GroEL accelerates the refolding of hen lysozyme without changing its folding mechanism

The chaperonin GroEL binds folding intermediates of four-disulfidehen lysoz yme transiently within its central cavity, Using stopped flow fluorescence we show that GroEL binds early intermediates in folding and accelerates the slow kinetic phase that reflects the reversal of non-native interactions i nvolving tryptophan residues and the formation of the native state. Pulsed hydrogen exchange monitored by electrospray ionization mass spectrometry de monstrates that GroEL does not alter the folding mechanism, nor are protect ed species unfolded by the chaperonin. The data suggest a mechanism for Gro EL-assisted folding in which the reorganization of non-native tertiary inte ractions is facilitated but domain folding is unperturbed.