Envelope lipids regulate the in vitro assembly of the HIV-1 capsid

Abstract

During maturation of type 1 human immunodeficiency virus, a fraction of the capsid protein (CA) molecules in the budding virus particle form a conical capsid. However, the location and role of the remaining CA molecules are unknown. It has been recently reported that the C-terminal domain of CA is able to interact with lipid bilayers, suggesting that the CA molecules that do not form the capsid could be attached to the lipid envelope of the virus. Here, we have studied in vitro the effect of different envelope lipids on the CA polymerization process. Our results show that the negatively charged lipids phosphatidic acid and phosphatidylserine partially inhibit CA polymerization, whereas the nonbilayer forming lipid phosphatidylethanolamine facilitates CA assembly. These results suggest that specific lipids of the viral envelope could have a regulatory role in the maturation of type 1 human immunodeficiency virus.

FIGURE 1

Polymerization of CA in the presence of lipids. (A) Polymerization of CA in the absence of lipids (black), or in the presence of PC (blue), PS (red), PA (orange), and a lipid mixture mimicking the lipid composition of the HIV-1 envelope (green) (9). (B) Polymerization of CA in the absence of lipids (black), or in the presence of a mixture of 33% Cho, 33% PC, and 33% SM (magenta) or 25% PE and 75% PC (green). CA concentration was 22 μM in all cases. Phosphate buffer 50 mM, NaCl 2.25 M, pH 8, was used. Lipid small unilamellar vesicles, 0.3 mM, were employed, and the blanks subtracted. Different CA stocks were employed in panels A and B, resulting in the small differences observed for polymerization in buffer. Each curve is the average of at least three experiments. Errors were smaller than 5%.

FIGURE 2

Transmission electron microscopy of CA polymers assembled in the presence of 25% PE-75% PC. Samples were treated with glutaraldehyde 0.1% for 30 min., deposited on ionized Formvar/carbon-coated copper grids, negatively stained with uranyl acetate 2%, and visualized in a JEM-1010 microscope. CA concentration was 60 μM. Lipid prepared as small unilamellar vesicles, and at 0.1 mM, were employed. The scale bar represents 100 nm.