The prefusion structure of herpes simplex virus glycoprotein B.

Cell entry of enveloped viruses requires specialized viral proteins that mediate fusion with the host membrane by substantial structural rearrangements from a metastable pre- to a stable postfusion conformation. This metastability renders the herpes simplex virus 1 (HSV-1) fusion glycoprotein B (gB) highly unstable such that it readily converts into the postfusion form, thereby precluding structural elucidation of the pharmacologically relevant prefusion conformation. By identification of conserved sequence signatures and molecular dynamics simulations, we devised a mutation that stabilized this form. Functionally locking gB allowed the structural determination of its membrane-embedded prefusion conformation at sub-nanometer resolution and enabled the unambiguous fit of all ectodomains. The resulting pseudo-atomic model reveals a notable conservation of conformational domain rearrangements during fusion between HSV-1 gB and the vesicular stomatitis virus glycoprotein G, despite their very distant phylogeny. In combination with our comparative sequence-structure analysis, these findings suggest common fusogenic domain rearrangements in all class III viral fusion proteins.

A glycerophospholipid-specific pocket in the RVFV class II fusion protein drives target membrane insertion.

The Rift Valley fever virus (RVFV) is transmitted by infected mosquitoes, causing severe disease in humans and livestock across Africa. We determined the x-ray structure of the RVFV class II fusion protein Gc in its postfusion form and in complex with a glycerophospholipid (GPL) bound in a conserved cavity next to the fusion loop. Site-directed mutagenesis and molecular dynamics simulations further revealed a built-in motif allowing en bloc insertion of the fusion loop into membranes, making few nonpolar side-chain interactions with the aliphatic moiety and multiple polar interactions with lipid head groups upon membrane restructuring. The GPL head-group recognition pocket is conserved in the fusion proteins of other arthropod-borne viruses, such as Zika and chikungunya viruses, which have recently caused major epidemics worldwide.

Formation of insoluble oligomers correlates with ST6Gal I stable localization in the golgi.

The ST6Gal I is a sialyltransferase that functions in the late Golgi to modify the N-linked oligosaccharides of glycoproteins. The ST6Gal I is expressed as two isoforms with a single amino acid difference in their catalytic domains. The STcys isoform is stably retained in the cell and is predominantly found in the Golgi, whereas the STtyr isoform is only transiently localized in the Golgi and is cleaved and secreted from a post-Golgi compartment. These two ST6Gal I isoforms were used to explore the role of the bilayer thickness mechanism and oligomerization in Golgi localization. Analysis of STcys and STtyr proteins with longer transmembrane regions suggested that the bilayer thickness mechanism is not the predominant mechanism used for ST6Gal I Golgi localization. In contrast, the formation and quantity of Triton X-100-insoluble oligomers was correlated with the stable or transient localization of the ST6Gal I isoforms in the Golgi. Nearly 100% of the STcys and only 13% of the STtyr were found as Triton-insoluble oligomers when Golgi membranes of COS-1 cells expressing these proteins were solubilized at pH 6.3, the pH of the late Golgi. In contrast, both proteins were found in the soluble fraction when these membranes were solubilized at pH 8.0. Analysis of other mutants suggested that a conformational change in the catalytic domain rather than increased disulfide bond-based cross-linking is the basis for the increased ability of STcys protein to form oligomers and the stable localization of STcys protein in the Golgi.