Review
The Role of Cellular Factors in Promoting HIV Budding

https://doi.org/10.1016/j.jmb.2011.04.055Get rights and content

Abstract

Human immunodeficiency virus type 1 (HIV-1) becomes enveloped while budding through the plasma membrane, and the release of nascent virions requires a membrane fission event that separates the viral envelope from the cell surface. To facilitate this crucial step in its life cycle, HIV-1 exploits a complex cellular membrane remodeling and fission machinery known as the endosomal sorting complex required for transport (ESCRT) pathway. HIV-1 Gag directly interacts with early-acting components of this pathway, which ultimately triggers the assembly of the ESCRT-III membrane fission complex at viral budding sites. Surprisingly, HIV-1 requires only a subset of ESCRT-III components, indicating that the membrane fission reaction that occurs during HIV-1 budding differs in crucial aspects from topologically related cellular abscission events.

Introduction

Viruses are obligate intracellular parasites due to their dependence on host cell factors to complete their “life cycle.” Human immunodeficiency virus type 1 (HIV-1) is no exception, as it relies exclusively on cellular machinery to express the components required for the assembly of progeny virions. Furthermore, HIV-1 is exquisitely dependent on cellular factors for its ultimate release from virus-producing cells.

HIV-1 assembly and release are driven by the viral Gag protein, which associates with the inner leaflet of the plasma membrane and oligomerizes into a spherical protein shell that deforms the attached membrane. Ultimately, the growing bud pinches off from the cell surface, leading to the release of an immature virion. During or after budding, Gag is cleaved by the viral protease, which is necessary for the morphological maturation of the virion and for infectivity. The major cleavage products derived from Gag are the matrix protein, the capsid protein, and the nucleocapsid protein (NC). The matrix protein remains associated with the lipid envelope of the virion, the capsid protein rearranges to form the typically cone-shaped mature HIV-1 capsid, and the NC covers the viral RNA genome within the capsid. The processing of HIV-1 Gag by the viral protease also yields a C-terminal peptide called p6, whose location in the mature virion has not yet been definitively determined.1

Because HIV-1 buds from the plasma membrane and thereby acquires a lipid envelope, the membrane covering the Gag shell ultimately must be severed from the plasma membrane to release the nascent virion into the extracellular medium. Although membrane fission could conceivably occur spontaneously once the assembling Gag shell approaches completion, retroviruses such as HIV-1 in fact exploit a cellular membrane fission machinery to facilitate their release. Specifically, the detachment of the virion depends on the host cell's endosomal sorting complex required for transport (ESCRT) pathway, which promotes membrane scission from the cytosolic side of bud necks, such as those formed during retroviral assembly. This review focuses on how HIV-1 engages the ESCRT pathway and on how various components of the pathway contribute to HIV-1 release.

Section snippets

Retroviral Late Domains

The first indication that the release of retroviruses does not occur spontaneously came from a study that shows that the unstructured p6 region of HIV-1 Gag is specifically required for the detachment of budded virions from the cell surface.2 In adherent cells, viral particles lacking p6 assembled at the plasma membrane and initiated the budding process but remained trapped at the cell surface via a membranous tether.2 The phenotype of p6 mutants differs somewhat in T-cell lines and primary

The ESCRT Pathway

The ESCRT pathway exists in all eukaryotes and consists of five heterooligomeric complexes (ESCRT-0, ESCRT-I, ESCRT-II, ESCRT-III, and VPS4), as well as accessory proteins such as ALIX.21 ESCRT complexes are thought to be recruited sequentially to endosomal membranes, although there is evidence that upstream ESCRT complexes may act in parallel.22 ESCRT-0, ESCRT-I, and ESCRT-II are recruited as preformed complexes, whereas the ESCRT-III complex transiently assembles on endosomes from soluble

ESCRT-I

The PTAP L domain of HIV-1 Gag binds directly to TSG101, the central component of ESCRT-I, a stable cytosolic heterotetramer formed by one copy each of TSG101, VPS28, VPS37, and MVB12.21, 30 Human cells express four versions of VPS37 (termed VPS37A–VPS37D) and two versions of MVB12 (termed MVB12A and MVB12B), which are encoded by different genes.30, 31, 32 Thus, eight combinations of TSG101, VPS28, VPS37, and MVB12 are theoretically possible, and all of these can indeed form stable ESCRT-I

ESCRT-II

Together with ESCRT-I, ESCRT-II mediates the deformation of endosomal membranes into buds during MVB biogenesis.23, 39 Yeast ESCRT-II binds to the ESCRT-I component Vps28 and to the ESCRT-III component Vps20 and thereby links ESCRT-I to ESCRT-III.40 Human ESCRT-I and ESCRT-II also interact, albeit in a different manner,41 and human ESCRT-II binds directly to the ESCRT-III component CHMP6.41, 42 Together, these observations indicate that ESCRT-II also connects ESCRT-I to ESCRT-III in humans.

ESCRT-III

The ESCRT-III complex is the main engine that carries out membrane scission in the ESCRT pathway.23, 43 The core ESCRT-III complex is formed by Vps20, SNF7, Vps24, and Vps2 in yeast and by their homologs CHMP6, CHMP4A–CHMP4C, CHMP3, and CHMP2A and CHMP2B in humans. As its yeast ortholog Vps20, human CHMP6 is N-myristoylated and thought to nucleate the oligomerization of CHMP4 proteins on membranes.42, 44, 45 The remaining human ESCRT-III core components CHMP3 and CHMP2 appear to function as

ALIX

Although HIV-1 relies predominantly on a PTAP-type L domain to recruit the CHMP4 fission factor via ESCRT-I, it can also engage CHMP4 in an alternative manner. As noted earlier, HIV-1 p6 harbors a secondary L domain of the LYPxnL type, which functions as a docking site for ALIX, another early-acting factor in the ESCRT pathway.8, 15, 16 ALIX is composed of a banana-shaped Bro1 domain, a V-shaped middle domain, and a presumably unstructured proline-rich C-terminal domain (PRD) that harbors

NEDD4L

PPxY-type L domains serve as docking sites for the WW domains of NEDD4 family ubiquitin ligases and ultimately depend on the ESCRT pathway for function.17, 78 However, how NEDD4-type ubiquitin ligases connect to the ESCRT membrane fission machinery remains unknown. The nine human family members all possess an N-terminal C2 domain involved in membrane binding, multiple WW domains, and a C-terminal catalytic HECT domain. PPxY-mediated virus budding requires the engagement of a NEDD4 family member

Concluding Remarks

Although remarkable progress has been made in identifying the major cellular players involved in HIV-1 release, important questions remain to be addressed. For instance, it remains a mystery how ESCRT-I connects to ESCRT-III during HIV-1 budding, given that ESCRT-II is not required. Also, the mechanism by which ESCRT-III catalyzes membrane fission without being consumed in the reaction remains to be fully determined. Unlike topologically equivalent cellular budding events during yeast MVB

Acknowledgements

Work in the authors' laboratory was supported by grant R37AI029873 from the National Institute of Allergy and Infectious Diseases.

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