Transmembrane Helical Protein Folding: Lipid Modulation and Folding Transition States

Abstract

General folding principles have emerged from studies on water-soluble proteins, but it is unclear how these ideas will translate to transmembrane proteins, which expose rather than hide their hydrophobic surfaces. We combine kinetic and thermodynamic studies of the reversible unfolding of helical membrane proteins to provide a definitive value for the reaction free energy and a means to probe the transition state. Efficient systems also need to be developed to stabilise, unfold and re-fold a wider range of alpha helical membrane proteins. We have been developing in vitro lipid bicelle and bilayer folding systems for membrane proteins. Bicelle properties, as well as the stored curvature elastic stress of model bilayers can be used to optimise the rate, yield and stability of folded protein. We have shown that events such as transmembrane helix insertion, as well as tertiary and quaternary structure formation are altered by the stored curvature stress of the bilayer. Either changing the lipid chains or introducing a different headgroup such as phosphoethanolamine alters the curvature stress of a phosphatoidylcholine lipid bilayer. Our results on a variety of monomeric, multidomain and multi-subunit proteins will be described.