The host proteins SERINC3 and SERINC5 are HIV-1 restriction factors that reduce infectivity when incorporated into the viral envelope. The HIV-1 accessory protein Nef abrogates incorporation of SERINCs via binding to intracellular loop 4 (ICL4). CryoEM maps of full-length human SERINC3 and an ICL4 deletion construct reveal that hSERINC3 is comprised of twoα- helical bundles connected by a ∼40-residue, tilted, “crossmember” helix. The design resembles non-ATP-dependent lipid transporters. Consistently, purified hSERINCs reconstituted into proteoliposomes flip phosphatidylserine (PS), phosphatidylethanolamine and phosphatidylcholine. SERINC3 and SERINC5 reduce infectivity and expose PS on the surface of HIV-1 and also MLV, which is counteracted by Nef and GlycoGag, respectively. Antiviral activities by SERINCs and the scramblase TMEM16F correlate with the exposure of PS and with altered conformation of the envelope glycoprotein. We conclude that SERINCs are lipid transporters, and we demonstrate that lipid flipping is directly correlated with loss of infectivity.One Sentence SummaryThe HIV-1 restriction factor SERINC3 has a molecular design similar to non-ATP dependent lipid transporters, a function supported by the observation of flipping activity in proteoliposomes and exposure of phosphatidylserine on HIV-1 and MLV particles, which is correlated with loss of infectivity.
