Expression, purification, and activities of full‐length and truncated versions of the integral membrane protein Vpu from HIV‐1

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Abstract
Vpu is an 81‐residue accessory protein of HIV‐1. Because it is a membrane protein, it presents substantial technical challenges for the characterization of its structure and function, which are of considerable interest because the protein enhances the release of new virus particles from cells infected with HIV‐1 and induces the intracellular degradation of the CD4 receptor protein. The Vpu‐mediated enhancement of the virus release rate from HIV‐1‐infected cells is correlated with the expression of an ion channel activity associated with the transmembrane hydrophobic helical domain. Vpu‐induced CD4 degradation and, to a lesser extent, enhancement of particle release are both dependent on the phosphorylation of two highly conserved serine residues in the cytoplasmic domain of Vpu. To define the minimal folding units of Vpu and to identify their activities, we prepared three truncated forms of Vpu and compared their structural and functional properties to those of full‐length Vpu (residues 2–81). Vpu2–37 encompasses the N‐terminal transmembrane α‐helix; Vpu2–51 spans the N‐terminal transmembrane helix and the first cytoplasmic α‐helix; Vpu28–81 includes the entire cytoplasmic domain containing the two C‐terminal amphipathic α‐helices without the transmembrane helix. Uniformly isotopically labeled samples of the polypeptides derived from Vpu were prepared by expression of fusion proteins in E. coli and were studied in the model membrane environments of lipid micelles by solution NMR spectroscopy and oriented lipid bilayers by solid‐state NMR spectroscopy. The assignment of backbone resonances enabled the secondary structure of the constructs corresponding to the transmembrane and the cytoplasmic domains of Vpu to be defined in micelle samples by solution NMR spectroscopy. Solid‐state NMR spectra of the polypeptides in oriented lipid bilayers demonstrated that the topology of the domains is retained in the truncated polypeptides. The biological activities of the constructs of Vpu were evaluated. The ion channel activity is confined to the transmembrane α‐helix. The C‐terminal α‐helices modulate or promote the oligomerization of Vpu in the membrane and stabilize the conductive state of the channel, in addition to their involvement in CD4 degradation.