Submillisecond folding of the peripheral subunit-binding domain

Folding and unfolding rates have been measured for the peripheral subunit-binding domain, a small three-helix protein. The protein folds very fast with rates too rapid to be measured using traditional stopped-flow techniques. Folding and unfolding rates were measured as a function of temperature using dynamic NMR lineshape analysis. At the lowest temperature at which there is sufficient broadening to measure rates, 41°C, the folding rate is 16,050 s^-1. Thus, the halftime required for folding is 43 μs. At the same temperature, the unfolding rate is 2800 s^-1. Identical rates were measured using resolved resonances from Val16 in the loop and Val21 at the end of the 3₁₀-helix. Folding rates have been correlated with protein topology, and this correlation is consistent with the rapid folding of the peripheral subunit-binding domain. The results presented here show that the peripheral subunit-binding domain is the third fastest folding protein for which rates have been estimated. The folding rate is the fastest that has been directly measured and provides further support for the importance of chain topology as a major determinant of folding rates.