Smaller and faster: the 20-residue Trp-cage protein folds in 4 micros.

We have used laser temperature jump spectroscopy to measure the folding speed of the 20-residue Trp-cage, the smallest polypeptide known to exhibit truly cooperative folding behavior. The observed folding time (4 mus at room temperature) makes this not only the smallest foldable protein, but also the fastest, with a folding speed that exceeds contact-order predictions and approaches anticipated diffusional "speed limits" for protein folding.