Structural studies of the HIV-1 accessory protein Vpu in Langmuir monolayers: Synchrotron X-ray reflectivity

Vpu is an 81 amino acid integral membrane protein encoded by the HIV-I geno me with:a N-terminal hydrophobic domain and a C-terminal hydrophilic domain . It enhances the release of virus from the infected cell and triggers degr adation of the virus receptor CD4. Langmuir monolayers of mixtures of Vpu a nd the phospholipid 1,2-dilignoceroyl-sn-glycero-3-phosphocholine (DLgPC) a t the water-air interface were studied by synchrotron radiation-based x-ray reflectivity over a range of mole ratios at constant surface pressure and for several surface pressures at a maximal mole ratio of Vpu/DLgPC. Analysi s of the x-ray reflectivity data by both stab model-refinement and model-in dependent box-refinement methods firmly establish the monolayer electron de nsity profiles. The electron density profiles as a function of increasing V pu/DLgPC mote ratio at a constant, relatively high surface pressure indicat ed that the amphipathic helices of the cytoplasmic domain lie on the surfac e of the phospholipid headgroups and the hydrophobic transmembrane helix is oriented approximately normal to the plane of monolayer within the phospho lipid hydrocarbon chain layer. At maximal Vpu/DLgPC mole ratio, the tilt of the transmembrane helix with respect to the monolayer normal decreases wit h increasing surface pressure and the conformation of the cytoplasmic domai n varies substantially with surface pressure.