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.