title: Functional consequences of preorganized helical structure in the intrinsically disordered cell-cycle inhibitor p27(Kip1).
abstract: p27(Kip1) contributes to cell-cycle regulation by inhibiting cyclin-dependent kinase (Cdk) activity. The p27 Cdk-inhibition domain has an ordered conformation comprising an alpha-helix, a 3(10) helix, and beta-structure when bound to cyclin A-Cdk2. In contrast, the unbound p27 Cdk-inhibition domain is intrinsically disordered (natively unfolded) as shown by circular dichroism spectroscopy, lack of chemical-shift dispersion, and negative heteronuclear nuclear Overhauser effects. The intrinsic disorder is not due to the excision of the Cdk-inhibition domain from p27, since circular dichroism spectra of the full-length protein are also indicative of a largely unfolded protein. Both the inhibition domain and full-length p27 are active as cyclin A-Cdk2 inhibitors. Using circular dichroism and proline mutagenesis, we demonstrate that the unbound p27 Cdk-inhibition domain is not completely unfolded. The domain contains marginally stable helical structure that presages the alpha-helix, but not the 3(10) helix, adopted upon binding cyclin A-Cdk2. Increasing or reducing the stability of the partially preformed alpha-helix in the isolated p27 domain with alanine or proline substitutions did not affect formation of the p27-inhibited cyclin A-Cdk2 complex in energetic terms. However, stabilization of the helix with alanine hindered kinetically the formation of the inhibited complex, suggesting that p27 derives a kinetic advantage from intrinsic structural disorder.
authors.affiliation: Department of Biochemistry and Molecular Biology, Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1870, USA.
authors.forename: Ewa A
authors.initials: EA
authors.lastname: Bienkiewicz
authors.forename: Joshua N
authors.initials: JN
authors.lastname: Adkins
authors.forename: Kevin J
authors.initials: KJ
authors.lastname: Lumb
mesh_terms: D019943:Amino Acid Substitution; D001665:Binding Sites; D018797:Cell Cycle Proteins / Q000737:chemistry* / Q000378:metabolism; D002942:Circular Dichroism; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors; D006801:Humans; D007700:Kinetics; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D010766:Phosphorylation; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics; D025521:Tumor Suppressor Proteins / Q000737:chemistry* / Q000378:metabolism
chemical_list: D018797:Cell Cycle Proteins; D011994:Recombinant Proteins; D025521:Tumor Suppressor Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases