Impact of bacterial chaperonin GroEL–GroES on bacteriorhodopsin folding and membrane integration

Our understanding of molecular chaperone function in membrane protein biogenesis lags far behind that in soluble protein biogenesis. Through a combined approach including isothermal titration calorimetry, UV–Vis spectroscopy, and fluorescence spectroscopy, the behavior of ATP-dependent chaperonin GroEL–GroES, a paradigmatic chaperone of soluble protein folding, was investigated in the refolding of membrane protein bacteriorhodopsin (BR) and its membrane insertion. We found that BR bound asymmetrically to the double-ring GroEL, with a much higher affinity when it was partially denatured. GroEL alone showed a clear influence on BR refolding, but the presence of ATP was necessary to significantly enhance both the rate and yield of the GroEL-mediated folding, in contrast to the adverse effect of GroES on the folding yield. However, synergy between ATP and GroES was shown to be required not only for releasing high-affinity BR species from GroEL, but also for unfolding and rescuing the misfolded conformers complexed to GroEL. This is consistent with the observation that maximum rate enhancement of BR refolding or assembly with the prepared inverted membrane vesicles was achieved when the complete chaperonin system was used. Our results support the iterative unfolding mechanism of GroEL activity previously proposed for soluble proteins, whereby GroEL might perform repeated unfolding and release of BR, thus offering additional opportunities for timely folding or membrane integration. This work provides important information on the convergence of folding of membrane and soluble proteins in light of folding pathways and the role of molecular chaperones.