Intermediates in the folding of the membrane protein bacteriorhodopsin.

Assembly of proteins within lipid bilayers is essential for the biogenesis and function of biological membranes. Little is known, however, about the underlying mechanism of assembly, and it is not clear whether it is possible to observe individual folding steps for integral membrane proteins either in vivo or in vitro. Fluorescence spectroscopy is used here to follow the time course of folding events for bacteriorhodopsin in mixed detergent/lipid micelles. Transient folding-intermediates are detected and binding of the retinal chromophore occurs at a late stage, when it binds to an apoprotein intermediate.