Solid-state NMR investigations on the structure and topological equilibria of polypeptides associated with biological membranes

Abstract

Peptides and proteins have been labelled with 15N specifically, selectively or uniformly by chemical or biochemical methods and reconstituted into oriented lipid bilayers. Thereafter, the orientation of polypeptide α-helices with respect to the bilayer surface has been determined by proton-decoupled 15N solid-state NMR spectroscopy. Hydrophobic peptides such as the channel-forming domains of Vpu of HIV-1 or M2 of Influenza A adopt stable transmembrane alignments. This orientation is in agreement with models suggesting the transient channel formation by transmembrane helical bundles. The size distribution of such oligomers is dependent on a multitude of experimental parameters. In contrast, a wide variety of peptide antibiotics and other amphipathic α-helices adopt stable orientations along the bilayer surface. For other peptides, equilibria are observed between in-plane and transmembrane topologies. These include designed sequences that change alignment in a pH dependent manner as well as peptides whose lengths do not match the bilayer thickness. A thermodynamic model is presented that describes in-plane-to-transmembrane topological transitions. Topological equilibria are also observed for larger membrane proteins such as some of the pore-forming domains of colicins.