Structure and Dynamics of the HIV-1 Vpu Transmembrane Domain Revealed by Solid-State NMR with Magic-Angle Spinning

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Abstract
We report solid-state nuclear magnetic resonance (NMR) measurements on the peptide Vpu(1-40), comprising residues 1-40 of the 81-residue type 1 integral membrane protein Vpu encoded by the HIV-1 genome. On the basis of a combination of 13C and 15N NMR chemical shifts under magic-angle spinning (MAS), effects of local mobility on NMR signal intensities, site-specific MAS NMR line widths, and NMR-detected hydrogen-deuterium exchange, we develop a model for the structure and dynamics of the Vpu(1-40) monomer in phospholipid bilayer membranes. Our data are largely consistent with earlier structural studies of Vpu peptides by Opella and co-workers, in which solution NMR and solid-state NMR without MAS were used, but our data provide new information about local variations in the degree of mobility and structural order. In addition, our data indicate that the transmembrane alpha-helix of Vpu(1-40) extends beyond the hydrophobic core of the bilayer. We find no evidence for heterogeneity in the conformation and intermolecular contacts of the transmembrane alpha-helix, with the exception of two distinct chemical shifts observed for the C alpha and C beta atoms of A18 that may reflect distinct modes of helix-helix interaction. These results have possible implications for the supramolecular structure of Vpu oligomers that form cation-selective ion channels.