Identification of a host protein essential for assembly of immature HIV-1 capsids

To form an immature HIV-1 capsid, 1,500 HIV-1 Gag (p55) polypeptides must assemble properly along the host cell plasma membrane. Insect cells and many higher eukaryotic cell types support efficient capsid assembly1, but yeast2 and murine cells3,4 do not, indicating that host machinery is required for immature HIV-1 capsid formation. Additionally, in a cell-free system that reconstitutes HIV-1 capsid formation, post-translational assembly events require ATP and a subcellular fraction5, suggesting a requirement for a cellular ATP-binding protein. Here we identify such a protein (HP68), described previously as an RNase L inhibitor6, and demonstrate that it associates post-translationally with HIV-1 Gag in a cell-free system and human T cells infected with HIV-1. Using a dominant negative mutant of HP68 in mammalian cells and depletion–reconstitution experiments in the cell-free system, we demonstrate that HP68 is essential for post-translational events in immature HIV-1 capsid assembly. Furthermore, in cells the HP68–Gag complex is associated with HIV-1 Vif, which is involved in virion morphogenesis and infectivity. These findings support a critical role for HP68 in post-translational events of HIV-1 assembly and reveal a previously unappreciated dimension of host–viral interaction.