Early Events During Folding of Wild-type Staphylococcal Nuclease and a Single-tryptophan Variant Studied by Ultrarapid Mixing

A continuous-flow mixing device with a dead time of 100 μs coupled with intrinsic tryptophan and 1-anilinonaphthalene-8-sulfonate (ANS) fluorescence was used to monitor structure formation during early stages of the folding of staphylococcal nuclease (SNase). A variant with a unique tryptophan fluorophore in the N-terminal β-barrel domain (Trp76 SNase) was obtained by replacing the single Trp140 in wild-type SNase with His in combination with Trp substitution of Phe76. A common background of P47G, P117G and H124L mutations was chosen in order to stabilize the protein and prevent accumulation of cis proline isomers under native conditions. In contrast to WT ∗ SNase, which shows no changes in tryptophan fluorescence prior to the rate-limiting folding step (∼100 ms), the F76W/W140H variant shows additional changes (enhancement) during an early folding phase with a time constant of 75 μs. Both proteins exhibit a major increase in ANS fluorescence and identical rates for this early folding event. These findings are consistent with the rapid accumulation of an ensemble of states containing a loosely packed hydrophobic core involving primarily the β-barrel domain while the specific interactions in the α-helical domain involving Trp140 are formed only during the final stages of folding. The fact that both variants exhibit the same number of kinetic phases with very similar rates confirms that the folding mechanism is not perturbed by the F76W/W140H mutations. However, the Trp at position 76 reports on the rapid formation of a hydrophobic cluster in the N-terminal β-sheet region while the wild-type Trp140 is silent during this early stage of folding. Quantitative modeling of the (un)folding kinetics and thermodynamics of these two proteins versus urea concentration revealed that the F76W/W140H mutation selectively destabilizes the native state relative to WT ∗ SNase while the stability of transient intermediates remains unchanged, leading to accumulation of intermediates under equilibrium conditions at moderate denaturant concentrations.