PE2 cleavage mutants of Sindbis virus: correlation between viral infectivity and pH-dependent membrane fusion activation of the spike heterodimer

Two Conserved Phenylalanine Residues in the E1 Fusion Loop of Alphaviruses Are Essential for Viral Infectivity

Alphaviruses infect cells by a low pH-dependent fusion reaction between viral and host cell membranes that is mediated by the viral E1 glycoprotein. Most reported alphavirus E1 sequences include two phenylalanines (F87 and F95) in the fusion loop, yet the role of these residues in viral infectivity remains to be defined. Following introduction of wild type (WT), E1-F87A, and E1-F95A chikungunya virus (CHIKV) RNA genomes into cells, viral particle production was similar in magnitude. However, CHIKV E1-F87A and E1-F95A virions displayed impaired infectivity compared with WT CHIKV particles. Although WT, E1-F87A, and E1-F95A particles bound cells with similar efficiencies, E1-F87A and E1-F95A particles were unable to undergo fusion and entry into cells. Introduction of an F95A mutation in the E1 fusion loop of Mayaro virus or Venezuelan equine encephalitis virus also resulted in poorly infectious virions. We further tested whether an E1-F87A or E1-F95A mutation could be incorporated into a live-attenuated vaccine strain, CHIKV 181/25, to enhance vaccine safety. Infection of immunocompromised Ifnar1-/- and Irf3-/-Irf5-/-Irf7-/- mice with 181/25E1-F87A or 181/25E1-F95A resulted in 0% mortality, compared with 100% mortality following 181/25 infection. Despite this enhanced attenuation, surviving Ifnar1-/- and Irf3-/-Irf5-/-Irf7-/- mice were protected against virulent virus re-challenge. Moreover, single-dose immunization of WT mice with either 181/25, 181/25E1-F87A, or 181/25E1-F95A elicited CHIKV-specific antibody responses and protected against pathogenic CHIKV challenge. These studies define a critical function for residues E1-F87 and E1-F95 in alphavirus fusion and entry into target cells and suggest that incorporation of these mutations could enhance the safety of live-attenuated alphavirus vaccine candidates. IMPORTANCE Alphaviruses are human pathogens that cause both debilitating acute and chronic musculoskeletal disease and potentially fatal encephalitis. In this study, we determined that two highly conserved phenylalanine residues in the alphavirus E1 glycoprotein are required for fusion of viral and host cell membranes and viral entry into target cells. We further demonstrated that mutation of these phenylalanines results in a substantial loss of viral virulence but not immunogenicity. These data enhance an understanding of the viral determinants of alphavirus entry into host cells and could contribute to the development of new antivirals targeting these conserved phenylalanines or new live-attenuated alphavirus vaccines.

Interactions involved in pH protection of the alphavirus fusion protein

The alphavirus membrane protein E1 mediates low pH-triggered fusion of the viral and endosome membranes during virus entry. During virus biogenesis E1 associates as a heterodimer with the transmembrane protein p62. Late in the secretory pathway, cellular furin cleaves p62 to the mature E2 protein and a peripheral protein E3. E3 remains bound to E2 at low pH, stabilizing the heterodimer and thus protecting E1 from the acidic pH of the secretory pathway. Release of E3 at neutral pH then primes the virus for fusion during entry. Here we used site-directed mutagenesis and revertant analysis to define residues important for the interactions at the E3-E2 interface. Our data identified a key residue, E2 W235, which was required for E1 pH protection and alphavirus production. Our data also suggest additional residues on E3 and E2 that affect their interacting surfaces and thus influence the pH protection of E1 during alphavirus exit.

The role of E3 in pH protection during alphavirus assembly and exit

Alphaviruses are small enveloped viruses whose surface is covered by spikes composed of trimers of E2/E1 glycoprotein heterodimers. During virus entry, the E2/E1 dimer dissociates within the acidic endosomal environment, freeing the E1 protein to mediate fusion of the viral and endosome membranes. E2 is synthesized as a precursor, p62, which is cleaved by furin in the late secretory pathway to produce mature E2 and a small peripheral glycoprotein, E3. The immature p62/E1 dimer is acid resistant, but since p62 is cleaved before exit from the acidic secretory pathway, low pH-dependent binding of E3 to the spike complex is believed to prevent premature fusion. Based on analysis of the structure of the Chikungunya virus E3/E2/E1 complex, we hypothesized that interactions of E3 residues Y47 and Y48 with E2 are important in this binding. We then directly tested the in vivo role of E3 in pH protection by alanine substitutions of E3 Y47 and Y48 (Y47/48A) in Semliki Forest virus. The mutant was nonviable and was blocked in E1 transport to the plasma membrane and virus production. Although the Y47/48A mutant initially formed the p62/E1 heterodimer, the dimer dissociated during transport through the secretory pathway. Neutralization of the pH in the secretory pathway successfully rescued dimer association, E1 transport, and infectious particle production. Further mutagenesis identified the critical contact as the cation-π interaction of E3 Y47 with E2. Thus, E3 mediates pH protection of E1 during virus biogenesis via interactions strongly dependent on Y47 at the E3-E2 interface.

Alphavirus structure: activation for entry at the target cell surface

A wealth of new data about the 3D organization of alphavirus particles was obtained in the last few years. This includes the crystal structures of the envelope glycoprotein complexes at neutral and at acid pH, as well as electron microscopy reconstructions of intact virions at neutral pH to resolutions between 7Å and 4Å. The combination has provided unprecedented detail in the description of the alphavirus virion. This review surveys the main features discovered and the implications for the biology of the virus, in particular for the process of disassembly of the glycoprotein shell during entry. The major outstanding questions in this area are also identified and discussed.

A key interaction between the alphavirus envelope proteins responsible for initial dimer dissociation during fusion

Alphaviruses such as Semliki Forest virus (SFV) are enveloped viruses whose surface is covered by an organized lattice composed of trimers of E2-E1 heterodimers. The E1 envelope protein, a class II fusion protein, contains the hydrophobic fusion loop and refolds to drive virus fusion with the endosome membrane. The E2 protein is synthesized as a precursor p62, whose processing by furin primes the heterodimer for dissociation during virus entry. Dissociation of the E2-E1 heterodimer is an essential step during low-pH-triggered fusion, while the dissociation of the immature p62-E1 dimer is relatively pH resistant. Previous structural studies described an "acid-sensitive region" in E2 that becomes disordered at low pH. Within this region, the conserved E2 H170 is in position to form a hydrogen bond with the underlying E1 S57. Here we experimentally tested the role of this interaction in regulating dimer dissociation in mature and immature virus. Alanine substitutions of E1 S57 and E2 H170 destabilized the heterodimer and produced a higher pH threshold for exposure of the E1 fusion loop and for fusion of the immature virus. E1 S57K or S57D mutations were lethal and caused transport and assembly defects that were partially abrogated by neutralization of the exocytic pathway. The lethal phenotype of E1 S57K was rescued by second-site mutations at E2 H170/M171. Together, our results define a key role for the E1 S57-E2 H170 interaction in dimer stability and the pH dependence of fusion and provide evidence for stepwise dissociation of the E2-E1 dimer at low pH.

Antibody to the E3 glycoprotein protects mice against lethal venezuelan equine encephalitis virus infection

Six monoclonal antibodies were isolated that exhibited specificity for a furin cleavage site deletion mutant (V3526) of Venezuelan equine encephalitis virus (VEEV). These antibodies comprise a single competition group and bound the E3 glycoprotein of VEEV subtype I viruses but failed to bind the E3 glycoprotein of other alphaviruses. These antibodies neutralized V3526 virus infectivity but did not neutralize the parental strain of Trinidad donkey (TrD) VEEV. However, the E3-specific antibodies did inhibit the production of virus from VEEV TrD-infected cells. In addition, passive immunization of mice demonstrated that antibody to the E3 glycoprotein provided protection against lethal VEEV TrD challenge. This is the first recognition of a protective epitope in the E3 glycoprotein. Furthermore, these results indicate that E3 plays a critical role late in the morphogenesis of progeny virus after E3 appears on the surfaces of infected cells.

Alphavirus Entry and Membrane Fusion

The study of enveloped animal viruses has greatly advanced our understanding of the general properties of membrane fusion and of the specific pathways that viruses use to infect the host cell. The membrane fusion proteins of the alphaviruses and flaviviruses have many similarities in structure and function. As reviewed here, alphaviruses use receptor-mediated endocytic uptake and low pH-triggered membrane fusion to deliver their RNA genomes into the cytoplasm. Recent advances in understanding the biochemistry and structure of the alphavirus membrane fusion protein provide a clearer picture of this fusion reaction, including the protein’s conformational changes during fusion and the identification of key domains. These insights into the alphavirus fusion mechanism suggest new areas for experimental investigation and potential inhibitor strategies for anti-viral therapy.

Epistatic roles of E2 glycoprotein mutations in adaption of chikungunya virus to Aedes albopictus and Ae. aegypti mosquitoes

Between 2005 and 2007 Chikungunya virus (CHIKV) caused its largest outbreak/epidemic in documented history. An unusual feature of this epidemic is the involvement of Ae. albopictus as a principal vector. Previously we have demonstrated that a single mutation E1-A226V significantly changed the ability of the virus to infect and be transmitted by this vector when expressed in the background of well characterized CHIKV strains LR2006 OPY1 and 37997. However, in the current study we demonstrate that introduction of the E1-A226V mutation into the background of an infectious clone derived from the Ag41855 strain (isolated in Uganda in 1982) does not significantly increase infectivity for Ae. albopictus. In order to elucidate the genetic determinants that affect CHIKV sensitivity to the E1-A226V mutation in Ae. albopictus, the genomes of the LR2006 OPY1 and Ag41855 strains were used for construction of chimeric viruses and viruses with a specific combination of point mutations at selected positions. Based upon the midgut infection rates of the derived viruses in Ae. albopictus and Ae. aegypti mosquitoes, a critical role of the mutations at positions E2-60 and E2-211 on vector infection was revealed. The E2-G60D mutation was an important determinant of CHIKV infectivity for both Ae. albopictus and Ae. aegypti, but only moderately modulated the effect of the E1-A226V mutation in Ae. albopictus. However, the effect of the E2-I211T mutation with respect to mosquito infections was much more specific, strongly modifying the effect of the E1-A226V mutation in Ae. albopictus. In contrast, CHIKV infectivity for Ae. aegypti was not influenced by the E2-1211T mutation. The occurrence of the E2-60G and E2-211I residues among CHIKV isolates was analyzed, revealing a high prevalence of E2-211I among strains belonging to the Eastern/Central/South African (ECSA) clade. This suggests that the E2-211I might be important for adaptation of CHIKV to some particular conditions prevalent in areas occupied by ECSA stains. These newly described determinants of CHIKV mosquito infectivity for Ae. albopictus and Ae. aegypti are of particular importance for studies aimed at the investigation of the detailed mechanisms of CHIKV adaptations to its vector species.

Biochemical and biophysical comparison of cleaved and uncleaved soluble, trimeric HIV-1 envelope glycoproteins

Human immunodeficiency virus type 1 (HIV-1) entry into host cells is mediated by the trimeric envelope glycoprotein complex (Env). Accordingly, the Env proteins are the targets for neutralizing antibodies (NAbs) and are the focus of vaccines intended to induce NAbs. Because the Env complex is labile, soluble recombinant Env (gp140) trimers require engineering to stabilize them sufficiently for use as immunogens. Trimeric forms of gp140 trimers can be created that are either cleavage-competent or cleavage-defective at the junction between the gp120 and gp41 subunits. As functional trimers are cleaved at this site, the question arises as to whether cleavage affects the antigenic structure of the Env complex in a way that is relevant to vaccine design. Here, we present a comparative analysis of the antigenicity profiles of cleaved and uncleaved gp140 trimers derived from the KNH1144 (subtype A) virus that are otherwise closely sequence-matched. While cleavage did not affect the exposure of NAb epitopes on the gp140 trimers, non-neutralizing antibodies to gp41 epitopes bound much more strongly to uncleaved trimers. Hence cleavage does alter the structure of the HIV-1 Env complex.

Heparan sulfate binding can contribute to the neurovirulence of neuroadapted and nonneuroadapted Sindbis viruses

Cell culture-adapted laboratory strains of Sindbis virus (SB) exhibit efficient initial attachment to cell surface heparan sulfate (HS) receptors. In contrast, non-cell-adapted strains, such as the SB consensus sequence virus TR339, interact weakly with HS and cell surfaces. Regardless of their HS binding phenotype, most SB strains do not cause fatal disease in adult mice, whether inoculated subcutaneously (s.c.) or intracranially (i.c.). However, laboratory strains of SB can be rendered neurovirulent for adult mice by introduction of a glutamine (Gln)-to-histidine (His) mutation at position 55 of the E2 envelope glycoprotein. In the current work, we have determined that E2 His 55-containing viruses require a second-site mutation (Glu to Lys) at E2 position 70 that confers efficient HS binding in order to exhibit virulence for adult mice and that virulence is correlated with very high infectivity for many cell types. Furthermore, introduction of E2 Lys 70 or certain other HS-binding mutations alone also increased morbidity and/or mortality over that of TR339 for older mice inoculated i.c. However, all viruses containing single HS-binding mutations were attenuated in s.c. inoculated suckling mice in comparison with TR339. These results suggest that HS binding may attenuate viral disease that is dependent on high-titer viremia; however, efficient cell attachment through HS binding can increase virulence, presumably through enhancing the replication of SB within specific host tissues such as the brain.

Class II virus membrane fusion proteins

Enveloped animal viruses fuse their membrane with a host cell membrane, thus delivering the virus genetic material into the cytoplasm and initiating infection. This critical membrane fusion reaction is mediated by a virus transmembrane protein known as the fusion protein, which inserts its hydrophobic fusion peptide into the cell membrane and refolds to drive the fusion reaction. This review describes recent advances in our understanding of the structure and function of the class II fusion proteins of the alphaviruses and flaviviruses. Inhibition of the fusion protein refolding reaction confirms its importance in fusion and suggests new antiviral strategies for these medically important viruses.

Resuscitating mutations in a furin cleavage-deficient mutant of the flavivirus tick-borne encephalitis virus

Cleavage of the viral surface protein prM by the proprotein convertase furin is a key step in the maturation process of flavivirus particles. A mutant of tick-borne encephalitis virus (TBEV) carrying a deletion mutation within the furin recognition motif of protein prM (changing R-T-R-R to R-T-R) was previously shown to be noninfectious in BHK-21 cells. We now demonstrate how natural selection can overcome this lethal defect in two different growth systems by distinct resuscitating mutations. In BHK-21 cells, a spontaneous codon duplication created a minimal furin cleavage motif (R-R-T-R). This mutation restored infectivity by enabling intracellular prM cleavage. A completely different mutation pattern was observed when the mutant virus was passaged in mouse brains. The "pr" part of protein prM, which is removed by cleavage, contains six conserved Cys residues. The mutations selected in mice changed the number of Cys residues to five or seven by substitution mutations near the original cleavage site, probably causing a major perturbation of the structural integrity of protein prM. Although viable in mice, such Cys mutants could not be passaged in BHK-21 cells under normal growth conditions (37 degrees C), but one of the mutants exhibited a low level of infectivity at a reduced incubation temperature (28 degrees C). No evidence for the cleavage of protein prM in BHK-21 cells was obtained. This suggests that under certain growth conditions, the structural perturbation of protein prM can restore the infectivity of TBEV by circumventing the need for intracellular furin-mediated cleavage. This is the first example of a flavivirus using such a molecular mechanism.

An interaction site of the envelope proteins of Semliki Forest virus that is preserved after proteolytic activation

Semliki Forest virus (SFV) membrane fusion is mediated by the viral E1 protein at acidic pH and regulated by the dimeric interaction of E1 with the E2 membrane protein. During low pH-triggered fusion, the E2/E1 heterodimer dissociates, freeing E1 to drive membrane fusion. E2 is synthesized as a precursor, p62, which is processed to mature E2 by the cellular protease furin. Both the dissociation of the p62/E1 dimer and the fusion reaction of p62 virus have a more acidic pH threshold than that of the mature E2 virus. We have previously isolated SFV mutations that allow virus growth in furin-deficient cells. Here we have used such pci mutations to compare the interactions of the p62/E1 and E2/E1 dimers. Our data suggest that there is an important p62/E1 dimer interaction site identified by an E2 R250G mutation and that this interaction is maintained after processing to the mature E2 protein.

Reversible acid-induced inactivation of the membrane fusion protein of Semliki Forest virus

Previously, it has been shown that the exposure of Semliki Forest virus (SFV) to a mildly acidic environment induces a rapid and complete loss of the ability of the virus to bind and fuse to target membranes added subsequently. In the present study, incubation of SFV at low pH followed by a specific reneutralization step resulted in a partial reversion of this loss of viral fusion capacity, as assessed in a liposomal model system. Also, the ability of the viral E1 fusion protein to undergo liposome-stimulated trimerization was restored. Furthermore, acid-treated and neutralized SFV largely retained infectivity. Exposure of SFV to low pH induced dissociation of the E1/E2 heterodimer, which was not reversed upon neutralization. It is concluded that the SFV E1 fusion protein, after acid-induced dissociation from E2, rapidly adopts an intermediate, nontrimeric conformation in which it is no longer able to interact with target membrane lipids. Neutralization restores the ability of E1 to interact with membranes. This interaction, however, remains strictly dependent on low pH.

Antiviral activity of human lactoferrin: inhibition of alphavirus interaction with heparan sulfate

Human lactoferrin is a component of the non-specific immune system with distinct antiviral properties. We used alphaviruses, adapted to interaction with heparan sulfate (HS), as a tool to investigate the mechanism of lactoferrin's antiviral activity. Lactoferrin inhibited infection of BHK-21 cells by HS-adapted, but not by non-adapted, Sindbis virus (SIN) or Semliki Forest virus (SFV). Lactoferrin also inhibited binding of radiolabeled HS-adapted viruses to BHK-21 cells or liposomes containing lipid-conjugated heparin as a receptor analog. On the other hand, low-pH-induced fusion of the viruses with liposomes, which occurs independently of virus-receptor interaction, was unaffected. Studies involving preincubation of virus or cells with lactoferrin suggested that the protein does not bind to the virus, but rather blocks HS-moieties on the cell surface. Charge-modified human serum albumin, with a net positive charge, had a similar antiviral effect against HS-adapted SIN and SFV, suggesting that the antiviral activity of lactoferrin is related to its positive charge. It is concluded that human lactoferrin inhibits viral infection by interfering with virus-receptor interaction rather than by affecting subsequent steps in the viral cell entry or replication processes.

A conserved histidine in the ij loop of the Semliki Forest virus E1 protein plays an important role in membrane fusion

The enveloped alphavirus Semliki Forest virus (SFV) infects cells via a low pH-triggered membrane fusion reaction mediated by the E1 protein. E1 is a class II fusion protein that contains the hydrophobic fusion peptide loop and converts to a stable homotrimer during the fusion reaction. Intriguingly, the fusion loop is closely associated with a loop connecting the i and j beta-strands. This ij loop plays a role in the cholesterol dependence of membrane fusion and is specifically susceptible to proteolysis in the protease-resistant E1 homotrimer. The SFV ij loop contains a histidine residue at position 230. Sequence comparisons revealed that an analogous histidine is completely conserved in all alphavirus and flavivirus fusion proteins. An E1 H230A mutant was constructed using the SFV infectious clone. Although cells infected with H230A RNA produced virus particles, these virions were completely noninfectious and were blocked in both cell-cell fusion and lipid mixing assays. The H230A virions efficiently bound to cell surface receptors and responded to low pH by undergoing acid-dependent conformational changes including dissociation of the E1/E2 dimer, exposure of the fusion loop, association with target liposomes, exposure of acid-conformation-specific epitopes, and formation of the stable E1 homotrimer. Studies with a soluble fragment of E1 showed that the mutant protein was defective in lipid-dependent conformational changes. Our results indicate that the E1 ij loop and the conserved H230 residue play a critical role in alphavirus-membrane fusion and suggest the presence of a previously undescribed late intermediate in the fusion reaction.

Mutations that promote furin-independent growth of Semliki Forest virus affect p62-E1 interactions and membrane fusion

The enveloped alphavirus Semliki Forest virus (SFV) infects cells via a low pH-triggered membrane fusion reaction mediated by the E1 protein. E1's fusion activity is regulated by its heterodimeric interaction with a companion membrane protein E2. Mature E2 protein is generated by furin processing of the precursor p62. Processing destabilizes the heterodimer, allowing dissociation at acidic pH, E1 conformational changes, and membrane fusion. We used a furin-deficient cell line, FD11, to select for SFV mutants that show increased growth in the absence of p62 processing. We isolated and characterized 7 such pci mutants (p62 cleavage independent), which retained the parental furin cleavage site but showed significant increases in their ability to carry out membrane fusion in the p62 form. Sequence analysis of the pci mutants identified mutations primarily on the E2 protein, and suggested sites important in the interaction of p62 with E1 and the regulation of fusion.

Attenuation of Sindbis virus variants incorporating uncleaved PE2 glycoprotein is correlated with attachment to cell-surface heparan sulfate

Sindbis virus virions incorporating uncleaved precursor envelope protein PE2 bind efficiently to cell-surface heparan sulfate (HS) because the furin cleavage site (a consensus HS-binding domain) is retained in the mature virus particle. However, they are essentially nonviable. Resuscitating mutations selected in the E3 or E2 protein preserve the PE2 noncleaving phenotype and HS binding, but facilitate fusion, and thereby restore wild-type infectivity on cultured cells. Here, we have demonstrated that the resuscitated PE2 noncleaving virus was almost avirulent in vivo, but mutated during the infection. Mutants had increased virulence and cleavage of PE2, with reduced HS binding capacity. We hypothesize that HS binding leads to sequestration of PE2 noncleaving virus particles and suppression of serum viremia, thereby selecting for evolution of the virus into a PE2-cleaving, low HS-binding phenotype.

Neuronal apoptosis pathways in Sindbis virus encephalitis

Sindbis virus infects neurons of the brain and spinal cord leading to neuronal apoptosis and encephalitis in mice. During postnatal development, neurons of mice remain susceptible to infection but become refractory to SV-induced programmed cell death. Failure to undergo programmed cell death results in a persistent infection. However, some neurovirulent strains of Sindbis virus overcome the age-dependent protective function in neurons, leading to enhanced apoptotic cell death in the central nervous system and higher mortality rates. Sindbis virus infections can also cause hind-limb paralysis due to the death of infected spinal cord motor neurons. However, spinal cord neuron death in older mice appears to occur by mechanisms that differ from classical apoptosis observed in newborn mice based on the morphology of dying neurons at these two sites. Sindbis virus infections of mosquitoes and some mosquito cell lines, on the other hand, do not induce cell death but persistent infections, a phenomenon also observed occasionally in cultured mammalian cells as well as in brains of infected mice surviving lethal infections. Thus, both viral and cellular factors contribute to the varied outcomes of infection. The molecular mechanisms that govern the susceptibility or resistance of particular cell types to SV-induced cell death are not well understood. Furthermore, the cellular execution machinery that produces the characteristic morphological distinctions between brain and spinal cord (i.e. apoptotic versus non-apoptotic) remain to be discovered.

Furin processing and proteolytic activation of Semliki Forest virus

The alphavirus Semliki Forest virus (SFV) infects cells via a low-pH-dependent membrane fusion reaction mediated by the E1 envelope protein. Fusion is regulated by the interaction of E1 with the receptor-binding protein E2. E2 is synthesized as a precursor termed "p62," which forms a stable heterodimer with E1 and is processed late in the secretory pathway by a cellular furin-like protease. Once processing to E2 occurs, the E1/E2 heterodimer is destabilized so that it is more readily dissociated by exposure to low pH, allowing fusion and infection. We have used FD11 cells, a furin-deficient CHO cell line, to characterize the processing of p62 and its role in the control of virus fusion and infection. p62 was not cleaved in FD11 cells and cleavage was restored in FD11 cell transfectants expressing human furin. Studies of unprocessed virus produced in FD11 cells (wt/p62) demonstrated that the p62 protein was efficiently cleaved by purified furin in vitro, without requiring prior exposure to low pH. wt/p62 virus particles were also processed during their endocytic uptake in furin-containing cells, resulting in more efficient virus infection. wt/p62 virus was compared with mutant L, in which p62 cleavage was blocked by mutation of the furin-recognition motif. wt/p62 and mutant L had similar fusion properties, requiring a much lower pH than control virus to trigger fusion and fusogenic E1 conformational changes. However, the in vivo infectivity of mutant L was more strongly inhibited than that of wt/p62, due to additional effects of the mutation on virus-cell binding.

Adaptation of alphaviruses to heparan sulfate: interaction of Sindbis and Semliki forest viruses with liposomes containing lipid-conjugated heparin

Passage of Sindbis virus (SIN) in BHK-21 cells has been shown to select for virus mutants with high affinity for the glycosaminoglycan heparan sulfate (HS). Three loci in the viral spike protein E2 (E2:1, E2:70, and E2:114) have been identified that mutate during adaptation and independently confer on the virus the ability to bind to cell surface HS (W. B. Klimstra, K. D. Ryman, and R. E. Johnston, J. Virol. 72:7357-7366, 1998). In this study, we used HS-adapted SIN mutants to evaluate a new model system involving target liposomes containing lipid-conjugated heparin (HepPE) as an HS receptor analog for the virus. HS-adapted SIN, but not nonadapted wild-type SIN TR339, interacted efficiently with HepPE-containing liposomes at neutral pH. Binding was competitively inhibited by soluble heparin. Despite the efficient binding of HS-adapted SIN to HepPE-containing liposomes at neutral pH, there was no fusion under these conditions. Fusion did occur, however, at low pH, consistent with cellular entry of the virus via acidic endosomes. At low pH, wild-type or HS-adapted SIN underwent fusion with liposomes with or without HepPE with similar kinetics, suggesting that interaction with the HS receptor analog at neutral pH has little influence on subsequent fusion of SIN at low pH. Finally, Semliki Forest virus (SFV), passaged frequently on BHK-21 cells, also interacted efficiently with HepPE-containing liposomes, indicating that SFV, like other alphaviruses, readily adapts to cell surface HS. In conclusion, the liposomal model system presented in this paper may serve as a novel tool for the study of receptor interactions and membrane fusion properties of HS-interacting enveloped viruses.