Cig2 is a cyclin that functions as the catalytic subunit of cyclin-dependent kinases (Cdks) in the fission yeast Schizosaccharomyces pombe. It is a B-type, S phase cyclin that is required for both the G1/S and G2/M cell cycle transitions and is expressed in a sharp spike that peaks during the G1/S period. Cig2 binds to Cdc2 (Cdk), and the resulting Cdc2/Cig2 complex controls the G1/S transition of the cell cycle. Disruption of Cig2 delays the onset of mitosis. The expression of the Cig2 gene is dependent on Mlu1-binding factor (MBF), and the protein is destroyed during anaphase by the APC/cyclosome (APC/C) and Skp1/Cullin-1/F-box (SCF), thereby ensuring the spike expression pattern of Cig2. SCF regulates Cig2 levels in a dual manner, transcriptionally and post-translationally, while APC/C only destroys the protein.
