Lipid Bilayer Induces Contraction of the Denatured State Ensemble of a Helical-Bundle Membrane Protein

Abstract

Defining the denatured state ensemble (DSE) and intrinsically disordered proteins is essential to understanding protein folding, chaperone action, degradation, translocation and cell signaling. While a majority of studies have focused on water-soluble proteins, the DSE of membrane proteins is much less characterized. Here, we reconstituted the DSE of a helical-bundle membrane protein GlpG of Escherichia coli in native lipid bilayers and measured the DSEs conformation and compactness. The DSE was obtained using steric trapping, which couples spontaneous denaturation of a doubly biotinylated GlpG to binding of two bulky monovalent streptavidin molecules. Using limited proteolysis and mass spectrometry, we mapped the flexible regions in the DSE. Using our paramagnetic biotin derivative and double electron-electron resonance spectroscopy, we determined the dimensions of the DSE. Finally, we employed our Upside model for molecular dynamics simulations to generate the DSE including the collapsed and fully expanded states in a bilayer. We find that the DSE is highly dynamic involving the topology changes of transmembrane segments and their unfolding. The DSE is expanded relative to the native state, but only to 55-90% of the fully expanded condition. The degree of expansion depends on the chemical potential with regards to local packing and the lipid composition. Our result suggests that the E. colis native lipid bilayer promotes the association of helices in the DSE of membrane proteins and, probably in general, facilitating interhelical interactions. This tendency may be the outcome of a general lipophobic effect of proteins within the cell membranes. SignificanceHere, we delineate the conformation of the denatured state ensemble (DSE) of a membrane protein confined in a native lipid bilayer and assay whether the bilayer permits full expansion or nonspecific collapse of the DSE. Using the intramembrane protease GlpG as a model, we find that the denatured state is a dynamic ensemble involving topological changes and local unfolding of transmembrane segments. The bilayer tends to contract the DSE relative to the fully lipid-solvated, expanded conformations while the degree of compactness is determined by the balance between protein-lipid, lipid-lipid and protein-protein interactions. These findings provide new insights into the lipid bilayer as a solvent that mediates folding, chaperone action, turnover and protein-protein interactions in cell membranes.