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 assembly 1 , but yeast 2 and murine cells 3 , 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 fraction 5 , suggesting a requirement for a cellular ATP-binding protein. Here we identify such a protein (HP68), described previously as an RNase L inhibitor 6 , 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.