Mechanisms for enveloped virus budding: can some viruses do without an ESCRT?

Many enveloped viruses complete their replication cycle by forming vesicles that bud from the plasma membrane. Some viruses encode "late" (L) domain motifs that are able to hijack host proteins involved in the vacuolar protein sorting (VPS) pathway, a cellular budding process that gives rise to multivesicular bodies and that is topologically equivalent to virus budding. Although many enveloped viruses share this mechanism, examples of viruses that require additional viral factors and viruses that appear to be independent of the VPS pathway have been identified. Alternative mechanisms for virus budding could involve other topologically similar process such as cell abscission, which occurs following cytokinesis, or virus budding could proceed spontaneously as a result of lipid microdomain accumulation of viral proteins. Further examination of novel virus-host protein interactions and characterization of other enveloped viruses for which budding requirements are currently unknown will lead to a better understanding of the cellular processes involved in virus assembly and budding.