The Envelope Glycoproteins of Gammaretroviruses and Type-D Betaretroviruses are Tetherin Antagonists

Abstract

Tetherin/BST2 is an interferon-inducible antiviral factor that restricts the egress of numerous enveloped viruses including HIV-1. Consequently, many viruses have evolved mechanisms to actively or passively evade restriction by tetherin. Most studies conducted to date focused on the tetherin-evasion mechanism of complex retroviruses like HIV and SIV, which encode accessory proteins like Vpu and Nef respectively to counteract tetherin-mediated restriction. However, there is a wide gap in knowledge in understanding how simple retroviruses (that includes alpharetroviruses, some betaretroviruses and gammaretroviruses) that lack obvious accessory proteins like HIV-1 Vpu and SIV-Nef, evade restriction by tetherin. In this dissertation, I have established that Simian retrovirus type-3, a prototypical type-D betaretrovirus, isolated from Asian macaques, is restricted by human tetherin but not by rhesus macaque tetherin. This differential sensitivity indicated that SRV-3 has a mechanism to evade tetherin-mediated restriction. I have identified the SRV-3 envelope (Env) glycoprotein as the viral determinant of tetherin antagonism, and have also found that SRV-3 envelope expression in-trans was sufficient to rescue a heterologous virus from tetherin. SRV-3 Env resulted in cell-surface down-modulation of rhesus tetherin, and this mechanism of tetherin-antagonism is independent of the SRV-3 Env trafficking pathway. The target specificity of SRV-3 Env overlapped a stretch of five residues (G14DIWK18) in the rhesus tetherin cytoplasmic tail that are absent from human tetherin. Additionally, I was able to show that SRV-3 Env physically interacts with rhesus tetherin by targeting the G14DIWK18 motif. SRV-3 belongs to a large supergroup of retroviruses, called the RDR Interference Supergroup. Due to this reason, I screened additional RDR envelope glycoproteins for their ability to antagonize a panel of tetherin homologs. All the RDR envelopes tested were sensitive to human tetherin but exhibited anti-tetherin activity when tested against a panel of tetherin homologs from squirrel monkey, baboon, dog and cat. I also found that several non-RDR gammaretroviral envelope glycoproteins also have anti-tetherin function. Thus, tetherin-antagonism is not just restricted to the envelope glycoproteins of retroviruses in the RDR interference supergroups but extends to other non-RDR gammaretroviruses as well. To my knowledge, this is the first characterization of gamma-type envelopes as tetherin antagonists. Thus, in the absence of a dedicated tetherin antagonist, many simple retroviruses in the beta- and gammaretrovirus genera may evade tetherin-mediated restriction through neo-functionalization of their envelope glycoproteins. We speculate that the evolutionary success of the gamma-type envelope may be due, at least in part, to this anti-tetherin function.