Human Cyclin K, a Novel RNA Polymerase II-Associated Cyclin Possessing Both Carboxy-Terminal Domain Kinase and Cdk-Activating Kinase Activity

Abstract

The gene coding for human cyclin K was isolated as a CPR (cell-cycle progression restoration) gene by virtue of its ability to impart a Far ؊ phenotype to the budding yeast Saccharomyces cerevisiae and to rescue the lethality of a deletion of the G 1 cyclin genes CLN1, CLN2, and CLN3. The cyclin K gene encodes a 357-aminoacid protein most closely related to human cyclins C and H, which have been proposed to play a role in regulating basal transcription through their association with and activation of cyclin-dependent kinases (Cdks) that phosphorylate the carboxyl-terminal domain (CTD) of the large subunit of RNA polymerase II (RNAP II). Murine and Drosophila melanogaster homologs of cyclin K have also been identified. Cyclin K mRNA is ubiquitously expressed in adult mouse and human tissues, but is most abundant in the developing germ cells of the adult testis and ovaries. Cyclin K is associated with potent CTD kinase and Cdk kinase (CAK) activity in vitro and coimmunoprecipitates with the large subunit of RNAP II. Thus, cyclin K represents a new member of the "transcription" cyclin family which may play a dual role in regulating Cdk and RNAP II activity.

Cyclin K Inhibits HIV-1 Gene Expression and Replication by Interfering with Cyclin-dependent Kinase 9 (CDK9)-Cyclin T1 Interaction in Nef-dependent Manner

“…Cyclin K Inhibits HIV-1 LTR-driven Gene Expression in Presence of Nef-Cyclin K was first identified as a novel C type cyclin that is associated with an in vitro kinase activity toward CTD of RNA polymerase II (30). As this activity is important for HIV-1 gene expression, we wanted to examine the effect of CycK on HIV-1 LTR-driven viral gene expression.…”

“…CycK was first identified as a protein that could restore progression through the cell cycle and was most closely related to human cyclins C and H (30). The kinase partner of CycK was identified as CDK9 and was reported to function as a CTD kinase in vitro (17).…”

“…EGFP-Nef plasmid was constructed by cloning the complete GFP-Nef fusion gene sequence from pEGFPN2-Nef in EcoRI and NotI sites of pcDNA6HisC (Invitrogen). Cyclin K (CCNK) gene encoding the 354-amino acid isoform (30) was cloned in-frame into BamHI and EcoRI sites of pcDNA6HisC to obtain pcDNA6HisC-Cyclin K (pc-CycK). The cloning of pc-CycK was confirmed by restriction digestion and by DNA sequencing in an ABI 310 Genetic Analyzer (Applied Biosystems).…”

“…Recently, CycK has been also shown to inhibit viral replication in a T-cell line by inhibiting LTR-mediated transcription (29), although the mechanism of inhibition remains to be elucidated. CycK was first identified as a protein that could restore progression of the cell cycle and was most closely related to human Cyclins C and H (30). The kinase partner of CycK was identified as CDK9 (17), although a recent report also indicates CycK interaction with CDK12 and -13 (31).…”

Regulation of RNA polymerase II activity by CTD phosphorylation and cell cycle control

“…P-TEFb is a positive-acting RNAP II transcription elongation factor, originally characterized in Drosophila embryonic extracts (Marshall et al, 1996;Price, 2000), that counteracts inhibition of RNAP II transcription elongation by two negative-acting factors factors, DSIF and NELF (Wada et al, 1998;Garber and Jones, 1999;Yamaguchi et al, 1999;Zorio and Bentley, 2001). CDK9 can associate with four different cyclin subunits, three different T-type cyclins (cyclin T1, T2a, and T2b) (Peng et al, 1998b) and cyclin K. Interestingly, cyclin K was originally identi®ed in a yeast screen based on its ability to restore cell cycle progression and to rescue the lethality of G1 cyclin gene deletions (Edwards et al, 1998;Fu et al, 1999). Similar to CDK7 and CDK8, CTD kinase activity and protein levels of the CDK9-cyclin T complex appear to not signi®cantly¯uctuate during the cell cycle in most eukaryotic cells (Garriga et al, 1998).…”