New insights into HIV assembly and trafficking

Abstract

Assembly and release of human immunodeficiency virus type 1 (HIV-1) particles is mediated by the viral Gag polyprotein precursor. Gag is synthesized in the cytosol and rapidly translocates to membrane to orchestrate particle production. The cell biology of HIV-1 Gag trafficking is currently one of the least understood aspects of HIV-1 replication. In this review, we highlight the current understanding of the cellular machinery involved in Gag trafficking and virus assembly.

FIGURE 1. Schematic representation of the linear organization of the HIV-1 Gag polyprotein precursor Pr55Gag

The myristylated ( ) matrix (MA) domain, capsid (CA), nucleocapsid (NC), p6 domains, and the spacer peptides SP1 and SP2 are indicated.

FIGURE 2. Cryoelectron micrographs and schematic representations of immature and mature HIV-1 particles

Central slices through cryo-EM tomograms of immature (A) and mature (B) virus particles that are ~130 nm in diameter (from Ref. 73). Schematic representation of immature (C) and mature (D) HIV-1 particles. Parts of figure were adapted from Ref. and used with permission from Elsevier.

FIGURE 3. Schematic representation of the HIV-1 assembly and release associated with different membrane interfaces

A: as per the canonical view, HIV-1 Gag traffics to the inner (cytoplasmic) leaflet of the PM, independent of the cell type, where it initiates virus assembly and budding. B: an alternative proposal is that HIV-1 Gag assembles, in a cell-type-dependent manner, on the cytoplasmic face of intracellular vesicles such as LE/MVBs, and the ensuing virus particles bud into the intraluminal space. The virus-harboring vesicles then traffic to and fuse with the PM (e.g., exosome pathway), thereby resulting in the extracellular release of the virions. C: another model contends that such intracellular compartments in macrophages, where HIV-1 Gag assembles at and buds into, are connected to the PM via nanoscale tubules. The accompanying IA-SEM image shows the transverse section of a virion-containing channel in an HIV-1-infected macrophage (from Ref. 13). D: in another version of the cell-type-dependent HIV-1 assembly and release events, Gag trafficking and assembly are proposed to target virological synapses that promote efficient virus transmission. The accompanying IA-SEM image shows such a synapse, where filopodial extensions of a CD4+ T cell (TC) come in contact with HIV-1 virions that are sequestered within a network of PM-accessible compartments in a dendritic cell (DC) (from Ref. 60). E: in polarized T cells, HIV-1 Gag has been proposed to localize at the PM of uropod structures, which then become the favored contact sites between infected and uninfected T-cells. The accompanying epifluorescence microscope image shows the copatching (yellow) of Gag with the uropod-specific marker CD43 at the uropod of a T cell (brightfield image on top) (from Ref. 141). Parts of figure were adapted from Refs. , , and and used with the authors’ permission.

FIGURE 4. HIV-Gag association with PI(4,5)P2 at the PM

A: the NH2-terminal myristic acid moiety (green) of MA is depicted in its sequestered conformation PI(4,5)P2 embedded in the inner leaflet of the PM lipid bilayer is shown with its 1′ (orange) and 2′ (yellow) acyl chains in the lipid bilayer. B: binding between MA and PI(4,5)P2 leads to the flipping out of the myristate moiety into an exposed conformation and its subsequent insertion into the lipid bilayer, and, based on the model of Saad et al. (207), the extrusion of the 2′ acyl chain from the lipid bilayer and its sequestration by MA. C: according to Saad et al. (207), during the formation of the complex between HIV-1 MA and PI(4,5)P2, the 2′-unsaturated acyl chain of PI(4,5)P2 (yellow) binds to the hydrophobic cleft in MA and the myristyl group (green) of MA inserts into the lipid bilayer (image generously provided by Dr. M. Summers).

FIGURE 5

Schematic representation of topological equivalence of HIV-1 budding and release (A), MVB biogenesis (B), and cytokinesis (C). D: the ESCRT machinery components implicated in HIV-1 budding and release. D was adapted from Ref. and used with permission from Nature Publishing group; for details, see text and Ref .