A hydrophobic cluster forms early in the folding of dihydrofolate reductase

The rapid kinetic phase that leads from unfolded species to transient folding intermediates in dihydrofolate reductase from Escherichia coli was examined by site‐directed mutagenesis and by physicochemical means. The absence of this fluorescence‐detected phase in the refolding of the Trp‐74Phe mutant protein strongly implies that this early phase in refolding can be assigned to just one of the five Trp residues in the protein, Trp‐74. In addition, water‐soluble fluorescence quenching agents, iodide and cesium, have a much less significant effect on this early step in refolding than on the slower phases that lead to native and nativelike conformers. These and other data imply that an important early event in the folding of dihydrofolate reductase is the formation of a hydrophobic cluster which protects Trp‐74 fromsolvent.