The process of folding proteins into membranes: Challenges and progress

Just 25 years ago the Anfinsen thermodynamic hypothesis was shown to be valid for membrane proteins. Despite the complex biological machinery required for their in vivo assembly and in face of the chemically heterogeneous, anisotropic nature of their biological lipid bilayer “solvent”, the evidence continues to suggest that membrane proteins are equilibrium structures. The progress in finding conditions in vitro to investigate the physical origins of their stabilities is the focus of this article. We catalogue in vitro folding studies in detergent micelles and in lipid bilayers. We consider the unique technical obstacles to folding studies of membrane proteins, and we highlight the progress that has been made in quantitative descriptions of membrane protein stability.