title: Phosphorylation by cyclin C/cyclin-dependent kinase 2 following mitogenic stimulation of murine fibroblasts inhibits transcriptional activity of LSF during G1 progression.
abstract: Transcription factor LSF is required for progression from quiescence through the cell cycle, regulating thymidylate synthase (Tyms) expression at the G(1)/S boundary. Given the constant level of LSF protein from G(0) through S, we investigated whether LSF is regulated by phosphorylation in G(1). In vitro, LSF is phosphorylated by cyclin E/cyclin-dependent kinase 2 (CDK2), cyclin C/CDK2, and cyclin C/CDK3, predominantly on S309. Phosphorylation of LSF on S309 is maximal 1 to 2 h after mitogenic stimulation of quiescent mouse fibroblasts. This phosphorylation is mediated by cyclin C-dependent kinases, as shown by coimmunoprecipitation of LSF and cyclin C in early G(1) and by abrogation of LSF S309 phosphorylation upon suppression of cyclin C with short interfering RNA. Although mouse fibroblasts lack functional CDK3 (the partner of cyclin C in early G(1) in human cells), CDK2 compensates for this absence. By transient transfection assays, phosphorylation at S309, mediated by cyclin C overexpression, inhibits LSF transactivation. Moreover, overexpression of cyclin C and CDK3 inhibits induction of endogenous Tyms expression at the G(1)/S transition. These results identify LSF as only the second known target (in addition to pRb) of cyclin C/CDK activity during progression from quiescence to early G(1). Unexpectedly, this phosphorylation prevents induction of LSF target genes until late G(1).
authors.affiliation: Department of Biology, Boston University, 5 Cummington Street, Boston, MA 02215, USA.
authors.forename: Utsav H
authors.initials: UH
authors.lastname: Saxena
authors.forename: Christina M H
authors.initials: CM
authors.lastname: Powell
authors.forename: Jill K
authors.initials: JK
authors.lastname: Fecko
authors.forename: Roxanne
authors.initials: R
authors.lastname: Cacioppo
authors.forename: Hubert S
authors.initials: HS
authors.lastname: Chou
authors.forename: Geoffrey M
authors.initials: GM
authors.lastname: Cooper
authors.forename: Ulla
authors.initials: U
authors.lastname: Hansen
mesh_terms: D000818:Animals; D056745:Cyclin C; D019927:Cyclin E / Q000235:genetics / Q000378:metabolism; D051357:Cyclin-Dependent Kinase 2 / Q000235:genetics / Q000378:metabolism*; D056849:Cyclin-Dependent Kinase 3; D018844:Cyclin-Dependent Kinases / Q000235:genetics / Q000378:metabolism; D016213:Cyclins / Q000235:genetics / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D004790:Enzyme Induction; D005347:Fibroblasts / Q000166:cytology / Q000502:physiology*; D016193:G1 Phase / Q000502:physiology*; D006801:Humans; D051379:Mice; D041681:NIH 3T3 Cells; D010766:Phosphorylation; D012694:Serine / Q000378:metabolism; D013940:Thymidylate Synthase / Q000235:genetics / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic
chemical_list: C503675:Ccnc protein, mouse; D056745:Cyclin C; D019927:Cyclin E; D016213:Cyclins; D004268:DNA-Binding Proteins; C501905:Tfcp2 protein, mouse; D014157:Transcription Factors; D012694:Serine; D013940:Thymidylate Synthase; C495893:Cdk2 protein, mouse; C490914:Cdk3 protein, mouse; D051357:Cyclin-Dependent Kinase 2; D056849:Cyclin-Dependent Kinase 3; D018844:Cyclin-Dependent Kinases