Cellular proteins and pathways involved in subviral particle release of foamy viruses

点击翻译按钮获取中文摘要

In several viral systems, different types of virus-like particles (VLPs) are produced depending on which viral proteins are expressed. Orthoretroviruses require only expression of the viral core precursor protein (Gag) for secretion of VLPs, however, in some cases an enhancement is observed upon glycoprotein (Env) coexpression. Foamy viruses (FV) are the only genus of the retrovirus subfamily spumaretrovirinae that show many unique features in their replication strategy and bearing homology to those of hepadnaviruses but setting them apart from orthoretroviruses. FV particle export is special because coexpression of FV Env is strictly required for this process in addition to FV Gag, which by itself only forms non-enveloped capsids in the cytoplasm. This indicates that both Gag and Env contain determinants essential for viral particle egress which is supported by the observation that, similar to hepatitis B virus S-antigen, prototype FV (PFV) Env expression by itself leads to release of capsid-less glycoprotein containing subviral particles (SVPs). We have observed recently that PFV Env, which undergoes a highly unusual biosynthesis, is ubiquitylated at its cytoplasmic domains (CDs) and ubiquitylation regulates the balance between viral and SVP release. In collaboration with the MPI-CBG in Dresden and the Institute of Virology in Erlangen we want to determine the mechanism underlying this regulation by posttranslational modification of the glycoprotein. Therefore the intracellular distribution and trafficking of various PFV Env mutants and ubiquitin fusions thereof as well as the influence of specific proteasome inhibitors on viraland subviral particle release will be studied. In addition we recently characterized a classical PSAP late-assembly (L) domain in PFV Gag linking PFV particle export to the vacuolar protein sorting machinery and multivesicular body that were demonstrated recently to be used by a variety of viruses for particle egress. In contrast, none of the currently known L-domains can be detected in the CDs of PFV Env and it is unclear which cellular interaction partners and pathways are essential for the release of PFV SVPs. In a proteomic approach using highly purified PFV SVPs and pull-downs of GST fusion proteins containing the CDs of PFV Env we want to identify, in collaboration with the MPI-CBG in Dresden and the Institute of Virology in Heidelberg, cellular proteins that are specifically incorporated into SVPs or interact with the glycoprotein CDs during budding and particle release. In addition we will characterize glycoprotein domains and sequences essential for SVP export by mutagenesis analysis of a ubiquitylation-deficient PFV glycoprotein variant that secretes high amounts of SVPs. Taken together the project should lead to the characterization of yet unknown viral sequences with L-domain or related activity, the identification of further cellular proteins or pathways potentially involved in viraland SVP egress and should further the understanding on how posttranslational modifications of viral glycoproteins regulate these processes.