title: bHLH proteins involved in Drosophila neurogenesis are mutually regulated at the level of stability.
abstract: Proneural bHLH activators are expressed in all neuroectodermal regions prefiguring events of central and peripheral neurogenesis. Drosophila Sc is a prototypical proneural activator that heterodimerizes with the E-protein Daughterless (Da) and is antagonized by, among others, the E(spl) repressors. We determined parameters that regulate Sc stability in Drosophila S2 cells. We found that Sc is a very labile phosphoprotein and its turnover takes place via at least three proteasome-dependent mechanisms. (i) When Sc is in excess of Da, its degradation is promoted via its transactivation domain (TAD). (ii) In a DNA-bound Da/Sc heterodimer, Sc degradation is promoted via an SPTSS phosphorylation motif and the AD1 TAD of Da; Da is spared in the process. (iii) When E(spl)m7 is expressed, it complexes with Sc or Da/Sc and promotes their degradation in a manner that requires the corepressor Groucho and the Sc SPTSS motif. Da/Sc reciprocally promotes E(spl)m7 degradation. Since E(spl)m7 is a direct target of Notch, the mutual destabilization of Sc and E(spl) may contribute in part to the highly conserved anti-neural activity of Notch. Sc variants lacking the SPTSS motif are dramatically stabilized and are hyperactive in transgenic flies. Our results propose a novel mechanism of regulation of neurogenesis, involving the stability of key players in the process.
authors.affiliation: Institute of Molecular Biology and Biotechnology, Foundation for Research and Technology Hellas, and Department of Biology, University of Crete, 70013 Heraklion, Crete, Greece.
authors.forename: Marianthi
authors.initials: M
authors.lastname: Kiparaki
authors.affiliation: Institute of Molecular Biology and Biotechnology, Foundation for Research and Technology Hellas, and Department of Biology, University of Crete, 70013 Heraklion, Crete, Greece.
authors.forename: Ioanna
authors.initials: I
authors.lastname: Zarifi
authors.affiliation: Institute of Molecular Biology and Biotechnology, Foundation for Research and Technology Hellas, and Department of Biology, University of Crete, 70013 Heraklion, Crete, Greece delidaki@imbb.forth.gr.
authors.forename: Christos
authors.initials: C
authors.lastname: Delidakis
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D030801:Animals, Genetically Modified; D051792:Basic Helix-Loop-Helix Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D015153:Blotting, Western; D002460:Cell Line; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D029721:Drosophila Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004331:Drosophila melanogaster / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D018507:Gene Expression Regulation, Developmental; D009154:Mutation; D055495:Neurogenesis; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D011485:Protein Binding; D055503:Protein Multimerization; D051880:Receptors, Notch / Q000235:genetics / Q000378:metabolism; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism; D014157:Transcription Factors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D051792:Basic Helix-Loop-Helix Proteins; D004268:DNA-Binding Proteins; D029721:Drosophila Proteins; D046988:Proteasome Endopeptidase Complex; D051880:Receptors, Notch; D012097:Repressor Proteins; D014157:Transcription Factors; C090080:Da protein, Drosophila; C121960:E(spl)m7-HLH protein, Drosophila; C497759:N protein, Drosophila; C089197:sc protein, Drosophila