Submillisecond folding of the peripheral subunit-binding domain

Folding and unfolding rates have been measured for the peripheral subunit-b inding domain, a small three-helix protein. The protein folds very fast, wi th rates too rapid to be measured using traditional stopped-now techniques. Folding and unfolding rates were measured as a function of temperature usi ng dynamic NMR lineshape analysis. At the lowest temperature at which there is sufficient broadening to measure rates, 41 degrees C, the folding rate is 16,050 s(-1). Thus, the half time required for folding is 43 mu s. At th e 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 3(10)-helix. Folding rates have been correlated with protein top ology, and this correlation is consistent with the rapid folding of the per ipheral subunit-binding domain. The results presented here show that the pe ripheral subunit-binding domain is the third fastest folding protein for wh ich rates have been estimated. The folding rate is the fastest that has bee n directly measured and provides further support for the importance of chai n topology as a major determinant of folding rates. (C) 1999 Academic Press .