title: Multi-site phosphorylation regulates NeuroD4 activity during primary neurogenesis: a conserved mechanism amongst proneural proteins.
abstract: BACKGROUND
  Basic Helix Loop Helix (bHLH) proneural transcription factors are master regulators of neurogenesis that act at multiple stages in this process. We have previously demonstrated that multi-site phosphorylation of two members of the proneural protein family, Ngn2 and Ascl1, limits their ability to drive neuronal differentiation when cyclin-dependent kinase levels are high, as would be found in rapidly cycling cells. Here we investigate potential phospho-regulation of proneural protein NeuroD4 (also known as Xath3), the Xenopus homologue of Math3/NeuroM, that functions downstream of Ngn2 in the neurogenic cascade.
  
  
  RESULTS
  Using the developing Xenopus embryo system, we show that NeuroD4 is expressed and phosphorylated during primary neurogenesis, and this phosphorylation limits its ability to drive neuronal differentiation. Phosphorylation of up to six serine/threonine-proline sites contributes additively to regulation of NeuroD4 proneural activity without altering neuronal subtype specification, and number rather than location of available phospho-sites is the key for limiting NeuroD4 activity. Mechanistically, a phospho-mutant NeuroD4 displays increased protein stability and enhanced chromatin binding relative to wild-type NeuroD4, resulting in transcriptional up-regulation of a range of target genes that further promote neuronal differentiation.
  
  
  CONCLUSIONS
  Multi-site phosphorylation on serine/threonine-proline pairs is a widely conserved mechanism of limiting proneural protein activity, where it is the number of phosphorylated sites, rather than their location that determines protein activity. Hence, multi-site phosphorylation is very well suited to allow co-ordination of proneural protein activity with the cellular proline-directed kinase environment.
authors.affiliation: Department of Oncology, University of Cambridge, Hutchison/MRC Research Centre, Cambridge Biomedical Campus, Cambridge, CB2 0XZ, UK. ljah2@cam.ac.uk.
authors.forename: Laura J A
authors.initials: LJ
authors.lastname: Hardwick
authors.affiliation: Department of Oncology, University of Cambridge, Hutchison/MRC Research Centre, Cambridge Biomedical Campus, Cambridge, CB2 0XZ, UK. ap113@cam.ac.uk.
authors.forename: Anna
authors.initials: A
authors.lastname: Philpott
mesh_terms: D000818:Animals; D051792:Basic Helix-Loop-Helix Proteins / Q000378:metabolism*; D015153:Blotting, Western; D002454:Cell Differentiation; D047108:Embryonic Development / Q000502:physiology; D017403:In Situ Hybridization; D016297:Mutagenesis, Site-Directed; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D055495:Neurogenesis / Q000502:physiology*; D009474:Neurons / Q000166:cytology; D010766:Phosphorylation; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014981:Xenopus / Q000196:embryology / Q000378:metabolism*; D029867:Xenopus Proteins / Q000378:metabolism*
chemical_list: D051792:Basic Helix-Loop-Helix Proteins; D009419:Nerve Tissue Proteins; D029867:Xenopus Proteins; C491279:NEUROD4 protein, Xenopus