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 ctk2, required for ctk1/lsk1 CTD kinase activation (By similarity). CTDK-I consists of three subunits, ctk1/lsk1, ctk2/lsc1 and ctk3 (also called alpha, beta and gamma). Belongs to the CTK3 family.