The Dynamic Envelope of a Fusion Class II Virus

Abstract

Semliki Forest virus is among the prototypes for Class II virus fusion and targets the endosomal membrane. Fusion protein E1 and its envelope companion E2 are both anchored in the viral membrane and form an external shell with protruding spikes. In acid environments, mimicking the early endosomal milieu, surface epitopes in the virus rearrange along with exposure of the fusion loop. To visualize this transformation into a fusogenic stage, we determined the structure of the virus at gradually lower pH values. The results show that while the fusion loop is available for external interaction and the shell and stalk domains of the spike begin to deteriorate, the E1 and E2 remain in close contact in the spike head. This unexpected observation points to E1 and E2 cooperation beyond the fusion loop exposure stage and implies a more prominent role for E2 in guiding membrane close encounter than has been earlier anticipated.Membrane fusion is important in biology to allow exchange of materials between compartments enclosed by lipid bilayer membranes. Naturally, viruses utilize such ways of genome delivery. Two different mechanisms of virus fusion are observed: the Class I mechanism, seen in viruses where the fusion protein structure is dominated by ␣-helix folding, like the influenza and the HIV, and the Class II mechanism, represented by viruses where the envelope proteins are dominated by ␤-sheet structures like the Tick Borne Encephalitis virus and the Semliki Forest virus (SFV) 2 in the Flavi and Toga virus families, respectively (1, 2).SFV infects its host cells by receptor-mediated endocytosis and fusion within acidic endosomes. It has been demonstrated that the membrane fusion of SFV is strictly dependent on the exposure of the virus to a low pH (3-5) and on the presence of cholesterol in the target membrane (6). Experimental mimicking of the endosomal acidification has shown that low pH triggers a series of conformational changes in the virus spikes (7-10). Mild acidification leads to exposure of the fusion peptide at the virion surface (11). The moderately hydrophobic fusion peptide loop can then interact with the target membrane and initiate fusion (12, 13). Optimal fusion kinetics of SFV has been observed at pH 5.5, while the pH threshold for fusion activation is reported to be around pH 6.2 (3, 5).Early data on the alphavirus imply that the fusion process involves a low pH-induced conformational change in the envelope whereby homotrimers of E1 are formed (9,10,14). This is supported by recent studies on the soluble ectodomain of the fusion protein, E1*. The crystal structure of E1* reveals an elongated molecule, mainly folded into ␤-sheets and interconnecting loops (15, 16). The structure resembles that of the TBE glycoprotein E ectodomain (17). The molecule comprises three domains; a ␤-sheet-folded domain I (DI) located in the central part of the molecule inbetween the projecting domain II (DII), carrying the putative fusion peptide loop (18,19) at the distal end, and the immunoglobulin-like domain III (DIII...

The Dynamic Envelope of a Fusion Class II Virus

“…A similar organization is deduced from studies on sugar deletion mutants of Sindbis virus (34). There seems to be no direct contact between the E1 molecules within the spike, which is held together only by pairwise E2-E2 contacts (31). Thus, the E1 protein in the virion is inter-and intramolecularly self-associated but in an alternative fashion to the fusion-related configuration of stable trimers for which the atomic structure is determined (12,13).…”

“…E1 is part of the spikes by heterodimeric interactions with E2. The fusion loop is thereby hidden by E2 under the rim of the spike wing (31)(32)(33). A similar organization is deduced from studies on sugar deletion mutants of Sindbis virus (34).…”

“…In the whole virus structure, revealed by electron cryomicroscopy (cryo-EM), the trimeric spikes protrude above a protein shell layer that surrounds the virus at some distance above its membrane (25)(26)(27)(28)(29)(30)(31). Modeling of the solved crystal structure of the ectodomain of fusion protein E1 (32,33) into cryo-EMderived whole virion structures implies that E1 constitutes the protein shell by intermolecular E1-E1 interactions and internal interactions between its domain III and I (31)(32)(33)(34).…”

“…Modeling of the solved crystal structure of the ectodomain of fusion protein E1 (32,33) into cryo-EMderived whole virion structures implies that E1 constitutes the protein shell by intermolecular E1-E1 interactions and internal interactions between its domain III and I (31)(32)(33)(34). E1 is part of the spikes by heterodimeric interactions with E2.…”

Bimolecular complementation reveals that glycoproteins gB and gH/gL of herpes simplex virus interact with each other during cell fusion

“…M embrane fusion allows exchange of materials between cellular compartments enclosed by lipid membranes (1). Similarly, entry of enveloped viruses into cells allows the viral contents to be exposed by fusion of its envelope with a target cell membrane.…”