The NCI study, Molecular Architecture of Native HIV-1 gp120 Trimers, published online July 30 in Nature, shows how the scientists used imaging techniques to produce a three-dimensional representation of the HIV spike, a key part of the HIV virus that changes shape after binding to immune system cells or to infection-fighting antibodies. Previous research has shown how HIV interacts with immune system cells and antibodies, but the new research characterizes, for the first time, the structure of the intact spike on virus particles, which can be used to design treatment and prevention approaches, says Dr. Sriram Subramaniam, head of the research team that carried out the study.
"Previous research showing how HIV interacts with immune system cells and antibodies has been important in vaccine design," Subramaniam says. "However, understanding the complete structure of the viral spike may reveal other vulnerable targets. This knowledge will be crucial to solving the puzzles associated with strategies at the heart of virus invasion."
The Subramaniam lab has been pioneering advances in electron tomography and related methods for three-dimensional electron microscopy, and is applying these emerging technologies to understanding not only virus-host interactions, but also structures inside the cell that distinguish cancer cells from normal cells. For this study, Subramaniam's team used cryo-electron tomography to freeze the HIV virus. The team took pictures of it from different angles, and then used advanced computer image-processing methods to average thousands of high-resolution images of individual spikes. This enabled them to interpret the three-dimensional images in terms of atomic structures.
The HIV spike is composed of two types of proteins, called gp120 and gp41, which interact with one another to form a protein pair. The final structure of the spike is achieved by the interaction of three of these protein pairs on the surface of the virus, forming a structure called a trimer. The head of the spike consists of gp120, and it is responsible for binding to the receptor on the target cell, which is a protein called CD4; gp41 spans the virus' outer membrane, forming the stalk of the spike. gp41 is responsible for events in which HIV injects its genetic material into the host cells. The trimeric spike is held together by strong contacts among the gp41 proteins at the bottom, but by only a few contacts among the gp120 proteins at the top.
The researchers found that, upon interaction with CD4 receptors, the gp120 proteins rearrange, causing the spike to spring open and exposing the gp41 stalk and other structures that are required for the virus to infect target cells. This rearrangement also draws the virus and the target cell closer together, which may help gp41 penetrate the target cell, allowing the virus' genetic material to be injected, according to the study.
In addition, the researchers also visualized the HIV spike to examine how an antibody called b12 can neutralize a broad range of HIV particles. The team showed that, in response to b12, the gp120 proteins rearrange in a similar manner as with CD4, but b12 prevents the spike from opening completely. The antibody locks the spike in a semi-open position, preventing the series of actions that enable HIV to infect the target cell.
Gaining understanding of the binding of one of the most effective broadly neutralizing antibodies could lays the foundation for designing more effective strategies for blocking HIV infection, Subramaniam says.
"The insights into this structure potentially change the paradigm for HIV vaccine development," he says. "Our work also represents a technical breakthrough in the general strategy for how we go about determining 3D structures of proteins when they are still in the physiological environment of a virus or cell. We are now working actively to increase the resolution of our structural analyses, and to understand the differences in binding between antibodies that neutralize, which are very rare in HIV-infected individuals, and non-neutralizing antibodies that are found in almost all AIDS patients. Knowing what these differences are will be critical to designing better strategies to neutralize HIV and to providing a new addition to the arsenal of strategies to combat HIV/AIDS."