title: Setting the pace of the Neurospora circadian clock by multiple independent FRQ phosphorylation events.
abstract: Protein phosphorylation plays essential roles in eukaryotic circadian clocks. Like PERIOD in animals, the Neurospora core circadian protein FRQ is progressively phosphorylated and becomes extensively phosphorylated before its degradation. In this study, by using purified FRQ protein from Neurospora, we identified 43 in vivo FRQ phosphorylation sites by mass spectrometry analysis. In addition, we show that CK-1a and CKII are responsible for most FRQ phosphorylation events and identify an additional 33 phosphorylation sites by in vitro kinase assays. Whole-cell metabolic isotope labeling and quantitative MS analyses suggest that circadian oscillation of the FRQ phosphorylation profile is primarily due to progressive phosphorylation at the majority of these newly discovered phosphorylation sites. Furthermore, systematic mutations of the identified FRQ phosphorylation sites led to either long or short period phenotypes. These changes in circadian period are attributed to increases or decreases in FRQ stability, respectively. Together, this comprehensive study of FRQ phosphorylation reveals that regulation of FRQ stability by multiple independent phosphorylation events is a major factor that determines the period length of the clock. A model is proposed to explain how FRQ stability is regulated by multiple phosphorylation events.
authors.affiliation: Department of Physiology, The University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390, USA.
authors.forename: Chi-Tai
authors.initials: CT
authors.lastname: Tang
authors.forename: Shaojie
authors.initials: S
authors.lastname: Li
authors.forename: Chengzu
authors.initials: C
authors.lastname: Long
authors.forename: Joonseok
authors.initials: J
authors.lastname: Cha
authors.forename: Guocun
authors.initials: G
authors.lastname: Huang
authors.forename: Lily
authors.initials: L
authors.lastname: Li
authors.forename: She
authors.initials: S
authors.lastname: Chen
authors.forename: Yi
authors.initials: Y
authors.lastname: Liu
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites / Q000235:genetics; D015153:Blotting, Western; D002940:Circadian Rhythm / Q000235:genetics / Q000502:physiology*; D003513:Cycloheximide / Q000494:pharmacology; D005656:Fungal Proteins / Q000235:genetics / Q000378:metabolism*; D013058:Mass Spectrometry / Q000379:methods; D008954:Models, Biological; D008969:Molecular Sequence Data; D009154:Mutation; D025282:Mycelium / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D009491:Neurospora / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation; D055550:Protein Stability / Q000187:drug effects; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology
chemical_list: C087405:FRQ protein, Neurospora crassa; D005656:Fungal Proteins; D011500:Protein Synthesis Inhibitors; D003513:Cycloheximide