Subviral Particle Release Determinants of Prototype Foamy Virus

Abstract

Glycoproteins of several viruses have the capacity to induce release of noninfectious, capsidless particulate structures containing only the viral glycoprotein. Such structures are often called subviral particles (SVP). Foamy viruses (FVs), a special type of retroviruses with a replication strategy combining features of both orthoretroviruses and hepadnaviruses, express a glycoprotein (Env) which has the ability to induce SVP release. However, unlike human hepatitis B virus, prototype FV (PFV) naturally secretes only small amounts of SVPs, because ubiquitination of the Env protein seems to suppress the intrinsic capacity for induction of SVP release. In this study, we characterized the structural determinants influencing PFV SVP release, examined the role of specific Env ubiquitination sites in the regulation of this process, and analyzed the requirement of the cellular vacuolar protein sorting (VPS) machinery for SVP egress. We observed that the cytoplasmic and membrane-spanning domains of both the leader peptide (LP) and the transmembrane (TM) subunit harbor essential as well as inhibitory domains. Furthermore, only ubiquitination at the most N-terminal lysine residues (K 14 and K 15 ) in LP reduced cell surface expression and suppressed SVP release to wild-type levels. This suggests that interaction of Env with cellular components required for SVP release suppression is effective only when Env is ubiquitinated at these lysine residues but not at others. Finally, SVP release was sensitive to dominant-negative mutants of late components, but not early components, of the cellular VPS machinery. PFV therefore differs from hepatitis B virus in using the same cellular pathway for egress of both virions and SVPs.Amplification and generation of new progeny viruses require the precise temporal and spatial coordination of multiple processes within the infected host cell. Not only viral but also cellular components, hijacked by the viruses, are essential for viral replication. Cellular mechanisms exploited by viruses for this purpose include cytoskeleton structures, transport machineries, and signaling pathways (reviewed in references 16 and 43). A lot has been learned in the past about the roles and functions of different viral components for these processes. However, we are only at the beginning of understanding the contributions of different cellular machineries and metabolic pathways involved in viral replication.One of the final steps in production of membrane-enveloped progeny viruses is the release of virions from infected cells, which is accomplished by budding of capsid structures across cellular membranes. In several viral systems, different types of virus-like particles (VLPs) are produced depending on which viral proteins are expressed, and a clear synergy in virus release efficiency is observed upon coexpression of multiple viral proteins. In some cases, the viral glycoproteins are actively involved in viral particle release and either are essential for or enhance this process. In the replication cycles of ...

Structure of a Spumaretrovirus Gag Central Domain Reveals an Ancient Retroviral Capsid

“…Protein samples from cellular lysates or purified particulate material were separated by SDS-PAGE on a 10% polyacrylamide gel and analyzed by immunoblotting as described previously [98]. Polyclonal rabbit antisera specific for PFV Gag [99] or residues1 to 86 of the PFV Env leader peptide (LP), [98] as well as hybridoma supernatants specific for PFV PR-RT (clone 15E10) or PFV integrase (IN) (clone 3E11) [100] were employed. After incubation with species-matched horseradish peroxidase (HRP)-conjugated secondary antibody, the blots were developed with Immobilon Western HRP substrate.…”

Analysis of Prototype Foamy Virus particle-host cell interaction with autofluorescent retroviral particles

“…Polyclonal antisera used were specific for PFV Gag [54] or the LP of PFV Env, aa 1 to 86 [53]. The chemiluminescence signal was digitally recorded using a LAS-3000 imager.…”

Non-Simian Foamy Viruses: Molecular Virology, Tropism and Prevalence and Zoonotic/Interspecies Transmission

“…Besides the well-known degradation process induction ubiquitination in PFV therefore exhibits a second function in the context of viral egress and infectivity. More recent analysis by the same group revealed that only the first two lysine residues are important with regard to ubiquitination and suppress SVP release to wild type levels [81]. As FV-Env is, in contrast to all other retroviruses capable of budding independent of any other FV protein, this mechanism might tightly regulate SVP release, at least in PFV.…”