More than one door - Budding of enveloped viruses through cellular membranes

Abstract

Enveloped viruses exit their host cell by budding from a cellular membrane and thereby spread from one cell to another. Virus budding in general involves the distortion of a cellular membrane away from the cytoplasm, envelopment of the viral capsid by one or more lipid bilayers that are enriched in viral membrane glycoproteins, and a fission event that separates the enveloped virion from the cellular membrane. While it was initially thought that virus budding is always driven by viral transmembrane proteins interacting with the inner structural proteins, it is now clear that the driving force may be different depending on the virus. Research over the past years has shown that viral components specifically interact with host cell lipids and proteins, thereby adopting cellular functions and pathways to facilitate virus release. This review summarizes the current knowledge of the cellular membrane systems that serve as viral budding sites and of the viral and cellular factors involved in budding. One of the best studied cellular machineries required for virus egress is the ESCRT complex, which will be described in more detail.

Figure 1

Various membrane systems are implicated in budding of enveloped viruses. Many enveloped viruses (e.g. retroviruses, paramyxoviruses, orthomyxoviruses, arenaviruses, filoviruses, coronaviruses and some rhabdoviruses) bud from the plasma membrane of infected cells (depicted in light blue). In the case of the retrovirus HIV‐1, the limiting membrane of multivesicular bodies has also been implicated in virus budding, and it was proposed that viruses exit cells in an exosome‐like fashion. Many other viruses bud into the lumen of organelles along the secretory pathway (ER, IC, Golgi and TGN). In such cases, viruses are often transported within secretory vesicles to the plasma membrane, where they are finally released from the cell (red, brown, black). Members of the Flaviviridae family bud on membranes of the ER or ER‐to‐Golgi IC, resulting in accumulation of enveloped virus particles in the lumen of the IC, followed by maturation in the Golgi or TGN, respectively (black). Similarly, Foamy Viruses, ‘unusual’ retroviruses, were proposed to bud into the lumen of the ER, from where enveloped particles are transported through the Golgi and TGN to the plasma membrane (red). Most DNA viruses replicate in the nucleus, and therefore have to cross the NE prior to budding at cellular membranes. In the case of Herpesviruses (brown), the viruses exit the nucleus by budding on the NE, followed by release of non‐enveloped viral capsids from the ER into the cytoplasm. In a second budding step, herpesviruses may bud on TGN‐derived vesicles, from which viruses are finally released. Vaccinia virus (pink), the prototype member of the Poxvirus family, is consecutively enveloped by several membrane layers, which are presumably derived from the ER and the TGN, respectively. NE‐nuclear envelope. ER‐endoplasmic reticulum. IC‐intermediate compartment. TGN‐trans‐Golg network. MVB‐multivesicular body.

Figure 2

Enveloped virus budding is driven by ‘pull’‐ or ‘push’‐mechanisms. (A) Viral membrane glycoproteins assemble on the cellular membrane that serves as budding site, thereby creating a ‘pulling’ force that drives membrane curvature and bud formation. (B) Inner viral structural proteins (I) or pre‐assembled viral nucleocapsids (II) attach to the cytoplasmic side of the budding site and create a ‘pushing’ force that drives membrane curvature and bud formation. (C) Pushing and pulling forces can act in concert to drive virus budding.

Figure 3

ESCRT plays a role in the formation of intralumenal MVB vesicles and in retroviral budding. (A) The endosomal sorting complex required for transport (ESCRT) functions in sorting of ubiquitinated cell surface receptors destined for degradation into the ILV of MVB. For several retroviruses (as well as other viruses – see text), the topologically equal process of virus budding (away from the cytoplasm) was shown to depend on ESCRT as well. Viral Gag proteins were shown to bind ESCRT, thereby recruiting it to the budding site. (B) ESCRT subcomplexes are sequentially recruited to the retroviral budding site. ESCRT‐0 binding to membranes is facilitated by the binding to PI(3)P and ubiquitinated proteins such as retroviral Gag, respectively. This is followed by recruitment of ESCRT‐I and/or AIP1/ALIX by retroviral proteins. AIP1/ALIX binds both ESCRT‐I and ‐III, which finally recruits the ATPase VPS4. VPS4 ATPase activity is believed to be required for the release of ESCRT‐III into the cytosol and to facilitate budding of the virus. The ESCRT‐II subcomplex is clearly required for ILV formation, but its involvement in virus budding is unclear. Ub, ubiquitin. MVB, multivesicular body.