HIV-1 Nef: Taking Control of Protein Trafficking.

The Nef protein of the human immunodeficiency virus is a crucial determinant of viral pathogenesis and disease progression. Nef is abundantly expressed early in infection and is thought to optimize the cellular environment for viral replication. Nef controls expression levels of various cell surface molecules that play important roles in immunity and virus life cycle, by directly interfering with the itinerary of these proteins within the endocytic and late secretory pathways. To exert these functions, Nef physically interacts with host proteins that regulate protein trafficking. In recent years, considerable progress was made in identifying host-cell-interacting partners for Nef, and the molecular machinery used by Nef to interfere with protein trafficking has started to be unraveled. Here, we briefly review the knowledge gained and discuss new findings regarding the mechanisms by which Nef modifies the intracellular trafficking pathways to prevent antigen presentation, facilitate viral particle release and enhance the infectivity of HIV-1 virions.

Golgi-dependent transport of vacuolar sorting receptors is regulated by COPII, AP1, and AP4 protein complexes in tobacco.

The cycling of vacuolar sorting receptors (VSRs) between early and late secretory pathway compartments is regulated by signals in the cytosolic tail, but the exact pathway is controversial. Here, we show that receptor targeting in tobacco (Nicotiana tabacum) initially involves a canonical coat protein complex II-dependent endoplasmic reticulum-to-Golgi bulk flow route and that VSR-ligand interactions in the cis-Golgi play an important role in vacuolar sorting. We also show that a conserved Glu is required but not sufficient for rate-limiting YXX-mediated receptor trafficking. Protein-protein interaction studies show that the VSR tail interacts with the µ-subunits of plant or mammalian clathrin adaptor complex AP1 and plant AP4 but not that of plant and mammalian AP2. Mutants causing a detour of full-length receptors via the cell surface invariantly cause the secretion of VSR ligands. Therefore, we propose that cycling via the plasma membrane is unlikely to play a role in biosynthetic vacuolar sorting under normal physiological conditions and that the conserved Ile-Met motif is mainly used to recover mistargeted receptors. This occurs via a fundamentally different pathway from the prevacuolar compartment that does not mediate recycling. The role of clathrin and clathrin-independent pathways in vacuolar targeting is discussed.