Viruses as vesicular carriers of the viral genome: a functional module perspective

Abstract

Enveloped viruses and cellular transport vesicles share obvious morphological and functional properties. Both are composed of a closed membrane, which is lined with coat proteins and encases cargo. Transmembrane proteins inserted into the membrane define the target membrane area with which the vesicle or virus is destined to fuse. Here we discuss recent insight into the functioning of enveloped viruses in the framework of the "functional module" concept. Vesicular transport is an exemplary case of a functional module, as defined as a part of the proteome that assembles to perform a specific autonomous function in a living cell. Cellular vesicles serve to transport cargo between membranous organelles inside the cell, while enveloped viruses can be seen as carriers of the viral genome delivering their cargo from an infected to an uninfected cell. The turnover of both vesicles and viruses involves an analogous series of submodular events. This comprises assembly of elements, budding from the donor compartment, uncoating and/or maturation, docking to and finally fusion with the target membrane to release the cargo. This modular perception enables us to define submodular building blocks so that mechanisms and elements can be directly compared. It will be analyzed where viruses have developed their own specific strategy, where they share functional schemes with vesicles, and also where they even have "hijacked" complete submodular schemes from the cell. Such a perspective may also include new and more specific approaches to pharmacological interference with virus function, which could avoid some of the most severe side effects.

Fig. 1

Cellular coated vesicles and enveloped viruses: basic composition. Both vesicles and viruses contain a membrane bilayer (thin black circle) derived from the donor membrane, which is lined by a coat (red circle) assembled from soluble, monomeric subunits. Inserted into the bilayer are transmembrane proteins (blue) required for targeting of the vesicle or virus. The interior contains cargo (grey ellipse), either protein or the viral genome. Insets: EM-pictures of COP I vesicles (left, by courtesy of Christoph Rutz and Britta Brügger, Biochemiezentrum, Heidelberg) and of influenza viruses (right, recreated 1918 influenza virus particles, taken from the Centers for Disease Control and Prevention's Public Health Image Library, identification number #8160). Note that the scheme is a drastic simplification to compare analogue structures in viruses and vesicles. Especially the term “coat” often comprises a multitude of different proteins, which might also have additional functions.

Fig. 2

Functional input/output cycles. The figure illustrates the notion of enveloped viruses as “vesicular carriers of the viral genome”. The input into both vesicular and viral transport modules is the uptake of cargo, either protein or the viral genome, from a donor compartment, the output is the release of cargo into another membrane-encased acceptor compartment. The individual steps of vesicle transport and virus replication are shown to follow a similar sequence of submodular events, comprising assembly of elements, budding from the donor membrane, uncoating of vesicles or maturation of viruses, tethering and docking of the vesicle or virus to the acceptor compartment, membrane fusion, and cargo release and uncoating. Note that some viruses bud into the lumen of membranous organelles of the exocytic pathway (ER/Golgi) and are subsequently secreted by the cell. Likewise, some viruses enter the cell via the endocytic pathway and fuse with the membrane of early and late endosome or with vesicles that transport cargo between them. Maturation of viruses often occurs also inside the cell, e.g. in the exocytic pathway or after endocytic uptake. Furthermore, for herpes viruses the term “maturation” refers to the acquisition of tegument and envelope by the nucleocapsid, which originates from budding of capsid through the inner nuclear membrane and their subsequent fusion with the outer nuclear membrane. See text for details.

Fig. 3

Example of an input submodule: assembly of influenza virus. Molecular level: molecular interactions (left; red arrows) between the viral RNA, the three polymerase proteins (PA, PB1, PB2) and the structural protein NP build the viral ribonucleoprotein-particle (vRNP). The glycoprotein HA (right) forms trimers, which associate with rafts in the plasma membrane. The coat protein M1 associates with membranes, where it binds weakly to the cytoplasmic tail of HA; these molecular interactions are illustrated by green arrows. Submodular level: oligomerization of M1 strengthens the weak interactions with HA and draws M1 to the viral budzone, preassembled viral envelopes. This submodular entity contains also the second viral glycoprotein neuraminidase (NA) and a few copies of the viral proton channel M2. Finally, the vRNPs bind to M1, a complete virus particle buds from the membrane and is released. The hallmarks of functional modules (grey lettering) are described in the text, 3.1, 3.2.

Fig. 4

Example of an output submodule: cell entry and disassembly of influenza virus. Submodular level: (A) The virus particle docks to a receptor on the target cell, thus causing endocytosis of the receptor with bound particle. Molecular details of receptor binding, framed in the green box, are shown in the lower part of the figure. (B) The low pH in the endosome (arising from ATPases in the endosomal membrane) activates HA, which catalyzes fusion of viral and endosomal membrane (red box, see the blow up at the bottom). The viral membrane contains the proton channel M2 which acidifies the interior of the virus particle, causing displacement of M1 from vRNPs (switch). vRNPs are released through the fusion pore into the cytoplasm of the cell. (C) In the nucleus, the compact structure of the vRNPs is partially disassembled allowing synthesis of mRNA and vRNA. The hallmarks of functional modules (grey lettering) are described in the text, 3.4, 3.5, 3.6. Molecular level: green box: interaction of HA with a sialic acid containing receptor. The peptide-backbone of HA (blue) with the amino acids (cyan) involved in recognition of the substrate sialic acid-galactose-N-acetyl-glucosamine (orange). Figure modified with Pymol from pdb file 2WR7, . Red box: conformational changes in HA leading to fusion of the viral with the endosomal membrane. The fusion peptide (red) becomes exposed on the molecule's surface after acidification (a) and inserts into the cellular membrane (b). A second conformational change then bends HA (c), which leads to hemifusion with exchange of lipids (d) and opening of a fusion pore (e). For clarity, the receptor-binding HA₁ subunit (grey) is omitted in b to e. Picture modified from Ref. .