title: An unusual hydrophobic core confers extreme flexibility to HEAT repeat proteins.
abstract: Alpha-solenoid proteins are suggested to constitute highly flexible macromolecules, whose structural variability and large surface area is instrumental in many important protein-protein binding processes. By equilibrium and nonequilibrium molecular dynamics simulations, we show that importin-beta, an archetypical alpha-solenoid, displays unprecedentedly large and fully reversible elasticity. Our stretching molecular dynamics simulations reveal full elasticity over up to twofold end-to-end extensions compared to its bound state. Despite the absence of any long-range intramolecular contacts, the protein can return to its equilibrium structure to within 3 A backbone RMSD after the release of mechanical stress. We find that this extreme degree of flexibility is based on an unusually flexible hydrophobic core that differs substantially from that of structurally similar but more rigid globular proteins. In that respect, the core of importin-beta resembles molten globules. The elastic behavior is dominated by nonpolar interactions between HEAT repeats, combined with conformational entropic effects. Our results suggest that alpha-solenoid structures such as importin-beta may bridge the molecular gap between completely structured and intrinsically disordered proteins.
authors.affiliation: Department of Theoretical and Computational Biophysics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Kappel
authors.forename: Ulrich
authors.initials: U
authors.lastname: Zachariae
authors.forename: Nicole
authors.initials: N
authors.lastname: Dölker
authors.forename: Helmut
authors.initials: H
authors.lastname: Grubmüller
mesh_terms: D020816:Amino Acid Motifs; D004548:Elasticity; D019277:Entropy; D005656:Fungal Proteins / Q000737:chemistry* / Q000378:metabolism; D057927:Hydrophobic and Hydrophilic Interactions; D056004:Molecular Dynamics Simulation; D017433:Protein Structure, Secondary; D012441:Saccharomyces cerevisiae; D028961:beta Karyopherins / Q000737:chemistry* / Q000378:metabolism
chemical_list: D005656:Fungal Proteins; D028961:beta Karyopherins
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