Structural changes of envelope proteins during alphavirus fusion

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Abstract
Alphaviruses are significant animal and human pathogens — as demonstrated in recent outbreaks of infection with the mosquito-borne Chikungunya virus in India and southeast Asia. The E1 and E2 glycoproteins of alphaviruses are central to the way the virus infects host cells. The E1/E2 heterodimers that form spikes on the virus surface dissociate in the acidic conditions found in the internal vesicles of host cells, and E1 triggers infection by fusing with the endosomal membrane. Félix Rey and colleagues present the structure of Chikungunya virus envelope glycoprotein at neutral pH, and Michael Rossmann and colleagues reveal the structure of the envelope proteins of Sindbis virus at low pH. Sindbis virus can cause fever in humans and is the most extensively studied alphavirus. Comparison of the two structures provides insight into how fusion activation is controlled and points to possible vaccine targets. The E1 and E2 glycoproteins of alphaviruses form heterodimers and assemble into spikes on the virus surface, which mediate receptor binding and endocytosis. When the virion encounters acidic pH in the endosome E1 and E2 dissociate and E1 triggers fusion with the endosomal membrane. Two papers now provide the first crystal structures for glycoprotein complexes incorporating E2 at acidic and neutral pH, respectively. Together they provide insight into how fusion activation is controlled in alphaviruses. Alphaviruses are enveloped RNA viruses that have a diameter of about 700 Å and can be lethal human pathogens1. Entry of virus into host cells by endocytosis is controlled by two envelope glycoproteins, E1 and E2. The E2–E1 heterodimers form 80 trimeric spikes on the icosahedral virus surface1,2, 60 with quasi-three-fold symmetry and 20 coincident with the icosahedral three-fold axes arranged with T = 4 quasi-symmetry. The E1 glycoprotein has a hydrophobic fusion loop at one end and is responsible for membrane fusion3,4. The E2 protein is responsible for receptor binding5,6 and protects the fusion loop at neutral pH. The lower pH in the endosome induces the virions to undergo an irreversible conformational change in which E2 and E1 dissociate and E1 forms homotrimers, triggering fusion of the viral membrane with the endosomal membrane and then releasing the viral genome into the cytoplasm3,4. Here we report the structure of an alphavirus spike, crystallized at low pH, representing an intermediate in the fusion process and clarifying the maturation process. The trimer of E2–E1 in the crystal structure is similar to the spikes in the neutral pH virus except that the E2 middle region is disordered, exposing the fusion loop. The amino- and carboxy-terminal domains of E2 each form immunoglobulin-like folds, consistent with the receptor attachment properties of E2.