On its way to making you miserable, a flu virus does a lot of work: enters a lung cell; unwinds its genetic material; copies itself many times; and sends newly made viruses out of the cell so they can infect other cells. A viral protein called M2 plays a critical role in several of these steps. Recently, NIAID grantee James Chou, Ph.D., of Boston’s Harvard Medical School, produced a highly detailed snapshot of M2. The picture—the first of this type of protein created through nuclear magnetic resonance (NMR)—offers clues about why once-powerful flu drugs no longer work and suggests ways to restore their effectiveness.
Scientists have studied M2 since the 1980s, when they discovered it to be the target of the antiviral drugs amantadine and rimantadine. Formerly useful weapons against flu, these drugs have now become less effective, thanks to the ease with which the virus mutates and thereby escapes their attack.
Researchers later learned that M2 is what’s known as an ion channel protein. The bulk of M2 consists of four twisting columns of amino acids–protein building blocks—that form a cone-like pore in the virus’s outer shell. As a flu virus travels into cells inside acid-filled bubbles called endosomes, the acid opens the M2 pore and allows positively charged particles to flow into the viral core. This helps loosen the densely packed viral DNA, a step that must occur before the virus can reproduce. Later in a virus’s lifecycle, M2 comes into play again by preventing a premature change in the shape of another viral protein that the progeny virus will need to successfully infect additional cells.
A full understanding of how M2 works requires a detailed picture of its shape, something that wasn’t easy to produce. Dr. Chou and his colleague Jason Schnell, Ph.D., used NMR, which measures distances and angles between atoms in free-floating molecules. NMR creates images of proteins or parts of proteins dissolved in liquid. In this way, Dr. Chou explains, an NMR image more closely mimics how a protein behaves in a living cell.
Dr. Chou’s NMR portrait of M2 showed the four-part channel in the closed position with a tail of amino acids anchoring it to the inner face of the viral shell. In the presence of acid, the columns of the pore destabilize and open, the researchers believe. When the investigators added rimantadine to the mix, the resulting image showed the drug binding at four external spots near the channel gate. This binding jammed the pore in the closed position.
“If we can understand how the drug blocks the channel and how mutations allow the virus to evade the effects of the drug, we could come up with better approaches to making drugs that will retain their ability to keep the pore closed,” says Dr. Chou.
Schnell JR and Chou JJ. Structure and mechanisms of the M2 proton channel of influenza A virus. Nature 451:591-95 (2008)
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Last Updated April 03, 2008