Folding intermediates of beta-lactamase recognized by GroEL.

beta-Lactamase, from which the disulfide bond was removed by two Cys-->Ala mutations, forms stable complexes with GroEL only during the first 30 s of folding, while wild-type beta-lactamase forms no stable complex under these conditions. The 3-phasic kinetics of folding are very similar between wild-type and mutant. After 4 s, Trp-210 is already juxtaposed to the disulfide bond, but proline cis-trans isomerization has not yet taken place and almost no enzymatic activity is observed. This shows that GroEL is unable to bind late folding intermediates and also discriminates between the degree of unfolding possible in wild-type disulfide-containing beta-lactamase and the Cys-Ala mutant.