Functional Bacteriorhodopsin Is Efficiently Solubilized and Delivered to Membranes by the Chaperonin GroEL

Soluble complexes between the tetradecameric chaperonin GroEL and integral membrane proteins can be efficiently formed by detergent dialysis. For example, GroEL14was found to bind a limit of two molecules of bacteriorhodopsin (BR). The GroEL-solubilized BR molecules were rapidly ejected from the chaperonin complexes on the addition of ATP or adenosine 5′[β,γ-imido]triphosphate but not AMP, indicating that conformational changes induced by nucleotide binding eliminate a binding site for the hydrophobic transmembrane domains. BR retains its native conformation in the GroEL complexes, as judged by the spectral characteristics of the bound retinal. Moreover, the chaperonin-solubilized BR could be transferred efficiently to liposomes and used to effect a light-driven proton gradient, indicating that both native conformation and vectorial insertion were accomplished. These results suggest new approaches to the study of purified integral membrane proteins in their natural membrane environment and raise the prospect that GroEL may have a role in the integration of proteins into the cytoplasmic membrane in vivo.