|The Premo™ FUCCI Cell Cycle Sensor consists of a fluorescent protein-based system that employs both a red (RFP) and a green (GFP) fluorescent protein fused to different regulators of the cell cycle: cdt1 and geminin. |
During the cell cycle, these two proteins are ubiquitinated by specific ubiquitin E3 ligases, which target them to the proteasome for degradation. The E3 ligases display temporal regulation of their activity which results in the biphasic cycling of geminin and cdt1levels during the cell cycle. In the G1 phase of the cell cycle, geminin is degraded; therefore, only cdt1 tagged with RFP is present and appears as red fluorescence within the nuclei. In the S, G2, and M phases, cdt1 is degraded and only geminin tagged with GFP remains, resulting in cells with green fluorescent nuclei.
During the G1/S transition, when cdt1 levels are decreasing and geminin levels increasing, both proteins are present, and when the green and red images are overlaid, nuclei appear yellow fluorescent. This dynamic color change, from red-through-yellow-to-green, represents the progression through cell cycle and division (Figures 1 and 2).