| |
NIAID Scientists Find a Key to Hepatitis C Entry into Cells
|
| |
| |
Download the PDF here
Understanding Structure of HCV Proteins Could Aid in Vaccine Development.
What
In a new paper published in Nature, scientists from the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health, describe the structure of a key protein on the surface of the hepatitis C virus (HCV) and how it interacts with its receptor found on some human cells. The findings provide new leads for developing an HCV vaccine.
Hepatitis C is one of the most common bloodborne infections in the United States. Although it may not cause any symptoms in its early stages, untreated chronic infections can lead to severe liver damage, cancer, and death. Concerningly, infections are on the rise among young adults, largely due to exposure resulting from shared drug-injectables. No vaccine is available to prevent HCV infection.
HCV is usually transmitted via blood, such as during birth or when drug-injection equipment is shared. Because HCV may not cause any symptoms for years after initial infection, infections often go undetected. According to the U.S. Centers for Disease Control and Prevention, an estimated 2.4 million people are living with hepatitis C infection in the United States. More than half of all people infected with HCV are thought to develop chronic infection. HCV is a leading cause of cirrhosis, liver failure requiring transplant, and the leading cause of death from liver disease. Although effective antiviral drugs are available to treat HCV infection, they are expensive and do not prevent reinfection.
In their new paper in Nature, researchers from NIAID and other organizations describe the interaction between a protein expressed on the surface of the HCV, known as HCV E2, and a receptor called CD81 found on the surface of some human cells. Prior research had shown that antibodies interfered with interactions between these two proteins. This suggested that the interaction between HCV E2 and CD81 allowed HCV to enter and infect human cells. However, exactly how this occurred was unknown.
The researchers determined the exact structure of HCV E2 and CD81 and studied how the two proteins interacted when exposed to each other under different conditions. They found that under acidic conditions, HCV E2 easily binds to the CD81 receptor. Once the interaction between virus and receptor begins, HCV E2 changes shape, facilitating its entrance into the cell by putting the virus in closer contact with the cell membrane.
Identifying these structures and the ways they interact with each other may provide the foundation for a vaccine against HCV, the researchers say. A vaccine potentially could cause a person to make specific antibodies that prevent HCV E2 from binding with CD81, stopping the virus from entering the cell, and preventing HCV infection.
-----------------------
Structural insights into hepatitis C virus receptor binding and entry
Nature Sept 15 2021
Abstract
Hepatitis C virus (HCV) infection is a causal agent of chronic liver disease, cirrhosis and hepatocellular carcinoma in humans, and afflicts more than 70 million people worldwide. The HCV envelope glycoproteins E1 and E2 are responsible for the binding of the virus to the host cell, but the exact entry process remains undetermined1. The majority of broadly neutralizing antibodies block interaction between HCV E2 and the large extracellular loop (LEL) of the cellular receptor CD81 (CD81-LEL)2. Here we show that low pH enhances the binding of CD81-LEL to E2, and we determine the crystal structure of E2 in complex with an antigen-binding fragment (2A12) and CD81-LEL (E2-2A12-CD81-LEL); E2 in complex with 2A12 (E2-2A12); and CD81-LEL alone. After binding CD81, residues 418-422 in E2 are displaced, which allows for the extension of an internal loop consisting of residues 520-539. Docking of the E2-CD81-LEL complex onto a membrane-embedded, full-length CD81 places the residues Tyr529 and Trp531 of E2 proximal to the membrane. Liposome flotation assays show that low pH and CD81-LEL increase the interaction of E2 with membranes, whereas structure-based mutants of Tyr529, Trp531 and Ile422 in the amino terminus of E2 abolish membrane binding. These data support a model in which acidification and receptor binding result in a conformational change in E2 in preparation for membrane fusion.
Discussion
Viral membrane fusion involves a two-step mechanism: priming (for example, proteolysis) and triggering (for example, acidification and/or receptor binding)28. Once triggered, the trimeric viral glycoprotein introduces a fusion loop or peptide into the cellular membrane, and this is followed by a conformational rearrangement that draws the two membranes together. HCV entry involves cell-type recognition and binding, translocation to the tight junctions and membrane fusion to the endosome. HCV fusion requires both E1 and E2 glycoproteins as well as low pH and is primed by CD81-LEL27. Thus far, there is little evidence of an eE2 trimer, although the E2 stem, transmembrane helix or E1 may influence oligomerization. The structural data we provide here demonstrate a mechanism by which HCV E2 binds to the cellular receptor CD81 at low pH, which results in the extension of an internal E2 loop towards the endosomal membrane. A fusion loop for HCV has yet to be identified, but the CD81-binding loop exhibits many necessary characteristics (that is, membrane binding, low-pH trigger and CD81-dependent extension). Furthermore, there is some additional evidence that a fusion loop may exist in E1 (residues 264-294)29. For fusion to occur, a conformational change must bring the viral, membrane-embedded transmembrane helix at the carboxyl terminus of E2 into contact with the host membrane-a distance of 35 Å according to our docking model (Fig. 4a). The intervening stem region of E2, which was omitted in this study, could span this distance and warrants further investigation.
Together, our results show that during entry to the host cell, E2 uses a hybrid triggering mechanism in which both acidification and CD81 interaction are necessary for optimal membrane binding.
| |
| |
| |
|
|
|
