title: Structure of a dominant-negative helix-loop-helix transcriptional regulator suggests mechanisms of autoinhibition.
abstract: Helix-loop-helix (HLH) family transcription factors regulate numerous developmental and homeostatic processes. Dominant-negative HLH (dnHLH) proteins lack DNA-binding ability and capture basic HLH (bHLH) transcription factors to inhibit cellular differentiation and enhance cell proliferation and motility, thus participating in patho-physiological processes. We report the first structure of a free-standing human dnHLH protein, HHM (Human homologue of murine maternal Id-like molecule). HHM adopts a V-shaped conformation, with N-terminal and C-terminal five-helix bundles connected by the HLH region. In striking contrast to the common HLH, the HLH region in HHM is extended, with its hydrophobic dimerization interfaces embedded in the N- and C-terminal helix bundles. Biochemical and physicochemical analyses revealed that HHM exists in slow equilibrium between this V-shaped form and the partially unfolded, relaxed form. The latter form is readily available for interactions with its target bHLH transcription factors. Mutations disrupting the interactions in the V-shaped form compromised the target transcription factor specificity and accelerated myogenic cell differentiation. Therefore, the V-shaped form of HHM may represent an autoinhibited state, and the dynamic conformational equilibrium may control the target specificity.
authors.affiliation: Department of Biophysics and Biochemistry, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo, Japan.
authors.forename: Ryohei
authors.initials: R
authors.lastname: Ishii
authors.forename: Kazunobu
authors.initials: K
authors.lastname: Isogaya
authors.forename: Azusa
authors.initials: A
authors.lastname: Seto
authors.forename: Daizo
authors.initials: D
authors.lastname: Koinuma
authors.forename: Yuji
authors.initials: Y
authors.lastname: Watanabe
authors.forename: Fumio
authors.initials: F
authors.lastname: Arisaka
authors.forename: So-ichi
authors.initials: S
authors.lastname: Yaguchi
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Ikushima
authors.forename: Naoshi
authors.initials: N
authors.lastname: Dohmae
authors.forename: Kohei
authors.initials: K
authors.lastname: Miyazono
authors.forename: Keiji
authors.initials: K
authors.lastname: Miyazawa
authors.forename: Ryuichiro
authors.initials: R
authors.lastname: Ishitani
authors.forename: Osamu
authors.initials: O
authors.lastname: Nureki
mesh_terms: D000595:Amino Acid Sequence; D018360:Crystallography, X-Ray; D006801:Humans; D008969:Molecular Sequence Data; D009154:Mutation; D011485:Protein Binding; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid; D014157:Transcription Factors / Q000737:chemistry* / Q000235:genetics
chemical_list: C485815:CCNDBP1 protein, human; D014157:Transcription Factors