The eukaryotic cell cycle is regulated by ubiquitin mediated degradation of cell cycle regulators. Degradation takes place at phase transitions like entry into and exit from mitosis as well as entry into S-phase. Cell cycle specific ubiquitin ligases identify degradation sequences called degrons in their target proteins. Fusion of such a degron to a fluorescent protein (or any other reporter) recapitulates the degradation event. In most cases degradation is either turned on or off. When expressed from a constitutive promoter cells either express the reporter, when degradation is off, or do not express it when degradation is on. It is thus possible to follow individual cells throughout the cell cycle or sort entire cell populations. Most reporters described so far can distinguish between cells in G1 and G0 to cells in the S-G2-M phases, which proved very useful for many different studies. We have recently developed a new reporter that yields a much higher temporal resolution. This reporter is based on the Cdc6 protein that plays a role in licensing of replication. Cdc6 and its fusion protein undergo two waves of degradation during the cell cycle – the first upon cell division and the second upon entry into S-phase. Unlike other proteins degraded in G1 Cdc6 becomes stabilized not at the end but in mid G1. This is thus the first reporter that enables to distinguish between early and late G1 opening many exciting new research possibilities. Moreover the cellular localization of Cdc6 and its fusion protein changes from nuclear to cytoplasmic, further improving our ability to measure the length of the various sub-phases.
