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Scientists at the Whitehead Institute for Biomedical Research have identified a spring-like mechanism that is a crucial weapon in the arsenal of the flu virus, enabling it to fuse with human cell membranes. This mechanism, described in the May 21 issue of Cell, represents a new target for antiviral drugs.
Preliminary evidence indicates that the same mechanism also may be important in infection with HIV-the virus that causes AIDS-and in common cellular processes such as the movement of neurotransmitters in the brain.
Dr. Peter S. Kim, a member of the Whitehead Institute and assistant investigator of the Howard Hughes Medical Institute, explains that the common theme in all of these processes is membrane fusion.
In flu, the central core of the virus is surrounded by a protective membrane. Infection begins when the viral membrane binds to the surface of the target cell membrane, causing the target cell membrane to fold inward into a small bubble (called an endosome). The bubble pinches off inside the cell with the virus floating in the middle. Infection is not truly complete, however, until the virus membrane fuses with the bubble membrane, spilling its contents into the body of the cell.
The nature of the fusion process in flu has been a mystery for more than two decades. It has been clear for some time that fusion depends on one particular protein in the viral membrane, called hemagglutinin (HA). In 1985, scientists even identified the region of HA that inserts itself into the "bubble" membrane to initiate fusion. They called this sequence the "fusion peptide."
The problem for drug designers and others has been the location of the fusion peptide in the molecule: common sense would dictate that the peptide should be near the top of the large HA molecule, ready to make contact with the target membrane. Instead, structural studies showed it was buried deep inside of HA very close to the viral membrane. Conceptually, understanding fusion was like trying to imagine plugging in a floor lamp with a one-foot cord when the outlet was in the ceiling.
Chavela M. Carr, an MIT graduate student in biology, and Dr. Kim solved the problem by looking at the pattern of protein building blocks (amino acids) in HA-these patterns can provide important clues about the three-dimensional structure of a molecule. In one region of HA, the Whitehead scientists found a surprising discrepancy between the pattern of the amino acid sequence and experimental evidence. They found a pattern commonly associated with a structure called a coiled-coil in a section of the molecule observed to be a long, uncoiled loop.
This observation and subsequent experiments led to the development of a novel model for membrane fusion in flu; the central tenet of the model is a spring-loaded mechanism that launches the fusion peptide to the surface of the virus.
Ms. Carr explained, "The piece of HA that carries the fusion peptide is like a bent spring-as you'd expect with a mechanical spring, tension causes the middle of the spring to uncoil. Two forces seem to hold the spring in the bent position: the fusion peptide itself, which acts as a hook at one end of the spring, and another subunit of the HA molecule that acts as a clamp.
"It's been known for some time that fusion follows a drop in pH [an increase in acidity level] inside the endosome, and that this change releases both the hook and the the clamp," she adds. "We've shown that the spring straightens out, allowing the fusion peptide to bury itself in the membrane of the endosome; this mediates fusion."
Dr. Kim said, "Ultimately, this model could help in the design of new antiviral drugs. For example, therapeutic agents might be designed to prevent activation of HA in a low pH environment. Alternatively, agents might be designed that cause the conformational change to occur early. Others have shown that acid pretreatment of the flu virus abolishes infectivity; knowing about the spring-loaded mechanism will help in the discovery of drugs that can accomplish this task in the body."
Since the initial discovery of the spring-loaded mechanism, Ms. Carr and her collaborators at the Whitehead Institute and MIT have focused on other systems that exhibit similar patterns. The HIV virus is a prime candidate. Like hemagglutinin, the coat protein of HIV is made up of two pieces, gp41 and gp120. Analysis of the amino acid sequence of gp41 reveals a pattern with striking similarities to the spring-like region of HA. Similarly, gp120 may be analogous to the clamp that holds the spring-loaded portion of HA in the bent position until the appropriate time for membrane fusion.
This model could explain several different lines of experimental evidence. First, researchers have shown that when HIV binds to CD4 receptors on human cells gp120 is shed, triggering the fusion reaction that leads to viral infection. Second, a research team from Duke University has reported that a synthetic coiled-coil peptide from gp41 inhibits replication of HIV; it may do so by interfering with the spring-loaded mechanism. Finally, scientists have demonstrated that cells bearing the gp41 subunit of HIV fuse together, suggesting that gp41 alone can initiate membrane fusion (and that the interaction between gp120 and gp41 prevents membrane fusion).
"We believe that many different viruses may use the spring-loaded mechanism to initiate fusion," Carr said. "We're just beginning studies to assess how general the process really is."
The Whitehead/MIT group also has discovered the molecular pattern associated with the spring in a ubiquitous cellular protein involved in membrane fusion in species from yeast to humans. In mammals, this protein plays a vital role in the transport of neurotransmitters (chemical signals that carry information from one nerve cell to another). It also is the primary target of toxins in diseases such as tetanus and botulism-the toxins cut the molecule in half, preventing cells from releasing neurotransmitters into the intracellular environment. Studies emerging from Ms. Carr's research could help scientists develop new strategies for fighting these life-threatening diseases.
A version of this article appeared in the May 26, 1993 issue of MIT Tech Talk (Volume 37, Number 34).