How complex cytoplasmic membrane proteins insert and fold into cellular membranes is not fully understood. One problem is the lack of suitable approaches that allow investigating the process by which polypeptides insert and fold into membranes. Here, we introduce a method to mechanically unfold and extract a single polytopic α-helical membrane protein, the lactose permease (LacY), from a phospholipid membrane, transport the fully unfolded polypeptide to another membrane and insert and refold the polypeptide into the native structure. Insertion and refolding of LacY is facilitated by the transmembrane chaperone/insertase YidC in the absence of the SecYEG translocon. Insertion into the membrane occurs in a stepwise, stochastic manner employing multiple coexisting pathways to complete the folding process. We anticipate that our approach will provide new means of studying the insertion and folding of membrane proteins and to mechanically reconstitute membrane proteins at high spatial precision and stoichiometric control, thus allowing the functional programming of synthetic and biological membranes.
Keywords: Membrane protein insertion and folding; YidC insertase; atomic force microscopy; folding pathway; mechanical reconstitution; single-molecule force spectroscopy.