In macrophages, HIV-1 has been shown to bud into intracellular structures that contain the late endosome marker CD63. We show that these organelles are not endosomes, but an internally sequestered plasma membrane domain. Using immunofluorescence microscopy and immunoelectron microscopy, we find that HIV-1 buds into a compartment that contains the tetraspanins CD81, CD9, and CD53. On uninfected macrophages, these proteins are seen at the cell surface and in intracellular vacuole-like structures with a complex content of vesicles and interconnected membranes that lack endosome markers, including CD63. Significantly, these structures are accessible to small tracers (horseradish peroxidase or ruthenium red) applied to cells at 4°C, indicating that they are connected to the cell surface. HIV assembles on, and accumulates within, these intracellular compartments. Furthermore, CD63 is recruited to the virus-containing structures and incorporated into virions. These results indicate that, in macrophages, HIV-1 exploits a previously undescribed intracellular plasma membrane domain to assemble infectious particles.

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Phosphatidylinositol 4-kinase␤ (PI4K␤) plays an essential role in maintaining the structural integrity of the Golgi complex. In a search for PI4K␤-interacting proteins, we found that PI4K␤ specifically interacts with the GTP-bound form of the small GTPase rab11. The PI4K␤-rab11 interaction is of functional significance because inhibition of rab11 binding to PI4K␤ abolished the localization of rab11 to the Golgi complex and significantly inhibited transport of vesicular stomatitis virus G protein from the Golgi complex to the plasma membrane. We propose that a novel function of PI4K␤ is to act as a docking protein for rab11 in the Golgi complex, which is important for biosynthetic membrane transport from the Golgi complex to the plasma membrane.