Correlation of the structural and functional domains in the membrane protein Vpu from HIV-1

Vpu is an 81-residue membrane protein encoded by the HIV-1 genome. NMR expe riments show that the protein folds into two distinct domains, a transmembr ane hydrophobic helix and a cytoplasmic domain with two in-plane amphipathi c alpha-helices separated by a linker region. Resonances in one-dimensional solid-state NMR spectra of uniformly N-15 labeled Vpu are clearly segregat ed into two bands at chemical shift frequencies associated with NH bonds in a transmembrane alpha-helix, perpendicular to the membrane surface, and wi th NH bonds in the cytoplasmic helices parallel to the membrane surface. So lid-state NMR spectra of truncated Vpu(2-51) (residues 2-51), which contain s the transmembrane alpha-helix and the first amphipathic helix of the cyto plasmic domain, and of a construct Vpu(28-81) (residues 28-81), which conta ins only the cytoplasmic domain, support this structural model of Vpu in th e membrane. Full-length Vpu (residues 2-81) forms discrete ion-conducting c hannels of heterogeneous conductance in lipid bilayers. The most frequent c onductances were 22 +/- 3 pS and 12 +/- 3 pS in 0.5 M KCl and 29 +/- 3 pS a nd 12 +/- 3 pS in 0.5 M NaCl. In agreement with the structural model, trunc ated Vpu(2-51), which has the transmembrane helix, forms discrete channels in lipid bilayers, whereas the cytoplasmic domain Vpu(28-81), which lacks t he transmembrane helix, does not. This finding shows that the channel activ ity is associated with the transmembrane helical domain. The pattern of cha nnel activity is characteristic of the self-assembly of conductive oligomer s in the membrane and is compatible with the structural and functional find ings.