Scientists watch proteins move on surface of HIV virus in real-time

In a new study published in the journal Science, researchers reveal that - for the first time - they have created technologies that allow them to see how HIV proteins move on the surface of the virus.

The ability to watch proteins on the surface of the HIV virus move may shed light on how the virus infects human immune cells and lead to strategies that prevent such infection.

The research team at Weill Cornell Medical College in New York, NY, says the discovery may shed light on how the HIV virus infects human immune cells, paving the way for strategies that prevent such infection.

Making the movements of HIV visible so that we can follow, in real-time, how surface proteins on the virus behave will hopefully tell us what we need to know to prevent fusion with human cells - if you can prevent viral entry of HIV into immune cells, you have won.

In their study, the researchers describe how they created fluorescent molecules, referred to as "beacons," and introduced them to the outer covering of the HIV virus - called the "envelope."

They then modified a technique called single-molecule fluorescence resonance energy transfer (smFRET) imaging to watch two of the beacons.

This imaging method uses fluorescent light to measure the distance viral particles travel. In this study, the team were able to measure distances - down to a billionth of an inch - between the two beacons, which lit-up in different colors. The team were then able to detect real-time shape-shifting of the virus as the two beacons moved.

Using this technology, the researchers were able to analyze the movements of envelope proteins - gp120 and gp41, known as "trimers" - on the surface of the HIV virus.

Such proteins are important in allowing the HIV virus to infect human cells that carry CD4 receptor proteins - the proteins that assist HIV in binding to a cell. The researchers explain that these proteins "open up like a flower" when CD4 is present, revealing a gp41 subunit.

Watching proteins on the HIV virus 'dance'

On watching the movements of the envelope proteins, the researchers found that the gp120 proteins continuously shape-shift, and that each protein's movement was "similar" and "distinct" in both timing and nature.

This answered the first big question of how opening of the envelope trimer is triggered. Many scientists believe that the particles remain in one conformation until they come across a CD4-positive cell. But we saw that the proteins dance when no CD4 was present - they change shape all the time.

On introducing synthetic CD4 to the virus, they found that some of the antibodies it contained reduced the effectiveness of the gp120 protein, meaning the ability of the HIV virus to infect human immune cells was reduced.

The team says they witnessed similar results when they introduced a small molecule to the virus, which is now being tested for its effectiveness in preventing HIV infection.

Commenting on team's findings, the researchers say:

"The practical outcome from this technology is that we can begin to understand how the biological system moves. So far we have detected three different conformations of the envelope trimer. We are working now to improve the technology to achieve the imaging precision we need to make broadly effective therapies."

A 'breakthrough for science'

In another study recently published in the journal Nature, a team led by researchers from the National Institute of Allergy and Infectious Diseases were able to view a 3D structure of one of the aforementioned conformations using X-ray crystallography.

The antibodies used in the crystallography study are ones that we observed to stop the dance of the HIV envelope proteins, pushing the trimer assembly into a quiescent, ground state.

"This concrete, atomic resolution picture of what the pre-fusion machinery looks like and where these antibodies bind provides an important step forward to understanding HIV's biology."

The researchers add that in future research, both the smFRET and X-ray crystallography methods can be used in sync to enhance understanding of the functions of HIV's surface proteins through analyzing their movements.

The approach is really a breakthrough for science because most research is done in a test tube where billions of molecules are present, all behaving independently. It is very difficult to extract direct information about these types of movements from indirect observations, such as those that don't use imaging technology.

"The single-molecule approach allows practical, interpretable, real-time information to be obtained about molecular processes in complex biological systems."