Membrane Binding and Virus Assembly by HIV-1 Gag

	[Enlarge Image] Colocalization of HIV-1 Gag and Env at the plasma membrane

The Gag protein of retroviruses and lentiviruses mediates viral particle assembly and budding from the plasma membrane. In order to function, Gag must be membrane bound. We have shown that a myristate + basic motif mediates binding and targeting of HIV-1 Gag to the plasma membrane. In addition, we have shown that HIV-1 Gag undergoes a myristoyl switch that regulates myristate exposure and consequently membrane binding of HIV-1 Gag.

We are currently investigating the biosynthetic pathways involved in targeting Gag to the plasma membrane. High resolution imaging techniques combined with biochemical pulse/chase analyses have been used to show that Gag initially targets to a perinuclear compartment that fuses with a multivesicular body (MVB)-like structure prior to delivery to the plasma membrane. In some cells, such as macrophages, Gag remains in the MVB, where it assembles and forms viral particles. In other cells, exocytosis of MVBs to the plasma membrane is rapid, and as a result, Gag assembles and buds from the plasma membrane.

The late domain of HIV-1 Gag binds to a host cell protein, Tsg101 that is part of a large complex, termed ESCRT involved in endosomal sorting. Binding of Tsg101 and ESCRT components to Gag is essential for forming a virus particle. We are investigating the consequences of Gag interaction with a vital host cell component. Our recent results indicate that association of Gag with Tsg101 interferes with normal endosomal sorting processes such as EGF Receptor downregulation. As a result, host cell physiology is altered.

	[Enlarge Image] HIV-1 Gag localizes to lipid raft-like domains

The signals within HIV-1 Gag that direct it to specific membranes include the myristate + basic motif at the N-terminus as well as a dileucine-like motif within the CA domain. It is not known how these signals are presented and how their targeting information is coordinated during Gag trafficking and assembly. We are investigating the roles of specific lipids and Gag targeting motifs in directing the sites of Gag localization and viral particle formation.