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. 2014 Sep 10;16(3):328-37.
doi: 10.1016/j.chom.2014.08.009.

Bat origins of MERS-CoV supported by bat coronavirus HKU4 usage of human receptor CD26

Qihui Wang  1 , Jianxun Qi  1 , Yuan Yuan  2 , Yifang Xuan  3 , Pengcheng Han  4 , Yuhua Wan  5 , Wei Ji  6 , Yan Li  1 , Ying Wu  1 , Jianwei Wang  7 , Aikichi Iwamoto  8 , Patrick C Y Woo  9 , Kwok-Yung Yuen  10 , Jinghua Yan  1 , Guangwen Lu  1 , George F Gao  11
Affiliations

Bat origins of MERS-CoV supported by bat coronavirus HKU4 usage of human receptor CD26

Qihui Wang et al. Cell Host Microbe. .

Abstract

The recently reported Middle East respiratory syndrome coronavirus (MERS-CoV) is phylogenetically closely related to the bat coronaviruses (BatCoVs) HKU4 and HKU5. However, the evolutionary pathway of MERS-CoV is still unclear. A receptor binding domain (RBD) in the MERS-CoV envelope-embedded spike protein specifically engages human CD26 (hCD26) to initiate viral entry. The high sequence identity in the viral spike protein prompted us to investigate if HKU4 and HKU5 can recognize hCD26 for cell entry. We found that HKU4-RBD, but not HKU5-RBD, binds to hCD26, and pseudotyped viruses embedding HKU4 spike can infect cells via hCD26 recognition. The structure of the HKU4-RBD/hCD26 complex revealed a hCD26-binding mode similar overall to that observed for MERS-RBD. HKU4-RBD, however, is less adapted to hCD26 than MERS-RBD, explaining its lower affinity for receptor binding. Our findings support a bat origin for MERS-CoV and indicate the need for surveillance of HKU4-related viruses in bats.

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Figures

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Graphical abstract
Figure 1
Figure 1
Comparison of the HKU4-RBD, HKU5-RBD, MERS-RBD, and SARS-RBD Sequences (A) Phylogenetic tree generated using MEGA (Tamura et al., 2013) with the indicated RBD sequences. (B) Structure-based sequence alignment. The secondary structure elements are defined based on an ESPript (Gouet et al., 1999) algorithm and are labeled as in a previous report on the MERS-RBD structure (Lu et al., 2013). Spiral lines indicate α or 310 helices, while arrows represent β strands. Helices α1 and η4 and strands β2 and β11 are not preserved in the HKU4-RBD structure and are marked in red. The element equivalent to the MERS-RBD helix α3 exhibits characteristics of a 310 helix in HKU4-RBD and is therefore labeled as η′. The external subdomain is highlighted by enclosure with a red box. The two deletions in HKU5-RBD are marked with blue lines. The Arabic numerals 1–4 indicate cysteine residues that pair to form disulfide bonds. See also Figure S3.
Figure 2
Figure 2
Characterization of Binding between HKU4-RBD and hCD26 by Flow Cytometry (A) Huh7 cells stained with an anti-hCD26 antibody. (B) Huh7 cells stained with MERS-RBD, HKU4-EBD, or HKU5-RBD. (C) Huh7 cells stained with HKU4-RBD in the presence of an anti-hCD26 antibody or an isocontrol antibody. (D) Huh7 cells stained with HKU4-RBD in the presence of hCD26 (hCD26-ecto) or hACE2 (hACE2-ecto) ectodomain protein. (E) BHK cells stained with an anti-hCD26 antibody. (F) BHK cells stained with MERS-RBD, HKU4-RBD, or HKU5-RBD. (G) hCD26-transfected BHK cells stained with an anti-hCD26 antibody. (H) hCD26-transfected BHK cells stained with MERS-RBD, HKU4-RBD, or HKU5-RBD.
Figure 3
Figure 3
Specific Interaction between HKU4-RBD and hCD26 Characterized by SPR (A–H) The indicated wild-type or mutant RBD proteins were immobilized on the chip and tested for binding with gradient concentrations of hCD26 or hACE2. The binding profiles are shown. (A) hCD26 binding to MERS-RBD. (B) hACE2 binding to SARS-RBD. (C) hCD26 binding to HKU4-RBD. (D) hACE2 binding to HKU4-RBD. (E) hCD26 binding to HKU5-RBD. (F) hCD26 binding to HKU4-RBD-K506A. (G) hCD26 binding to HKU4-RBD-E541A. (H) hCD26 binding to HKU4-RBD-SKL, a triple mutant of S540W, K547R, and L558W. The dissociation constants were calculated to be 35.7 and 0.42 μM for the hCD26/HKU4-RBD and hCD26/HKU4-RBD-SKL pairs, respectively, and >400 μM for the hCD26/HKU4-RBD-K506A and hCD26/HKU4-RBD-E541A pairs. See also Figures S1 and S2.
Figure 4
Figure 4
Huh7 Infection by Lentiviral Particles Pseudotyped with BatCoV HKU4 S (A) Proteolytic processing of the embedded BatCoV HKU4 S by trypsin. The HKU4 pseudovirus was treated with trypsin at the indicated concentrations and characterized with an antibody recognizing a FLAG epitope engineered at the S C terminus. The untreated MERS pseudovirus was included as a reference control. (B) A Huh7 cell infection assay with the indicated pseudoviruses. (C) An antibody blocking assay using HKU4 pseudoviruses treated with 10 μg/ml trypsin and an anti-hCD26 antibody. The recorded fluorescence intensities were plotted as histograms, and the error bars represent ± SD for triplicate experiments.
Figure 5
Figure 5
The Complex Structure of HKU4-RBD Bound to hCD26 (A) The overall structure. The two 1:1 complexes related by a two-fold axis (vertical arrow) are shown in cartoon and surface representations, respectively. The core and external subdomains of HKU4-RBD and the β-propeller and hydrolase domains of hCD26 are individually labeled and highlighted in orange, cyan, magenta, and green, respectively. The propeller blades (I–VIII) and the protein N/C termini are marked. (B) A magnified view of the HKU4-RBD structure and the ligand/receptor interface. The secondary structure elements are specified by ESPript and labeled for the viral ligand. Yellow sticks marked with Arabic numbers indicate disulfide bonds. For the receptor, only propeller blades IV and V that engage HKU4-RBD are shown, using a surface representation. (C–E) The important contact sites are marked with boxed letters A–E and are further delineated for interaction details as follows. (C) A solid network of H bond and salt bridge interactions. (D) A small patch of hydrophobic interactions. (E) Extra H bond contacts contributed by a carbohydrate moiety linked to hCD26 N229. The residues involved and the carbohydrates referred to are shown and labeled. See also Tables S1 and S2.
Figure 6
Figure 6
Comparison of the HKU4-RBD/hCD26 and MERS-RBD/hCD26 Pairs for Their Binding Modes and the Interaction Details (A) Overall similar receptor binding mode between HKU4-RBD and MERS-RBD. Superimposition of the structure of HKU4-RBD (cyan) bound to hCD26 (green) and a complex structure of MERS-RBD (orange) with hCD26 (magenta). The loops exhibiting variant conformations are highlighted. (B) A magnified view of the ligand/receptor (RBD in cartoon and hCD26 in surface) interface in the two binding pairs. The elements located within the vdw contact distance from the receptor are highlighted for the η2-α4 region (gray) in the core subdomain, and the β6-β7 (tint), β8 (red), and β9 (blue) strands in the external subdomain. Top: the HKU4-RBD/hCD26 structure. Bottom: the MERS-RBD/hCD26 structure. (C) Better hCD26 adaptation in MERS-RBD than in HKU4-RBD. For each element specified in (B), the amino acid sequences were aligned between HKU4-RBD and MERS-RBD. The number pairs listed above the sequence highlight the differences in vdw contacts. For clarity, only those providing ≥10 intermolecule contacts are labeled. The S540/W535, K547/R542, and L558/W553 pairs showing the most contact differences are marked with red boxes.
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