Expression, purification and crystallization of the complex of RNA polymerase II carboxyl-terminal repeat domain kinase subunits CTK2-CTK3 from Saccharomyces cerevisiae.

Carboxyl-terminal repeat domain (CTD) of the largest subunit Rpb1 of RNA polymerace II is essential for transcription regulation. Heptapeptide repeat of CTD of Rpb1 is phosphorylated by carboxyl-terminal repeat domain kinase (CTDK-I), composed of CTK1, CTK2 and CTK3, in order to regulate transcription and transcription associated processes. The yeast specific protein CTK3 binds to cyclin CTK2 to form a heterodimer serving as a regulational factor to control CTK1 activity by binding to CTK1. Structural information of CTK2-CTK3 complex is yet to be elucidated. Here, we report the co-expression of CTK2-CTK3 complex from Saccharomyces cerevisiae with N-terminal His6 -tag in CTK3 in Escherichia coli (E. coli), purification of the complex by four chromatographic steps and crystallization of the complex as well as the diffraction data collection and processing. This study provides some essential information and a guide for structural and functional study of CTK2-CTK3 complex and CTDK-I in the future.