LAUSR

To establish a lasting infection, lentiviruses, such as HIV-1, stably integrate their DNA into the chromatin of host cells. Integration is mediated by the viral integrase enzyme in a multimeric nucleoprotein complex called the intasome, which assembles onto viral DNA ends to form the strand transfer complex that mediates 3′-end processing and the strand transfer reaction. Structural studies of the architecture of the intasome have been hindered by the tendency of the integrase and intasomes to aggregate in vitro; however, Passos et al. and Ballandras-Colas et al. now provide high-resolution models of the HIV-1 and Maedi-Visna virus (MVV) intasomes, respectively, using cryo-electron microscopy. Structural studies of the HIV-1 intasome were facilitated by fusing the HIV-1 integrase to a DNA-binding protein to improve solubility, and MMV integrase was chosen because it does not aggregate in solution. Previous studies had found that several retroviral integrases form dimers and that the HIV-1 integrase forms tetramers; however, both HIV-1 and MMV integrases formed higher-order assemblies of 16 protomers, constituting a hexadecamer, or tetramer of tetramers. Thus, lentiviral intasomes are more intricate and complex than those of other retroviruses. Mutagenesis experiments that blocked the assembly of large integrase assemblies affected strand catalytic activity and virus replication, which suggests that higher-order intasomes are important for efficient DNA integration. Together, these studies provide novel insights into lentiviral integration and a platform for the development of HIV-1 integrase inhibitors.