Cyclin subunit of the CTDK-I complex, which hyperphosphorylates the C-terminal heptapeptide repeat domain (CTD) of the largest RNA polymerase II subunit. As part of the CTDK-I complex, involved in RNA polymerase II transcriptional elongation and pre-mRNA 3'-end processing. Together with ctk3, required for ctk1/lsk1 CTD kinase activation (By similarity). Together with ctk1/lsk1, required for the regulation of cytokinesis by phosphorylating 'Ser-2' residues found in the heptad repeats of the CTD. CTDK-I consists of three subunits, ctk1/lsk1, ctk2/lsc1 and ctk3 (also called alpha, beta and gamma) (By similarity). Interacts with ctk1/lsk1. This interaction is dependent on ctk1/lsk1 kinase activity. Nuclear localization is dependent on ctk1/lsk1. Cells are hyper-sensitive to actin depolymerizing drug, latrunculin A (Lat A). Deletion mutants are unable to complete septum formation upon LatA treatment, but are able to suppress the lethal, multiseptate phenotype conferred by the constitutive hyperactivation of the septation initiation network (SIN). Belongs to the cyclin family.