Ultrafast folding of WW domains without structured aromatic clusters in the denatured state

Ultrafast-folding proteins are important for combining experiment and simul ation to give complete descriptions of folding pathways. The WW domain fami ly comprises small proteins with a three-stranded antiparallel beta -sheet topology. Previous studies on the 57-residue YAP 65 WW domain indicate the presence of residual structure in the chemically denatured state. Here we a nalyze three minimal core WW domains of 38-44 residues. There was little sp ectroscopic or thermodynamic evidence for residual structure in either thei r chemically or thermally denatured states. Folding and unfolding kinetics, studied by using rapid temperature-jump and continuous-flow techniques, sh ow that each domain folds and unfolds very rapidly in a two-state transitio n through a highly compact transition state. Folding half-times were as sho rt as 17 mus at 25 degreesC, within an order of magnitude of the predicted maximal rate of loop formation. The small size and topological simplicity o f these domains, in conjunction with their very rapid two-state folding, ma y allow us to reduce the difference in time scale between experiment and th eoretical simulation.