Steric trapping strategy for studying the folding of helical membrane proteins.

Elucidating the folding energy landscape of membrane proteins is essential to the understanding of the proteins' stabilizing forces, folding mechanisms, biogenesis, and quality control. This task has not been trivial because the reversible control of folding is inherently difficult for membrane proteins in a lipid bilayer environment. Recently, novel methods have been developed, each of which has a unique strength in investigating specific aspects of membrane protein folding. Among such methods, steric trapping is a versatile strategy allowing a reversible control of membrane protein folding with minimal perturbation of native protein-water and protein-lipid interactions. In a nutshell, steric trapping exploits the coupling of spontaneous denaturation of a doubly biotinylated protein to the simultaneous binding of bulky monovalent streptavidin molecules. This strategy has been evolved to investigate multiple key elements of membrane protein folding such as thermodynamic stability, spontaneous denaturation rates, conformational features of the denatured states, and cooperativity. In this review, we describe the key methodological advancement, limitation, and outlook of the steric trapping strategy.