Nanomechanics of Membrane Proteins Probed by Atomic Force Microscopy

Abstract

Understanding of how membrane proteins fold in their native lipid bilayers is lacking - a major reason being that membrane proteins aggregate outside the lipid bilayer. Therefore, detergents and lipid vesicles are used to study these important proteins. However, the folding and function of a membrane protein in detergents or in lipid vesicles may not mimic its behavior inside the native lipid bilayer. It is, therefore, crucial that new techniques be employed to study membrane protein (un)folding inside their native lipid bilayers.Atomic force microscope based single-molecule force spectroscopy (SMFS) experiments have provided us with insight into the (un)folding of proteins hitherto inaccessible by conventional methods which probe the bulk behavior of molecules. SMFS has been used to study membrane protein folding inside the protein's native lipid bilayer. Since membrane proteins are inserted vectorially in the lipid membrane in vivo, it is of relevance to probe them by applying a directed force. Mechanically unfolding membrane proteins maps regimes of the energy landscape very different from those mapped in studies with chemical denaturants.SMFS experiments suggest that the mechanical characteristics of a membrane protein are linked to its functional properties. Changing the experimental conditions like temperature, mutations, or using different metal ions changes the mechanical characteristics of α-helical membrane proteins like bacteriorhodopsin and bovine rhodopsin. Recently, for the first time, we have also probed the mechanics of a β-barrel membrane protein, OmpG. Mechanical unfolding of OmpG shows that β-hairpins are more stable than individual β-strands. Moreover, the changes in the intramolecular interactions of OmpG associated with its pH dependent gating can be correlated with its mechanical properties, e.g., rigidity of certain structural segments.In summary, I will talk about the importance of mechanical properties of membrane proteins in (un)folding and function.