SERINC5 Inhibits HIV Fusion Through Inactivation of Env Glycoproteins and Interference with Productive Refolding of Env

The multispan membrane proteins, SERINC3 and SERINC5, have been recently shown to incorporate into HIV-1 particles and compromise their ability to fuse with target cells – an effect that is antagonized by the viral accessary protein Nef. Env glycoproteins from different HIV-1 strains exhibit variable levels of sensitivity to SERINC-mediated restriction. The mechanism by which SERINCs interfere with HIV-1 fusion remains unclear. Here, we show by real-time single particle imaging that incorporation of SERINC5 into virions in the absence of Nef inhibits the formation of small fusion pores between viruses and cells. This effect was not related to the SERINC5's ability to oligomerize in the membrane or target the virus to degradation in lysosomes. Strikingly, we found that SERINC5 promotes spontaneous inactivation of sensitive, but not resistant Env glycoproteins, and enhances the exposure of the conserved gp41 domains by delaying the HIV-1 fusion reaction. Super-resolution imaging revealed that SERINC5 also interferes with the formation of Env clusters on mature virions, a step that is thought to be required for efficient HIV-1 fusion. These results show that SERINC5 restricts HIV-1 fusion at a step prior to small pore formation by selectively inactivating sensitive Env glycoproteins and interfering with the function of the remaining active Env, likely by preventing the formation of large Env clusters and slowing down Env refolding. This work was partially supported by the NIH R01 grant GM054787 to G.B.M.