Organization of HIV-1 capsid proteins on a lipid monolayer

In an in vitro system that mimics the assembly of immature human immunodeficiency virus (HIV) particles, ordered arrays of HIV-1 capsid (CA) proteins encoded by the viral gag gene have been obtained by incubation of histidine-tagged capsid proteins (HisHIVCA) beneath lipid monolayers containing the nickelchelating lipid, 1,2-di-O-hexadacyl-sn-glycero-3-(1'- 2'-R-hydroxy-3'-N-(5-amino-1-carboxypentyl)iminodiacetic acid)propyl ether. The membrane-bound His-HIVCA proteins formed small crystalline arrays of primitive (pl) unit cells with dimensions of a = 74.2 Å, b = 126.2 y = 89.3°. The image-analyzed two-dimensional projection of His-HIVCA assemblies shows a cage-like lattice, consisting of hexamer and trimer units, surrounding protein-free cage holes. The hexamer-coordinated cage holes of 26.3-Å diameter are spaced at 74.2-Å intervals: these distances, and the hexamer-trimer arrangement, are consistent with previous, lower resolution studies on immature HIV-1 virus particles produced in vivo. Additionally, HIV-1 matrix protein trimer unit structures align to the His-HIVCA trimer units such that residues previously shown to interact with the HIV-1 gp120/gp41 envelope protein complex are oriented toward the hexamer cage holes. Our results form a bridge between results from conventional methods for the analysis of HIV particle structure.