Ephrin-B2 and ephrin-B3 as functional henipavirus receptors

Abstract

Members of the ephrin cell-surface protein family interact with the Eph receptors, the largest family of receptor tyrosine kinases, mediating bi-directional signaling during tumorogenesis and various developmental events. Surprisingly, ephrin-B2 and -B3 were recently identified as entry receptors for henipaviruses, emerging zoonotic paramyxoviruses responsible for repeated outbreaks in humans and animals in Australia, Southeast Asia, India and Bangladesh. Nipah virus (NiV) and Hendra virus (HeV) are the only two identified members in the henipavirus genus. While the initial human infection cases came from contact with infected pigs (NiV) or horses (HeV), in the more recent outbreaks of NiV both food-borne and human-to-human transmission were reported. These characteristics, together with high mortality and morbidity rates and lack of effective anti-viral therapies, make the henipaviruses a potential biological-agent threat. Viral entry is an important target for the development of anti-viral drugs. The entry of henipavirus is initiated by the attachment of the viral G envelope glycoprotein to the host cell receptors ephrin-B2 and/or -B3, followed by activation of the F fusion protein, which triggers fusion between the viral envelop and the host membrane. We review recent progress in the study of henipavirus entry, particularly the identification of ephrins as their entry receptors, and the structural characterization of the ephrin/Henipa-G interactions.

Figure 1. Structures of Henipavirus G glycoprotein head domains

A. The structure of the Nipah virus G glycoprotein monomer. Disulfide bonds are presented as yellow sticks. Glycosylation residues are illustrated as grey spheres. B. The structure of the Hendra virus G glycoprotein dimer

Figure 2. The structure of the Nipah G glycoprotein bound to ephrin-B3

A. Schematic representation of the NiV-G/ephrin-B3 complex structure. Ephrin-B3 is in cyan, NiV-G is in magenta. The G protein-contacting regions of ephrin-B3 are in blue. The N and C termini are indicated. B. View of the surface of the Nipah G glycoprotein from the point of view of the incoming ephrin. The ephrin-B3 G-H loop is in blue, and the Nipah G protein surface, which contacts ephrin, is in red.

Figure 3. Different ephrin conformations observed in the complexes of ephrins with different binding partners

The different ephrin structures are color coded as indicated in the figure A. The structural superimposition was performed using only the ephrin structures B. The structural superimposition was performed using only the viral G glycoprotein structures

Figure 4. Comparison of the ephrin-B2 residues, which interact with the Hendra G protein and with EphB2

A. Structure of ephrin-B2 in its complex with the Hendra virus G glycoprotein. Ephrin-B2 is in cyan and the G protein-contacting residues are in red. B. Structure of ephrin-B2 in its complex with EphB2. Ephrin-B2 is in the same orientation as in panel A. Ephrin-B2 is in yellow and the EphB2 contacting residues are in red.

Figure 5

Crystal structure of the Fab fragment of a neutralizing monoclonal antibody bound to the Hendra virus G glycoprotein