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Updated: Aug 14, 2026

Identification of Cyclin-dependent Kinase 1 Specific Phosphorylation Sites by an In Vitro Kinase Assay
Published on: May 3, 2018
Protein kinase C phosphorylates DNA topoisomerase I
D S Samuels1, Y Shimizu, N Shimizu
1Department of Molecular and Cellular Biology, University of Arizona, Tucson 85721.
Abstract:
The induction of mammalian cell proliferation requires the expression of a specific set of genes. Tumor promoters stimulate cell growth by activating the Ca2+ and phospholipid-dependent protein kinase, protein kinase C (PKC). DNA topoisomerase I, a nuclear enzyme involved in transcription, was phosphorylated by activated PKC in vitro. Phosphorylation by PKC stimulated the DNA relaxation activity of topoisomerase I two- to three-fold. Therefore, DNA topoisomerase I is a substrate for PKC-mediated activation by phosphorylation and may serve as a nuclear target of mitogenic signals generated by tumor promoters in vivo.
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