Thermal switching between enhanced and arrested reactivation of bacterial glucose-6-phosphate dehydrogenase assisted by GroEL in the absence of ATP.

Facilitation of protein folding by GroEL usually requires involvement of GroES and ATP. In their absence nascent proteins tend to be arrested on GroEL or, if released, fail to show enhancement of reactivation yield relative to that observed without the chaperonin. In contrast, the yield of reactivation of glucose-6-phosphate dehydrogenase (Glu-6-PDH) from Leuconostoc mesenteroides at 20 degrees C is increased 2-3-fold (to over 80%) by GroEL alone. ATP greatly enhances the rate of GroEL-assisted reactivation and slightly increases its yield to 90%. The efficiency of the GroEL-assisted reactivation of Glu-6-PDH is strongly dependent on temperature. A switch from enhanced to fully arrested reactivation occurs over a narrow temperature range from 25 to 30 degrees C in the presence of GroEL when ATP is absent. At physiological temperature therefore, reactivation is fully arrested by GroEL if ATP is absent and in its presence the protein is released in a form not committed to correct folding. The data shows that the committing step in Glu-6-PDH refolding occurs while the nascent protein is bound to GroEL, a step which is temperature-sensitive. The extreme temperature sensitivity of this step indicates a sharp structural transition in GroEL.