Bat IFITM3 Restriction Depends on S-Palmitoylation and a Polymorphic Site Within the CD225 Domain
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Published Online: 11 December, 2019 | Supp Info: http://doi.org/10.26508/lsa.201900542 Downloaded from life-science-alliance.org on 28 September, 2021 Research Article Bat IFITM3 restriction depends on S-palmitoylation and a polymorphic site within the CD225 domain Camilla TO Benfield1 , Farrell MacKenzie2, Markus Ritzefeld3, Michela Mazzon2 , Stuart Weston2, Edward W Tate3 , Boon Han Teo1, Sarah E Smith5, Paul Kellam4,5 , Edward C Holmes6, Mark Marsh2 Host interferon-induced transmembrane proteins (IFITMs) are haveorthologsofalltheseIFITMsaswellastwoadditionalgenes, broad-spectrum antiviral restriction factors. Of these, IFITM3 Ifitm6 and Ifitm7. Phylogenetic analysis of vertebrate IFITMs indicates potently inhibits viruses that enter cells through acidic endo- that IFITM1, IFITM2,andIFITM3 group with murine ifitm6 and ifitm7 in a somes, many of which are zoonotic and emerging viruses with clade of immunity-related IFITMs (IR-IFITMs), with IFITM5 and IFITM10 bats (order Chiroptera) as their natural hosts. We previously falling as separate lineages (2). IFITMs belong to the CD225/pfam04505 demonstrated that microbat IFITM3 is antiviral. Here, we show or “dispanin” protein superfamily (http://pfam.xfam.org/family/ that bat IFITMs are characterized by strong adaptive evolution PF04505)(3) that contains more than 2,000 members, including and identify a highly variable and functionally important both prokaryotic and eukaryotic proteins, all of which encode a site—codon 70—within the conserved CD225 domain of IFITMs. conserved CD225 protein domain. Mutation of this residue in microbat IFITM3 impairs restriction of As their name suggests, IFITMs are membrane proteins, allowing representatives of four different virus families that enter cells via them to police the cell surface and endocytic membranes that endosomes. This mutant shows altered subcellular localization and viruses must cross to invade cells. Studies of IFITM topology suggest reduced S-palmitoylation, a phenotype copied by mutation of a type II transmembrane configuration with a cytosolic N terminus, conserved cysteine residues in microbat IFITM3. Furthermore, we cytosolic conserved intracellular loop (CIL) domain, transmembrane show that microbat IFITM3 is S-palmitoylated on cysteine residues domain, and extracellular (or intraluminal) C terminus (4, 5), al- C71, C72, and C105, mutation of each cysteine individually impairs though there is evidence that other IFITM topologies exist (6, 7, 8). virus restriction, and a triple C71A-C72A-C105A mutant loses all The results of spectroscopic topological studies agree with the type restriction activity, concomitant with subcellular re-localization of II transmembrane configuration, as do bioinformatic predictions of microbat IFITM3 to Golgi-associated sites. Thus, we propose that IFITM3 secondary structure that reveal three alpha helices, with the S-palmitoylation is critical for Chiropteran IFITM3 function and C-terminal helix forming a single transmembrane domain (9, 10). identify a key molecular determinant of IFITM3 S-palmitoylation. The CD225 domain is highly conserved among IFITMs and comprises an intramembrane domain (IMD) and CIL domain. The hydrophobic DOI 10.26508/lsa.201900542 | Received 4 September 2019 | Revised 27 November 2019 | Accepted 28 November 2019 | Published online 11 IMD contains a 10-residue amphipathic helix (amino acid residues December 2019 59–68 of human IFITM3) that is required for the antiviral activity of both IFITM3 and IFITM1 (9). The subcellular localization of IFITMs is a key determinant of their antiviral profile. When expressed singly, IFITM3 and IFITM2 preferentially localize to early and late endo- Introduction somes and lysosomes, restricting viruses that enter via these endolysosomal compartments. In contrast, IFITM1 primarily local- Interferon-induced transmembrane proteins (IFITMs) are antiviral izes at the cell surface and can restrict viruses that enter through factors that act uniquely and early in viral replication cycles to restrict the plasma membrane (11, 12, 13, 14). Indeed, mutants of IFITM3 that the entry of a diverse range of primarily enveloped viruses into cells lack an N-terminal endocytic sorting motif 20YEML23 localize to the (1). Humans possess three IFN-inducible IFITM genes—IFITM1, IFITM2, plasma membrane and lose their ability to inhibit influenza A virus and IFITM3—encoding proteins with antiviral functions and two IFITM (IAV), alphavirus, and coronavirus infection by endosomal routes family members that lack antiviral function—IFITM5 and IFITM10. Mice (14, 15, 16, 17, 18). 1Department of Pathobiology and Population Sciences, Royal Veterinary College, University of London, London, UK 2 Medical Research Council Laboratory for Molecular Cell Biology, University College London, London, UK 3Department of Chemistry, Imperial College London, London, UK 4Department of Infectious Disease, Imperial College Faculty of Medicine, Wright Fleming Institute, St Mary’s Campus, London, UK 5Kymab Ltd, The Bennet Building (B930), Babraham Research Campus, Cambridge, UK 6Marie Bashir Institute for Infectious Diseases and Biosecurity, Charles Perkins Centre, School of Life and Environmental Sciences and Sydney Medical School, The University of Sydney, Sydney, New South Wales, Australia Correspondence: cbenfi[email protected] Stuart Weston’s present address is Department of Microbiology and Immunology, University of Maryland School of Medicine, Baltimore, MD, USA ©2019Benfield et al. https://doi.org/10.26508/lsa.201900542 vol 3 | no 1 | e201900542 1of18 Studies focusing on IFITM3 restriction of IAV and Semliki Forest the IFITM gene family is evolutionarily conserved and characterized virus (SFV) indicate that virus internalization is unaffected by IFITM3 by gene duplication, copy number variation, and the presence of expression and, for SFV at least, the viral envelope glycoprotein pseudogenes (2, 45). Whereas humans possess a single IFITM3 gene, undergoes low pH-induced conformational changes (14). However, a remarkable multiplicity of IFITM3-like genes exists in the genomes for both viruses, the viral core components are not delivered to the of some other primates; for example, there are 25 IFITM3-like cytoplasm, suggesting that membrane fusion fails. Experiments variants in the marmoset and eight in the African green monkey with IAV indicate that hemifusion (i.e., lipid-mixing between viral (2, 46). Interestingly, several of these variants contain N-terminal and cellular membranes) can occur in the presence of IFITM3, but polymorphisms which, when inserted into human IFITM3, prevent the subsequent formation of a fusion pore is inhibited (13, 19). Recent ubiquitination and endocytosis; however, the function of these work has shown that IFITM3-positive vesicles fuse with incoming duplicated IFITM3-like genes has not been tested (46). virus-bearing vesicles before hemifusion and that IFITM3 enhances The antiviral responses of bats (mammalian order Chiroptera) the rate of virus trafficking to lysosomes (20). The co-localization of are of particular interest because these animals have been in- viral cargo with IFITM3-positive endosomes is specific to restricted creasingly recognized as reservoir hosts from which numerous viruses, suggesting that IFITM-insensitive viruses such as Lassa viruses have ultimately emerged, with severe pathogenic and virus enter via different endosomal compartments and thereby socioeconomic consequences in humans and livestock. Indeed, a escape IFITM engagement and restriction (13, 20). Further examples recent analysis showed that bats host a significantly higher of virus-specific IFITM action include the ability of murine IFITM6 to proportion of zoonotic viruses than other mammalian orders restrict filoviruses, but not IAV (21), and amino acids within the (47). Moreover, Chiroptera are the only mammalian order that IFITM3 CIL domain that are preferentially needed for IAV but not harbor significantly more zoonotic viruses than predicted from dengue virus restriction (22). Other post-entry mechanisms for reporting effort and host traits such as geographic range and IFITM3 restriction have also been proposed (23, 24, 25). mammal sympatry. It is possible that this propensity to be IFITMs are heavily regulated by posttranslational modifications reservoirs for a large number of viruses, many of which may (PTMs). One major modification is S-palmitoylation, a reversible 16- remain asymptomatic, in part reflects aspects of bat immunology carbon lipid PTM that increases protein hydrophobicity and in- (48, 49, 50, 51, 52). Hence, the study of Chiropteran antiviral ef- fluences the behavior of proteins in membrane environments (26). fectors, such as IFITMs, can potentially reveal mechanisms of For human and murine IFITM3, S-palmitoylation can occur on viral tolerance as well as the evolutionary signatures of virus– cysteine residues 71, 72, and 105 and enhances IFITM3 antiviral host co-evolution. We previously showed that microbat IFITM3 activity (27, 28). Recent live-cell imaging showed that abrogating C72 retains sequence motifs for endocytosis and PTM, traffics to the palmitoylation slowed IFITM3 trafficking to membrane compart- plasma membrane before endocytic uptake, co-localizes with ments containing IAV particles (20). Multiple zinc finger DHHC endosomal markers, and at normal expression levels in primary (Asp-His-His-Cys) domain–containing palmitoyltransferases (ZDHHCs) microbat cells, inhibits infection by pH-dependent enveloped can palmitoylate IFITM3 with marked functional redundancy,