title: P22 Arc repressor: folding kinetics of a single-domain, dimeric protein.
abstract: The rate constants for refolding and unfolding of the P22 Arc repressor dimer have been determined by stop-flow fluorescence experiments. Under most conditions, refolding is described well as a two-state reaction with a bimolecular rate-limiting step (kf approximately 10(7) M-1 s-1). A unimolecular step appears to become co-rate limiting at high protein concentrations. The urea dependence of the refolding reaction suggests that about 75% of the total burial of hydrophobic surface occurs between the unfolded state and the transition state for folding. Hydrophobic interactions are also evidenced by the temperature dependence of the refolding reaction; the rate increases with temperature and Arrhenius plots are curved, as expected for a reaction that proceeds with a significant heat capacity change. The refolding of Arc also proceeds more rapidly as the salt concentration is raised, presumably because repulsive interactions between monomers are screened. At a protein concentration of 10 microM, the apparent rate constant for refolding of the Arc dimer is approximately 100 s-1, as fast as the refolding of many monomeric proteins. The rate constant for unfolding is approximately 0.1 s-1, corresponding to a half-life of less than 10 s for the folded Arc dimer. This rate of unfolding is very fast in comparison to that of other characterized proteins and implies that a free Arc molecule must unfold and refold hundreds of times per generation in the cell.
authors.affiliation: Department of Biology, Massachusetts Institute of Technology, Cambridge 02139.
authors.forename: M E
authors.initials: ME
authors.lastname: Milla
authors.forename: R T
authors.initials: RT
authors.lastname: Sauer
mesh_terms: D004268:DNA-Binding Proteins / Q000737:chemistry*; D007700:Kinetics; D009994:Osmolar Concentration; D017510:Protein Folding; D012097:Repressor Proteins / Q000737:chemistry*; D013050:Spectrometry, Fluorescence; D013696:Temperature; D014508:Urea; D014764:Viral Proteins / Q000737:chemistry*; D054334:Viral Regulatory and Accessory Proteins
chemical_list: D004268:DNA-Binding Proteins; D012097:Repressor Proteins; D014764:Viral Proteins; D054334:Viral Regulatory and Accessory Proteins; C016649:phage repressor proteins; D014508:Urea