THE PRE-MOLTEN GLOBULE, A NEW INTERMEDIATE IN PROTEIN-FOLDING

In vitro folding studies of several proteins revealed the formation, w ithin 2-4 msec, of transient intermediates with a large far-UV ellipti city but no amide proton protection. To solve the contradiction betwee n the secondary structure contents estimated by these two methods, we characterized the isolated C-terminal fragment F2 of the tryptophan sy nthase beta(2) subunit. In beta(2), F2 forms its tertiary interactions with the F1 N-terminal region. Hence, in the absence of F1, isolated F2 should remain at an early folding stage with no long-range interact ions. We shall show that isolated F2 folds into, and remains in, a ''s tate'' called the pre-molten globule, that indeed corresponds to a 2- to 4-msec intermediate. This condensed, but not compact, ''state'' cor responds to an array of conformations in rapid equilibrium comprising native as well as nonnative secondary structures. It fits the ''new vi ew'' on the folding process.