Biophys J, April 2001, p. 1837-1850, Vol. 80, No. 4

Structural Studies of the HIV-1 Accessory Protein Vpu in Langmuir Monolayers: Synchrotron X-ray Reflectivity

Songyan Zheng,^* Joseph Strzalka,^* Che Ma,^* Stanley J. Opella,^* Benjamin M. Ocko, and J. Kent Blasie^*

 ^*Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, and  Department of Physics, Brookhaven National Laboratory, Upton, New York 11973, USA

Vpu is an 81 amino acid integral membrane protein encoded by the HIV-1 genome with a N-terminal hydrophobic domain and a C-terminal hydrophilic domain. It enhances the release of virus from the infected cell and triggers degradation of the virus receptor CD4. Langmuir monolayers of mixtures of Vpu and the phospholipid 1,2-dilignoceroyl-sn-glycero-3-phosphocholine (DLgPC) at 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. Analysis of the x-ray reflectivity data by both slab model-refinement and model-independent box-refinement methods firmly establish the monolayer electron density profiles. The electron density profiles as a function of increasing Vpu/DLgPC mole ratio at a constant, relatively high surface pressure indicated that the amphipathic helices of the cytoplasmic domain lie on the surface of the phospholipid headgroups and the hydrophobic transmembrane helix is oriented approximately normal to the plane of monolayer within the phospholipid hydrocarbon chain layer. At maximal Vpu/DLgPC mole ratio, the tilt of the transmembrane helix with respect to the monolayer normal decreases with increasing surface pressure and the conformation of the cytoplasmic domain varies substantially with surface pressure.

Biophys J, April 2001, p. 1837-1850, Vol. 80, No. 4
© 2001 by the Biophysical Society   0006-3495/01/04/1837/14  $2.00

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