Equilibrium stability and sub-millisecond refolding of a designed single-chain Arc repressor.

Arc-L1-Arc is a single-chain variant of bacteriophage P22 Arc repressor in which a 15 residue linker joins the C-terminus of one subunit to the N-terminus of an otherwise identical subunit. Spectroscopic probes indicate that the native and denatured state of the single-chain protein are similar to those of the unlinked Arc dimer. In equilibrium experiments, Arc-L1-Arc denatures in a reaction without populated intermediate states as judged by the fits of the denaturation isotherms to a two-state model and by the coincidence of denaturation curves monitored by fluorescence and circular dichroism. Comparison of the equilibrium stabilities of Arc-L1-Arc and unlinked Arc gives an effective concentration of subunits in the denatured single-chain variant of 2.7 (+/- 0.7) mM. The kinetic refolding and unfolding reactions of Arc-L1-Arc also appear to proceed without populated intermediates. The rate constant for Arc-L1-Arc unfolding is about 2-fold faster than that of unlinked Arc, indicating that the linker mediates no significant contacts in the native structure that need to be broken to allow unfolding. As expected, the major effect of the linker occurs during the refolding reaction, where the effective subunit concentration calculated from the bimolecular and unimolecular refolding rate constants is 4.5 (+/- 1.8) mM. The transition states for the unfolding and refolding reactions of Arc-L1-Arc and wild-type Arc have similar solvent exposures as measured by the urea dependencies of the equilibrium and rate constants. In the absence of urea, the single-chain protein refolds very rapidly (kf approximately 10(4) s-1) in a reaction that is essentially complete in the sub-millisecond time regime.