Cyclin H is targeted to the nucleus by C-terminal nuclear localization sequences

Cdk-activating kinase (CAK) is a trimeric complex consisting of cdk7, cyclin H, and MAT1, which activates the cell-cycle-regulating cdks through T loop phosphorylation. In addition, other substrates of the CAK complex have been identified when CAK is assembled with the TFIIH core proteins, thereby regulating transcription and nucleotide excision repair. Little is known about the regulation of the CAK complex through cyclin H. In this study we further analyzed cyclin H regulation and identified two basic clusters in the C terminus of the protein as putative nuclear localization sequences (NLSs). Fusion constructs of full-length and truncated cyclin H sequences demonstrated the functionality of the NLSs. A peptide-binding assay revealed that at least one NLS interacts with the nuclear import receptors importin α/β. Phosphorylation in the vicinity of the NLSs by cyclin C/cdk8 or protein kinase CK2, however, does not influence the nuclear translocation of cyclin H.