Molecule of the Month: Eastern Equine Encephalitis Virus

Eastern equine encephalitis virus (EEEV) and its closely-related cousins Western equine encephalitis virus (WEEV) and Venezuelan equine encephalitis virus (VEEV) are viruses that can cause swelling of the brain (encephalitis) or joints, paralysis, and other neurological conditions in mammals. The different equine encephalitis viruses are all zoonotic, meaning that can be transmitted between animals and humans. Although EEEV is named for its detrimental health impact on horses and livestock, it primarily circulates in passerine birds, which include common species such as robins and sparrows. In most birds, EEEV is able to proliferate while not causing any symptoms. Mosquitoes that feed on infected birds can pass the virus on to mammalian hosts, including humans. Most human infections in people will go unnoticed, but around 5% of cases result in symptoms.

EEEV is a member of the Togavirus family of the alphavirus genus, a group of small membrane-enveloped viruses with a single-stranded RNA genome. On the viral surface, 240 transmembrane envelope glycoproteins called E1 (shown in green in the structure on the right, 6MX4) and E2 (shown in yellow) pack tightly in a regular lattice, forming trimeric spikes. Inside the membrane, another 240 capsid proteins (shown in blue in the cross-section view) help to pack and protect the RNA genome. In addition to E1, E2 and capsid, EEEV includes two other structural proteins – 6K and TF – which play important roles in infection but are not shown in the illustration.

Infection of cells by alphaviruses like EEEV rely on adhesive interactions between their surface glycoproteins and membrane proteins on the cell surface. After attaching to the cell, EEEV is internalized via endocytosis. The acidic environment of the endosome triggers a series of conformational changes in the envelope proteins that results in the fusion of the viral membrane with the endosome membrane. This fusion event releases the capsid with its genome cargo into the cytosol. Disassembly of the capsid releases the positive-sense viral RNA, which can then be directly read by ribosomes, leading to the production of viral proteins.

One of the features of EEEV that makes it so dangerous is its ability to infect a wide variety of cell types in diverse organisms, including birds, insects, and mammals. This ability to adapt to different hosts is thought to be due in part to the way that EEEV binds to receptors on the cell surface. While some viruses, such as HIV, appear to primarily bind to a specific cell surface protein, EEEV has been shown to be able to bind to several different molecules, including the very low density lipoprotein receptor (VLDLR), apolipoprotein E receptor 2 (ApoER2, also called LRP8), and heparan sulfate, a sugar that commonly decorates cell-surface proteins. Structures showing the binding of EEEV envelope proteins to heparan sulfate (pink, from 6ODF) and VLDLR (purple, from 8UFC) are shown in the illustration on the left. Researchers found that VLDLR, which has a long and flexible extracellular domain, can bind to EEEV envelope proteins in a variety of ways. Rather than relying on a single strong contact, attachment and subsequent infection by EEEV appears to require multiple weaker interactions between the virus and cell surface molecules. This strategy may allow EEEV to readily adapt to diverse hosts.