Molecular Dissection of the Semliki Forest Virus Homotrimer Reveals Two Functionally Distinct Regions of the Fusion Protein

Abstract

Semliki Forest virus (SFV) is an enveloped alphavirus that infects cells via a membrane fusion reactiontriggered by the acidic pH of endosomes. In response to low pH, the E1 proteins on the virus membrane undergo a series of conformational changes, resulting in the formation of a stable E1 homotrimer. Little is known about the structural basis of either the E1 conformational changes or the resulting homotrimer or about the mechanism of action of the homotrimer in fusion. Here, the E1 homotrimer was formed in vitro from either virus or soluble E1 ectodomain and then probed by various perturbants, proteases, or glycosidase. The preformed homotrimer was extremely stable to moderately harsh conditions and proteases. By contrast, mild reducing conditions selectively disrupted the N-terminal region of trimeric E1, making it accessible to proteolytic cleavage and producing E1 fragments that retained trimer interactions. Trypsin digestion produced a fragment missing a portion of the N terminus just proximal to the putative fusion peptide. Digestion with elastase produced several fragments with cleavage sites between residues 78 and 102, resulting in the loss of the putative fusion peptide and the release of membrane-bound E1 ectodomain as a soluble trimer. Elastase also cleaved the homotrimer within an E1 loop located near the fusion peptide in the native E1 structure. Mass spectrometry was used to map the C termini of several differentially produced and fully functional E1 ectodomains. Together, our data identify two separate regions of the SFV E1 ectodomain, one responsible for target membrane association and one necessary for trimer interactions.Enveloped viruses must circumvent the double barrier of their own membranes and the membrane of the host cell in order to deliver their genomes into the cytoplasm and cause infection. They bypass both membrane barriers simultaneously by means of a membrane fusion event, either at the cell surface or in the endocytic pathway (18). The well-characterized alphavirus Semliki Forest virus (SFV) is a positive-strand RNA virus that enters cells by receptor-mediated endocytosis and then fuses with the endosome membrane to infect the cell (see references 23 and 47 for reviews). SFV fusion is mediated by the viral E1 protein, a type I membrane glycoprotein of about 50 kDa. Upon exposure to the low pH of the endosomes, E1 dissociates from its stable dimeric interaction with the companion E2 protein. E1 then undergoes a series of conformational changes resulting in the exposure of previously hidden acid-conformation-specific epitopes and the formation of a highly stable E1 homotrimer. The protein associates with the target membrane, presumably via the putative fusion peptide (approximately residues 83 to 100), and the dual interaction of E1 with the target and virus membranes drives the fusion reaction. SFV fusion is strongly dependent on the presence of cholesterol and sphingolipid in the target membrane (reviewed in reference 24). Although treatment of virus at low pH in the absence of...

Prefusion Rearrangements Resulting in Fusion Peptide Exposure in Semliki Forest Virus

“…1). The pH threshold for these transformations conforms to reported data on E1 oligomerization, possibly trimerization (61,62). The loss of the neutralization domain, probed by mabE1n, occurs in parallel to proposed E1-E2 dissociation, but before the formation of a "stable" E1 oligomer.…”

The Fusion Peptide of Semliki Forest Virus Associates with Sterol-Rich Membrane Domains

“…We wished to explore the dual role of cholesterol and sphingolipid in the membrane interaction of the SFV E1 fusion protein. In order to follow membrane interaction in the absence of the E1 TM domain and membrane fusion, we used the previously described E1 ectodomain, E1*, a proteolytically truncated form of E1 active in membrane binding, homotrimer formation, and acid epitope exposure (12,13,25). The ectodomain preparation used for these studies contains both E1* and the E2 ectodomain, E2*, but these polypeptides are monomeric and removal of E2* has no effect on the activity of the E1 ectodomain (25).…”

“…During fusion, E1 interacts with the target bilayer and undergoes specific low-pH-triggered conformational changes that result in exposure of previously masked epitopes and formation of a highly stable E1 homotrimer that appears to be required for fusion (22). A proteolytically truncated ectodomain form of E1, E1*, has been used to follow E1's membrane interactions and conformational changes in the absence of virus fusion (12,13,20,25). The SFV E1 ectodomain has recently been crystallized and characterized structurally (27).…”

Novel Mutations That Control the Sphingolipid and Cholesterol Dependence of the Semliki Forest Virus Fusion Protein

“…To form E1 homotrimers, radiolabeled virus preparations were mixed with 1 mM complete liposomes prepared as described above in morpholineethanesulfonic acid (MES)-saline buffer. The mixture was adjusted to pH 5.5 and incubated for 5 min at 20°C (3,9). Samples were then adjusted to neutral pH and treated at 30°C for 4 min with SDS concentrations ranging from 0 to 4%.…”

“…Progeny virus production was quantitated with an infectious center assay in BHK cells at 28°C. At 37°C, both the wild type and the srf-3ϩ4 mutant grew to high titers (ϳ10 9 infectious centers per ml), while the srf-4ϩ5 mutant produced 4 to 5 logs less infectious virus (ϳ10 5 infectious centers per ml) even after 25 h of incubation (Table 1). At 28°C, both the wild type and srf-3ϩ4 grew more slowly, but produced titers of ϳ10 8 infectious centers per ml by 40 h, about 10-fold less virus than was produced at 37°C.…”