Abstract
In alphaviruses, here represented by Semliki Forest virus, infection requires an acid-responsive spike configuration to facilitate membrane fusion. The creation of this relies on the chaperone function of glycoprotein E2 precursor (p62) and its maturation cleavage into the small external E3 and the membrane-anchored E2 glycoproteins. To reveal how the E3 domain of p62 exerts its control of spike functions, we determine the structure of a p62 cleavage-impaired mutant virus particle (SQL) by electron cryomicroscopy. A comparison with the earlier solved wild type virus structure reveals that the E3 domain of p62(SQL) forms a bulky side protrusion in the spike head region. This establishes a gripper over part of domain II of the fusion protein, with a cotter-like connection downward to a hydrophobic cluster in its central beta-sheet. This finding reevaluates the role of the precursor from being only a provider of a shield over the fusion loop to a structural playmate in formation of the fusogenic architecture.
Highlights
When an enveloped virus infects its target cell, the mechanism usually involves a step with hairpin refolding of the viral fusion protein to promote merging of virus and target membranes
Here represented by Semliki Forest virus (SFV),3 the fusion proteins are of class II and essentially lack helical motifs
The pH Profile of Fusion Loop Exposure—A functional difference of the SFV wild type (WT) virus and the SQL is the threshold for fusion activation
Summary
E3 DOMAIN OF GLYCOPROTEIN E2 PRECURSOR IN SEMLIKI FOREST VIRUS PROVIDES A UNIQUE CONTACT WITH THE FUSION PROTEIN E1*. To reveal how the E3 domain of p62 exerts its control of spike functions, we determine the structure of a p62 cleavage-impaired mutant virus particle (SQL) by electron cryomicroscopy. A comparison with the earlier solved wild type virus structure reveals that the E3 domain of p62SQL forms a bulky side protrusion in the spike head region This establishes a gripper over part of domain II of the fusion protein, with a cotter-like connection downward to a hydrophobic cluster in its central -sheet. The furin cleavage of p62 into the small external glycoprotein E3 and the membrane-anchored spike glycoprotein E2 is not a prerequisite for transport to the plasma membrane and virus assembly, since virions are formed in furin-deficient cells [19], as well as with cleavage-impaired p62 mutants (18 –22) Such “nonmature” particles, or virion equivalents, are less sensitive to pH trigger for fusion than the maturation-cleaved wild type (WT) particles with E2 and are noninfectious under normal cell conditions. We used the Iris explorer software (NAG, Inc., Downers Grove, IL), supplemented with custom-made modules for the three-dimensional visualization, along with PymolTM (DeLano Scientific LLC; available on the World Wide Web)
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