title: Binding-dependent disorder-order transition in PKI alpha: a fluorescence anisotropy study.
abstract: The conformational flexibility of peptidyl ligands may be an essential element of many peptide-macromolecular interactions. Consequently, the alpha-carbonyl backbone flexibility of the 8 kDa protein kinase inhibitor (PKI alpha) peptide of cAMP-dependent protein kinase (cAPK) free in solution and bound to cAPK was assessed by time-resolved fluorescence anisotropy. Specifically, three full-length, single-site PKI alpha mutants (V3C, S28C, and S59C) were prepared, and fluorescein iodoacetamide (FI) was selectively conjugated to the side chains of each substituted cysteine. The time-resolved anisotropy decay profiles of the labeled mutants were well fit to a model-free nonassociative biexponential equation. Free in solution, the three labeled proteins had very similar anisotropy decays arising primarily from local alpha-carbonyl backbone movements. Only a small fraction of the anisotropy decay was associated with slower, whole-body tumbling, confirming that PKI alpha is highly disordered at all three locations. Complexation of the mutants with the catalytic (C) subunit of cAPK decreased the rate of whole-body tumbling for all three mutants. The effects on the rapid decay processes, however, were dependent upon the site of conjugation. The anisotropy decay profiles of both FI-V3C- and FI-S28C-PKI alpha were associated with significantly reduced contributions from the fast decay processes, while that of FI-S59C-PKI alpha was largely unaffected by binding to the C-subunit. The results suggest that the cAPK-binding domain of PKI alpha extends from the its N-terminus to residues beyond Ser28 but does not include the segment around Ser59, which is still part of a highly flexible domain when bound to the C-subunit.
authors.affiliation: Department of Chemistry and Biochemistry, Howard Hughes Medical Institute, University of California, San Diego, La Jolla 92093-0654, USA.
authors.forename: J A
authors.initials: JA
authors.lastname: Hauer
authors.forename: S S
authors.initials: SS
authors.lastname: Taylor
authors.forename: D A
authors.initials: DA
authors.lastname: Johnson
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001667:Binding, Competitive; D002352:Carrier Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D002384:Catalysis; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors* / Q000737:chemistry* / Q000378:metabolism; D003545:Cysteine / Q000235:genetics; D004791:Enzyme Inhibitors / Q000737:chemistry* / Q000378:metabolism; D005454:Fluorescence Polarization / Q000379:methods; D047908:Intracellular Signaling Peptides and Proteins; D051379:Mice; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding; D020033:Protein Isoforms / Q000737:chemistry / Q000378:metabolism; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012694:Serine / Q000235:genetics; D013050:Spectrometry, Fluorescence; D014633:Valine / Q000235:genetics
chemical_list: D002352:Carrier Proteins; D004791:Enzyme Inhibitors; D047908:Intracellular Signaling Peptides and Proteins; C107593:Pkia protein, mouse; D020033:Protein Isoforms; D011994:Recombinant Proteins; D012694:Serine; D017868:Cyclic AMP-Dependent Protein Kinases; D014633:Valine; D003545:Cysteine