition complex; APC, anaphase promoting complex; cell cycle control; centrosome; PCNA, proliferating cell nuclear antigen; MCM, minichromosome maintenance complex; nuclear export; FRAP, fluorescence recovery after photobleaching; licensing; ori, origin of replication
• ISSN: 1538-4101; 1551-4005
• DOI: 10.1080/15384101.2014.1000182

To maintain genome stability, the thousands of replication origins of mammalian genomes must only initiate replication once per cell cycle. This is achieved by a strict temporal separation of ongoing replication in S phase, and the formation of pre-replicative complexes in the preceding G1 phase, which "licenses" each origin competent for replication. The contribution of the loading factor Cdc6 to the timing of the licensing process remained however elusive due to seemingly contradictory findings concerning stabilization, degradation and nuclear export of Cdc6. Using fluorescently tagged Cdc6 (Cdc6-YFP) expressed in living cycling cells, we demonstrate here that Cdc6-YFP is stable and chromatin-associated during mitosis and G1 phase. It undergoes rapid proteasomal degradation during S phase initiation followed by active export to the cytosol during S and G2 phases. Biochemical fractionation abolishes this nuclear exclusion, causing aberrant chromatin association of Cdc6-YFP and, likely, endogenous Cdc6, too. In addition, we demonstrate association of Cdc6 with centrosomes in late G2 and during mitosis. These results show that multiple Cdc6-regulatory mechanisms coexist but are tightly controlled in a cell cycle-specific manner.