Scrambling a virus

Building a complex machine requires collecting all the parts and putting them together in a precise order. Building a virus requires a similar process; an infected cell takes it apart, copies its molecules, and assembles them into new viruses that go on to invade other tissues. Human cells sometimes protect themselves by interfering with this viral assembly line. For example, a human protein called MxA sequesters viral components so that they can't be packed into new copies of the virus. Oliver Daumke's team at the MDC, together with the groups of Otto Haller and Georg Kochs from the Virology Department at the University of Freiburg, have now obtained a detailed view of the architecture of MxA that helps explain how it missorts viral molecules. The study appeared in the May 27 edition of Nature.

The structure of the stalk region of a single MxA molecule (top). This region assembles with the stalks of other MxA proteins (middle). The bottom image shows how 32 full MxA molecules assemble via the stalk to form a ring-shaped machine, which captures the virus in the interior. On the outer side of the machine are the "motor modules" which use GTP energy. 

A protein's behavior is determined by its shape and chemistry. It starts as a string-like chain of amino-acids that folds into a precise three-dimensional shape. This structure can have docking sites for other small molecules so that it can capture energy and carry out its function in the cell. Song Gao from Oliver’s lab thought that a close study of the structure of MxA might show how it binds to viral molecules and sequesters them.

Researchers had already proposed that a region of the molecule folds into a stalk-like structure that helps four copies of MxA bind together. These quadruple clusters are necessary for it to dock onto viral proteins, to assemble in a ring-like structure around them and to sequester them. Song and his colleagues made mutant forms of MxA with small changes in the stalk region, discovering that the protein no longer formed clusters and carried away viral molecules. But finding out why would require a close look at the details of the stalk.

Since proteins are too small to be seen with even the most powerful microscopes, researchers turn to methods like X-ray crystallography to obtain information about the arrangement of their atoms and their shapes. In this procedure, proteins are crystallized and bombarded with X-rays, producing a diffraction pattern that can be turned into a three-dimensional model.

Song and his colleagues obtained crystals of the stalk region of MxA and used them to create a structural map of this part of the molecule. They discovered that the amino acid chain in this region forms long helices that fold back and forth four times to create a bundle – the way you might fold and tie a computer cable that's too long. The stalk made by these folds has three interfaces which can dock onto parallel structures in other MxA proteins. The way the copies assemble creates a docking site for viral proteins on one side; the other end acts as a motor module that is driven by the small “fuel” molecule GTP. Transferring energy to MxA affects the way how it handles viral molecules.

Some mutations in MxA disrupt this activity and others don't. The study produced an explanation: if a change in MxA's chemical code alters one of the binding sites in the stalk, it can no longer form clusters with other copies. This prevents it from binding to viral molecules.

The new picture of MxA is also helping researchers understand a wider range of proteins that have similar stalk structures. Dynamin, which also uses the energy of GTP, plays a key role in the transport of molecules through the cell. Many proteins are packed into membrane bubbles to be shuttled between cellular compartments; these bubbles need to be pinched off from one membrane and carried to their destination membrane. This is an important activity in all cells and plays a special role in neurons, where the transmission of nerve impulses critically depends on the action of dynamin.

Dynamin is involved in pinching, and defects in the molecule have been linked to nerve and muscle diseases. The strong resemblance between its stalk and that of MxA presents a detailed picture for its action: once dynamin assembles via the stalk to form a huge multi-copy machine around the neck of a newly created membrane bubble, the motor modules are precisely placed to be actived. Now they can make use of GTP energy to activate the motor activity of dynamin and pinch off the vesicle. So as well as explaining the antiviral activity of MxA, the study promises to provide important structural information about several other proteins involved in diseases.

- Russ Hodge  

Highlight Reference:

Gao S, von der Malsburg A, Paeschke S, Behlke J, Haller O, Kochs G, Daumke O. Structural basis of oligomerization in the stalk region of dynamin-like MxA. Nature. 2010 May 27;465(7297):502-6.

Full text of the paper
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