Visualization of retroviral replication in living cells reveals budding into multivesicular bodies.

Retroviral assembly and budding is driven by the Gag polyprotein and requires the host-derived vacuolar protein sorting (vps) machinery. With the exception of human immunodeficiency virus (HIV)-infected macrophages, current models predict that the vps machinery is recruited by Gag to viral budding sites at the cell surface. However, here we demonstrate that HIV Gag and murine leukemia virus (MLV) Gag also drive assembly intracellularly in cell types including 293 and HeLa cells, previously believed to exclusively support budding from the plasma membrane. Using live confocal microscopy in conjunction with electron microscopy of cells generating fluorescently labeled virions or virus-like particles, we observed that these retroviruses utilize late endosomal membranes/multivesicular bodies as assembly sites, implying an endosome-based pathway for viral egress. These data suggest that retroviruses can interact with the vps sorting machinery in a more traditional sense, directly linked to the mechanism by which cellular proteins are sorted into multivesicular endosomes.