Biophysical investigations of membrane perturbations by polypeptides using solid-state NMR spectroscopy (review)

Polypeptides have been prepared by solid-phase peptide synthesis and labelled with 15N at single sites to be used for static or magic angle spinning solid-state NMR spectroscopy. After reconstitution into oriented membranes, the alignment of polypeptide alpha-helices with respect to the bilayer surface is accessible by proton-decoupled 15N solid-state NMR spectroscopy. In addition, limiting values of rotational diffusion coefficients are obtained. The effects of membrane inserted peptides on the bilayer phospholipids have been investigated by 2H and 31P solid-state NMR spectroscopy. Long hydrophobic peptides such as the channel-forming domains of Vpu of HIV-1 or M2 of influenza A adopt stable alignments approximately parallel to the bilayer normal in agreement with models suggesting transmembrane helical bundle formation. The 15N chemical shift data agree with tilt angles of approximately 20 degrees and 33 degrees, respectively. In contrast, multi-charged amphipathic alpha-helices adopt stable orientations parallel to the bilayer surface. In the presence of these peptides, decreased order parameters of the fatty acyl chains, membrane thinning, and the loss of long-range order are observed. Peptides that change topology in a pH dependent manner are more potent in antibiotic assays under experimental conditions where they show in-plane alignments. This result suggests that their detergent-like properties, rather than the formation of transmembrane helical bundles, are responsible for their cell-killing activities. Topological equilibria are also observed within proteins or for polypeptides that do not match the hydrophobic thickness of the bilayer.