Understanding Nipah Virus G Glycoprotein

What Is Nipah Virus?

The Nipah virus (NiV) is classified within the Paramyxoviridae family and the Henipavirus genus. This enveloped RNA virus from the NiV family possesses a monomeric negative-sense genome that spans 18.2 kb and produces six structural proteins. In the early stages of infection humans got the disease from contact with pigs and experienced deadly brain inflammation that resulted in death rates between 38.5% and 100%. Fruit bats from the genus Pteropus serve as the natural host with transmission to intermediate hosts like pigs or humans happening through contaminated food or direct contact. Certain strains including the Bangladesh lineage possess the ability to transmit between humans.

Figure 1. Schematic representation of the viral structure and genome organization
(Source: Sun B, et al. 2018)

NiV has an incubation period lasting between 4 and 21 days which results in severe acute encephalitis and respiratory illnesses that lead to high mortality rates. The initial incubation period ends with the appearance of prodromal symptoms including fever and headache while patients experience myalgia. Within a week patients start showing encephalitis symptoms which include altered mental status together with absent reflexes, hypotonia, segmental myoclonus, gaze palsy and limb weakness. Patients quickly worsen after the onset of the disease and succumb to comatose states before death occurs in just a few days. Survivors experience lasting neurological impairments such as fatigue and focal neurological deficits and depression in 20% of cases. Some differences exist in clinical features between Malaysian and Indian outbreaks. Mortality rates in India and Bangladesh (70%) are higher than in Malaysia (40%). Respiratory diseases occur in 70% of patients in India and Bangladesh, while no severe respiratory manifestations were observed in Malaysia. Symptoms of respiratory involvement include coughing along with respiratory distress and unusual pneumonia. Old age combined with comorbidities and thrombocytopenia with elevated aminotransferases upon admission as well as brainstem involvement and seizures contribute to poor prognosis risk factors.

Structural Features and Receptor Binding Mechanism of the G Protein

As a core protein for viral entry, the G protein plays a pivotal role in receptor recognition, immune evasion, and vaccine design. NiV G protein is a type II transmembrane glycoprotein that functions as a receptor-binding protein. Its C-terminal globular head domain adopts a unique six-bladed β-propeller configuration, formed by six lobe modules interconnected via disulfide bonds to create a stable antiparallel β-sheet structure. This structural feature distinguishes it from attachment glycoproteins of other paramyxoviruses (such as the HN protein of parainfluenza virus), particularly in the conformational differences of the receptor-binding domain. Crystallographic studies reveal that the G protein head specifically binds to host cell receptors Ephrin-B2/B3 through a hydrophobic pocket and hydrogen-bond network, with residues Y120, P122, L124, and W125 forming a critical hydrophobic channel essential for receptor recognition. The G protein maintains a discoid structure when unbound but undergoes separation between its head and stalk domains upon Ephrin binding which reveals its interface for F protein interaction.

Ephrin B2 functions as a ligand for Eph receptors and performs essential functions in developmental processes within vascular and nervous systems as a cell surface glycoprotein. This protein shows increased expression within specific tumor cells. This receptor distribution well explains NiV's cellular and tissue tropism in vivo, along with its broad cellular tropism in cultured tumor cells. Ephrin B3 is considered an alternative receptor for NiV. Since ephrin B3 is expressed in some brain regions where the primary entry receptor ephrin B2 is absent, it may partially account for central nervous system pathologies observed in NiV patients.

Immune Evasion and Host Immune Regulation

The result of an infection depends on how well the host cell's innate immune responses react to attack in comparison to the microorganism's ability to avoid or suppress these defenses. Specifically, NiV among Henipaviruses produces multiple viral factors that accomplish this function throughout their life cycle. In vitro studies demonstrate that NiV infection of endothelial cell lines induces the production and secretion of multiple host antiviral proteins, type I interferons (IFN-I), inflammatory chemokines, and cytokines. Following initial attachment and fusion of NiV with the host cell membrane, cytoplasmic RNA helicases recognize the released viral genomic RNA, triggering robust activation of IFN-I responses.

Neutrophils are among the first immune cells recruited to infection sites. These cells employ neutrophil extracellular traps (NETs) along with antiviral substances to enclose the virus. Certain NiV proteins suppress IFN signaling through their interaction with the JAK/STAT pathway. Virus entry through ephrin-B2/B3 receptors leads to NiV uptake by antigen-presenting cells like macrophages and dendritic cells which convert it into viral peptides. T cells become activated when viral peptides bind to MHC molecules through TCR engagement. Activated helper T cells play a critical role in initiating B cell activation and proliferation while promoting mature antibody response development. Consequently, plasma cells and memory B cells are generated, producing NiV-specific antibodies that protect against infection. Meanwhile, activated cytotoxic T cells target and eliminate NiV-infected cells.

Figure 3. Mechanism of host immune response in NiV infection
(Source: Wong WF, et al. 2022)

The G protein employs a dual strategy for immune evasion: On one hand, its highly glycosylated surface structure masks neutralizing epitopes, reducing antibody recognition efficiency. On the other hand, the secreted G protein (sG) acts as a "molecular decoy" to neutralize host-specific antibodies while mimicking the CX3CL1 chemokine to interfere with CX3CR1⁺ immune cell recruitment. Studies reveal that the CX3C motif in G protein exhibits high homology with the chemotactic functional domain of host CX3CL1. This molecular mimicry mechanism not only facilitates viral attachment but also suppresses the interferon signaling pathway, creating a microenvironment favorable for viral replication.

NiV Antibodies

NiV Antigens