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Understanding HIV's Envelope Protein

What are the challenges to understanding this protein’s structure and how would revealing its structure impact the design of possible HIV vaccine candidates?

Over the past two years, researchers have isolated nearly two dozen new antibodies against HIV from the blood of infected individuals (see VAX Oct. 2009 Spotlight article, Vaccine Research Gains Momentum). When tested in the laboratory, these antibodies are capable of inactivating or neutralizing many of the HIV strains currently in circulation and are therefore referred to as broadly neutralizing antibodies (bNAbs). Many of these bNAbs can also neutralize HIV at relatively low concentrations, suggesting they are quite potent.

Now, scientists are using these antibodies to design vaccine candidates that would ideally be able to induce similar antibodies in people before they are exposed to HIV, thereby protecting them against infection (see VAX May 2010Primer on Understanding if Broadly Neutralizing Antibodies are the Answer). However, there are several significant challenges to designing a vaccine capable of eliciting such bNAbs.

Immunogen Design

Researchers start by understanding how these antibodies successfully bind to and neutralize HIV. All of the bNAbs bind to HIV’s Envelope protein, or Env for short, which is the protein that juts out from the surface of the virus in spike-like protrusions (see image, below). By studying how they bind to HIV, researchers hope to identify the non-infectious pieces of the virus they could put into a vaccine candidate to provoke the body’s immune system to make similar antibodies. The pieces of the virus used in a vaccine to invoke an immune response are referred to as immunogens. Because the antibodies bind to the HIV Envelope spikes that dot the surface of the virus, the immunogens will likely be parts of this protein.

 HIV's Envelope Protein
 
Three-dimensional image of HIV showing HIV Envelope spikes on the surface of the virus. These spikes make contact with human cells that the virus infects. The images of these spikes, which are actually three-armed structures known as trimers, were created using a technique called electron tomography that has also provided insights into how their shape changes after HIV makes contact with a human target cell. Image courtesy ofSriram Subramaniam and Donald Bliss at the US National Institutes of Health.
 

However, the process of selecting the pieces of HIV Envelope to put into a vaccine candidate is made more difficult by the fact that this protein is rather unstable. The HIV Envelope, also known as gp160, is actually composed of two different proteins that are weakly bound together. One of these proteins, known as gp120, is what forms the spike, and the other protein, known as gp41, is what makes up the base of the spike. Making matters even more complicated, each of the HIV Envelope spikes is actually composed of three identical gp120/gp41 proteins that are linked together. This three-pronged protein structure is referred to as the trimer.

The HIV Envelope trimer is what binds to human cells, allowing the virus to infect them. To infect human cells, the trimer spikes must be able to undergo complex changes in their conformation, and therefore they are very flexible. As a result, this trimeric protein is unstable, making it more difficult for researchers to fully understand the structure of HIV Envelope and see how some bNAbs bind to it. Researchers have been stymied by the instability and flexibility of the HIV Envelope trimer for many years. Their inability to stabilize the trimer has in turn hampered the development of AIDS vaccine candidates.

X-ray crystallography

To study the structure of proteins, researchers typically use a method known as X-ray crystallography. This method involves sending a beam of X-rays through a solid, crystalline structure of the protein. This allows researchers to determine the precise arrangement of the different atoms that make up the protein, and then to determine how these atoms interact with other proteins, such as antibodies. X-ray crystallography has been used to reveal the structure of several of the key enzymes HIV uses to infect cells and reproduce.

To use X-ray crystallography to study the HIV Envelope trimer, researchers first have to be able to develop a stable crystalline structure of the trimer bound to one of the bNAbs. This has been incredibly difficult because the trimer is so unstable and flops around in space. Researchers have tried several different methods to stabilize the trimer, including adding pieces of synthetic protein into the structure to prop it up and prevent it from shifting around, but, so far, none of these attempts have stabilized the trimer enough that a pure crystal of it bound to an antibody could be obtained.

Researchers have, however, successfully crystallized a single HIV gp120/gp41 protein, which is referred to as a monomer. Some of the bNAbs that have been identified will bind to the HIV monomer, while others only bind to the trimeric HIV Envelope structure. This, plus the fact that the trimeric form of HIV Envelope is what naturally exists, makes the quest to get a crystal structure of the trimer an important goal.