Dissecting the molecular mechanisms accounting for the Feline Immunodeficiency Virus Envelope Glycoprotein ability to antagonize feline tetherin restriction

Humans and other mammals have evolved different antiviral factors to defend themselves from retroviral infection. Among these, the so-called “host restriction factors” are host cellular proteins constitutively expressed or induced by interferon in response to viral infection. The species-specific expression and activity of these cellular factors limit viral host tropism and constitute a barrier to cross-species transmission events. In order to replicate, retroviruses need to overcome restriction factors and, thus, have evolved countermeasures to antagonize them. Recently, it has been reported that tetherin (BST2) is the cellular protein that blocks the particle release of some enveloped viruses. Given the relevance of the Feline Immunodeficiency Virus (FIV) model, our research group has recently identified and characterized a feline tetherin protein (cBST2) and demonstrated that the FIV Envelope glycoprotein is the viral gene product by which FIV counteracts tetherin restriction. With the aim to dissect the molecular basis of FIV Env/cBST2 functional interactions, we generated different FIV Env mutants and we tested its ability to overcome cBST2 restriction. We investigated by confocal analysis whether FIV Env affects cBST2 localization and/or sequesters it into specific intracellular compartments. Furthermore, cBST2 posttranslational modifications potentially relevant for its antiviral activity were also examined. Here we show that i) FIV Env specifically interacts with feline tetherin; ii) important motif within FIV Env are not involved in such an interaction but have a role in the Env ability to counteract cBST2; iii) cBST2 antiviral activity is strictly dependent on its glycosylation, which is essential for feline tetherin localization to the cell plasma membrane; iii) Env expression leads to a decrease in the cell surface levels of cBST2, without affecting its total amount. Overall our data suggest that cBST2 and FIV Env correct localization to the cell membrane is required for their functional interaction. Dissecting the role of BST2 in the innate immune response to viruses might contribute to the elucidation of cellular mechanisms relevant for tetherin antagonism in the viral pathogenesis.