Pandemic Protein

Picture a pandemic and you may think of smallpox, tuberculosis, cholera and the Black Death. Closer to home, the 2009 influenza pandemic is still fresh in people’s memories – and the H1N1 virus that caused it is still under study. Recently, scientists at St. Jude Children’s Research Hospital discovered that in the human viruses, the hemagglutinin protein responsible for fusing the virus to its target cell became more stable in an acidic environment than it was in earlier swine viruses (1) – exactly the property needed for airborne human-to-human transmission. They also demonstrated that when the protein was mutated to increase its activation pH, it lost the ability to spread via airborne particles. We spoke with Charles Russell, who led the project, to find out more.

What led you to examine the stability of hemagglutinin as a function of pH?

The influenza virus hemagglutinin protein has long served as a model for how a protein can cause membrane fusion. The high-resolution structure of hemagglutinin was determined over 30 years ago. Since then, researchers have studied how low pH in the endocytic entry pathway activates it to undergo structural changes that cause membrane fusion. So we know how the molecule changes its shape, but we had questions about how these structural changes affect the ability of an influenza virus to first infect one host (like birds or pigs) and then another (like humans).

What changes occur in hemagglutinin when moving to a more acidic environment?

Think of the influenza virus as a ball with spikes sticking out. The protein is attached to the virus’ membrane by a stalk; sticking out farther is a globular head domain with a pocket that points at and binds to receptors on a host cell. After binding, the cell “swallows” the virus into an endosome. Over time, the virus pumps protons into the endosome. When the pH has dropped enough, a complete hemagglutinin shape change is triggered.

First, the receptor binding domain heads pop off. Then the fusion peptide (part of the stalk), shoots out like a harpoon into the membrane of the endosome. After that, the protein bends back on itself so that it looks like a hairpin, fastening the virus to the host cell. You can think of hemagglutinin like a mousetrap. When the trap is set (the protein is folded and activated by cleavage), it waits until the mouse bites the cheese (the pH is low enough). After it is triggered, the spring in the trap snaps the wire down to catch the mouse (just like the hemagglutinin protein shoots its fusion peptide into the target endosomal membrane, then snaps back to pull both membranes together).

What implications might this have for disease prediction and prevention?

We now have a better idea of what animal viruses pose a threat of human transmission and pandemic. So we will be better prepared to control infections of risky viruses, and we will have a better idea which viruses to target with vaccines to prevent possible pandemics. It takes months to prepare an influenza vaccine, so we need to have a head start. There are also experimental drugs that bind to the hemagglutinin stalk. We know that, in some cases, resistance to these drugs occurs by decreasing hemagglutinin stability (raising the activation pH). But now, we also know that the penalty for such resistance is lower transmissibility – so we should continue to pursue antiviral drugs that stop hemagglutinin membrane fusion.

What are the next steps for your work?

We want to know whether the other influenza viruses that have caused human pandemics have also had altered hemagglutinin activation pH, and what the tolerable range of hemagglutinin activation pH is in influenza virus reservoirs, intermediate hosts, and humans. Finally, we want to know how altering the hemagglutinin acid stability with a mutation can help us make live influenza virus vaccines that are grown more efficiently and better stimulate protective immunity.