Abstract
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 two α- 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 Summary The 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.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
↵* Lead author: Mark Yeager, M.D., Ph.D., The Phillip and Patricia Frost Institute for Chemistry and Molecular Science 1201 Memorial Drive, University of Miami Miami, FL 31346, Phone: 858-344-1834, E-mail: yeager{at}miami.edu
The 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. Given the inherent asymmetry of PS in plasma membranes, we also used exposure of PS on HIV-1 and MLV viral particles as a read-out of lipid flipping, which was detected using annexin V antibodies. We observed a strong correlation of PS flipping, and thereby lipid flipping, with the loss of infectivity. However, the revised manuscript emphasizes that several factors besides loss of PS asymmetry may contribute to the restriction phenotype, and PS exposure was primarily used as an assay to measure lipid transport activity.
