Important Protein Structure

A team led by scientists at the Scripps Research Institute and the University of Amsterdam has achieved an important goal in virology: mapping key proteins that attach to the surface of the hepatitis C virus (HCV) and enable it to enter host cells at high resolution.

The findings, published recently in Science, detail the key vulnerable sites of the virus, which can now be effectively targeted by vaccines.

Dr. Gabriel Lander, a professor in the Department of Integrated Structural and Computational Biology at the Scripps Research Institute and senior co-author of the study, said: "This long-standing and interesting structural information on HCV incorporates a large number of previous observations into the structural context, paving the way for rational vaccine design against this incredible goal."

The study is the product of years of collaboration and includes the Ward laboratory, Dr. Gabriel Lander’s laboratory (also a professor in the Department of Integrated Structure and Computational Biology at the Scripps Research Institute); Dr. Rogier Sanders' laboratory at the University of Amsterdam; and Max Crispin's laboratory at the University of Southampton.

It is estimated that about 60 million people worldwide, including about 2 million Americans, have chronic hepatitis C virus infection. This virus infects liver cells and usually forms a "silent" infection over decades until liver damage is severe enough to cause symptoms. It is a major cause of chronic liver disease, liver transplantation, and primary liver cancer.

The origin of the virus is uncertain, but it is thought to have emerged at least several hundred years ago and eventually spread worldwide through blood transfusion in the second half of the 20th century. Although the virus was essentially eliminated from blood banks after its first discovery in 1989, it continues to spread mainly through needle sharing among intravenous drug users in developed countries and the use of unsterilized medical devices in developing countries. Major hepatitis C antivirals are effective, but too expensive for mass treatment.

An effective vaccine may ultimately eliminate HCV as a public health burden. However, no such vaccine has been developed to date, mainly because it is very difficult to study the envelope protein complex of hepatitis C virus, which consists of two viral proteins E1 and E2.

"The E1E2 composite structure is very fragile—it is like a bag of wet spaghetti and always changing shape—which is why imaging at high resolution is very challenging," said co-first author Dr. Lisa Eshun-Wilson.

In this study, the researchers found that they could use a combination of three broadly neutralizing anti-HCV antibodies to stabilize the natural conformation of the E1E2 complex. Broadly neutralizing antibodies are those that protect themselves from a variety of viral strains by binding to relatively invariable sites on the virus in a manner that interrupts the viral life cycle.

The researchers used cryo-electron microscopy to image antibody stable protein complexes. With the help of advanced image analysis software, researchers were able to generate E1E2 structural maps at near atomic-scale resolution, which are unprecedented in clarity and breadth.

Details include most E1 and E2 protein structures, including critical E1/E2 interfaces, and three antibody binding sites. Structural data also revealed "glycan" molecules associated with sugars on top of E1E2. Viruses usually use glycans to protect themselves from the immune system of infected hosts, but in this case, structural data show that glycans of hepatitis C virus clearly have another key role: helping to fix fragile E1E2 complexes together. Understanding these details of E1E2 will help researchers rationally design a vaccine that can robustly stimulate these antibodies to prevent HCV infection.