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The 35-residue subdomain of the villin headpiece (HP35) is a small ultrafast folding protein that is being intensely studied by experiments, theory, and simulations. We have solved the x-ray structures of HP35 and its fastest folding mutant [K24 norleucine (nL)] to atomic resolution and compared their experimentally measured folding kinetics by using laser temperature jump. The structures, which are in different space groups, are almost identical to each other but differ significantly from previously solved NMR structures. Hence, the differences between the x-ray and NMR structures are probably not caused by lattice contacts or crystal͞solution differences, but reflect the higher accuracy of the x-ray structures. The x-ray structures reveal important details of packing of the hydrophobic core and some additional features, such as crosshelical H bonds. Comparison of the x-ray structures indicates that the nL substitution produces only local perturbations. Consequently, the finding that the small stabilization by the mutation is completely reflected in an increased folding rate suggests that this region of the protein is as structured in the transition state as in the folded structure. It is therefore a target for engineering to increase the folding rate of the subdomain from Ϸ0.5 s ؊1 for the nL mutant to the estimated theoretical speed limit of Ϸ3 s ؊1 .kinetics ͉ temperature jump ͉ NMR R ecent developments in both experimental and computational methods have opened exciting new possibilities for combining experiments with all-atom molecular dynamics (MD) simulations of protein folding. The dramatic improvement in time resolution in protein folding kinetic studies brought about by the introduction of nanosecond optical triggering methods (1-4), together with the use of distributed computing, now make it possible to directly compare MD simulations with experiments on ultrafast folding proteins (5). Such simulations are important because, if accurate, they contain a complete atomic description of the folding mechanism. The computational cost of folding simulations demands small size and ultrafast folding. HP35 is a subdomain of the headpiece of actinbinding protein villin and is the smallest naturally occurring polypeptide that folds autonomously without any cofactor or disulfide bond (6). Unlike other ''miniproteins'' (7), HP35 is highly thermostable (6) and, as revealed by the NMR structure, globular with a well packed hydrophobic core (8). These properties resemble much larger single-domain helical proteins and have made HP36 (HP35 plus an N-terminal Met) the focus of several all-atom folding simulations (9-17), which have thus far been based on the NMR structure.Because of the delicate balance of stabilization forces involved in protein folding an accurate structure for the folded state is essential for MD simulations of folding and protein redesign calculations. We have previously shown that HP35 is one of the fastest folding proteins, with a folding rate (Ϸ0.2 s Ϫ1 ) approaching its theoretical limit (3 s Ϫ1 ...

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