title: Folding and binding of an intrinsically disordered protein: fast, but not 'diffusion-limited'.
abstract: Coupled folding and binding of intrinsically disordered proteins (IDPs) is prevalent in biology. As the first step toward understanding the mechanism of binding, it is important to know if a reaction is 'diffusion-limited' as, if this speed limit is reached, the association must proceed through an induced fit mechanism. Here, we use a model system where the 'BH3 region' of PUMA, an IDP, forms a single, contiguous α-helix upon binding the folded protein Mcl-1. Using stopped-flow techniques, we systematically compare the rate constant for association (k(+)) under a number of solvent conditions and temperatures. We show that our system is not 'diffusion-limited', despite having a k(+) in the often-quoted 'diffusion-limited' regime (10(5)-10(6) M(-1) s(-1) at high ionic strength) and displaying an inverse dependence on solvent viscosity. These standard tests, developed for folded protein-protein interactions, are not appropriate for reactions where one protein is disordered.
authors.affiliation: Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK.
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Rogers
authors.forename: Annette
authors.initials: A
authors.lastname: Steward
authors.forename: Jane
authors.initials: J
authors.lastname: Clarke
mesh_terms: D001665:Binding Sites; D004058:Diffusion; D008958:Models, Molecular; D011487:Protein Conformation; D017510:Protein Folding; D011506:Proteins / Q000737:chemistry*
chemical_list: D011506:Proteins