Ebola virus glycoprotein needs an additional trigger, beyond proteolytic priming for membrane fusion

Abstract

Background: Ebolavirus belongs to the family filoviridae and causes severe hemorrhagic fever in humans with 50-90% lethality. Detailed understanding of how the viruses attach to and enter new host cells is critical to development of medical interventions. The virus displays a trimeric glycoprotein (GP(1,2)) on its surface that is solely responsible for membrane attachment, virus internalization and fusion. GP(1,2) is expressed as a single peptide and is cleaved by furin in the host cells to yield two disulphide-linked fragments termed GP1 and GP2 that remain associated in a GP(1,2) trimeric, viral surface spike. After entry into host endosomes, GP(1,2) is enzymatically cleaved by endosomal cathepsins B and L, a necessary step in infection. However, the functional effects of the cleavage on the glycoprotein are unknown.

Principal findings: We demonstrate by antibody binding and Hydrogen-Deuterium Exchange Mass Spectrometry (DXMS) of glycoproteins from two different ebolaviruses that although enzymatic priming of GP(1,2) is required for fusion, the priming itself does not initiate the required conformational changes in the ectodomain of GP(1,2). Further, ELISA binding data of primed GP(1,2) to conformational antibody KZ52 suggests that the low pH inside the endosomes also does not trigger dissociation of GP1 from GP2 to effect membrane fusion.

Significance: The results reveal that the ebolavirus GP(1,2) ectodomain remains in the prefusion conformation upon enzymatic cleavage in low pH and removal of the glycan cap. The results also suggest that an additional endosomal trigger is necessary to induce the conformational changes in GP(1,2) and effect fusion. Identification of this trigger will provide further mechanistic insights into ebolavirus infection.

Figure 1. Representative deuteration of peptide fragments of ZEBOV-GP_1,2.

Plots of deuteration of peptides of ZEBOV-GP_1,2 (shown in dark blue) and ZEBOV-GP_1,2CL (shown in magenta) over a time period of 10–1000 sec. Representative peptides are taken from residues adjacent to the glycan cap of GP1 (Panel A: residues 82–94), the base subdomain core region of GP1 (Panel B: residues 166–176), a partially exposed region N-terminal of the trimeric interface of GP2 (Panel C: residues 572–581) and the trimeric interface of GP2 (Panel D: residues 582–593). Deuteration plots of the detected peptides of entire cleaved and uncleaved GP_1,2 are illustrated in Suppl. Figures S2A–S3C.

Figure 2. The fusion loop of ZEBOV-GP_1,2.

(A) Cartoon representation of the fusion loop of GP2 (shown in ball-and-stick representation with carbon atoms colored orange). GP1 subunits of the 3-fold related protomers are shown in different shades of blue. One of the GP2 subunits is shown in orange, and the other two are shown in grey. The crystallographically disordered loop that is cleaved by cathepsin L/B is shown as a dotted line. (B) Deuteration plots of the residues in the fusion loop in ZEBOV-GP_1,2 (shown in dark blue) and ZEBOV-GP_1,2CL (shown in magenta).

Figure 3. Residues important for attachment.

Ribbon representation of a model of the cleaved ZEBOV-GP_1,2CL trimer with residues R64, F88, K95, K114, K115 and K140, which have been identified by mutagenesis as important for attachment, shown in ball-and-stick with carbon atoms colored yellow. GP1 subunits are shown in different shades of blue and the GP2 subunits are shown in different shades of grey. Peptides containing residues R64, F88, K95, K114 and K115 do not undergo measurable conformational change upon priming of ZEBOV-GP_1,2.

Figure 4. Binding of KZ52 to ZEBOV-GP_1,2CL incubated at endosomal pH.

Plot of KZ52 binding at 1.0, 0.33, and 0.11 µg/ml (blue, light blue and grey, respectively) to ZEBOV-GP_1,2CL in ELISA. Bovine serum albumin (BSA) and denatured ZEBOV-GP1,2_CL (Denat.) were used as negative controls. Control ZEBOV-GP_1,2CL was maintained at pH 7.5.

Figure 5. Changing structures of ZEBOV-GP_1,2.

Cartoon representations of GP_1,2 in its viral surface form (PDB: 3CSY), putative receptor-binding form and post-fusion form (PDB: 2EBO). GP1s and GP2s are colored in different shades of blue and grey, respectively. The mucin-like domains were deleted from ZEBOV-GP_1,2 for crystallization and have been modeled here as not-to-scale balloons. It is currently unclear if GP1 remains attached to GP2 during the conformational changes that lead to fusion. The transmembrane regions at the bottom of GP_1,2 are not illustrated.