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Structural Biology

Cell Adhesion Receptor Caught on Bended Knee

The First Crystal Structure to Be Solved for Shape-Shifting Integrin

A team led by M. Amin Arnaout, HMS professor of medicine and director of the Structural Biology Program at Massachusetts General Hospital, has produced the first crystal structure of an integrin, a mediator of cell adhesion and numerous cell processes that plays a role in tumor angiogenesis, osteoporosis, and some infectious diseases. A shape-shifter as well as a molecular behemoth at nearly 2,000 amino acids, the integrin was a particularly difficult beast to capture, and Arnaout's team spent five years pursuing it. Their findings appear in the Sept. 6 Science Express.

cell structure

Crystal structure ties up loose ends. At left is a ribbon drawing of the crystallized extracellular segment of alpha-V beta-3. The alpha-V and beta-3 subunits appear in blue and yellow, respectively. The structure is severely bent at the kneelike "genu" (arrows). Disordered regions are in gray. At right is a computer model of the straightened alpha-V beta-3, which was developed by extension (135 degrees) and rotation (120 degrees) of the bent structure at the genu. The approximate location and shape of three small and disordered domains are shown in gray. Illustration courtesy of M. Amin Arnaout

Integrins are receptors that are expressed ubiquitously on the surface of cells to give them a certain stickiness by linking them to one another and to the extracellular matrix. Each integrin forms only a weak link, so they work in teams, clustering together where needed most. Unlike classical receptors, integrins naturally lie dormant until they are switched on by a signal from the cell. This level of control is especially crucial for blood cells, which need to circulate freely without sticking to the lining of blood vessels. Once activated, integrins can transmit signals inward to the cytoskeleton, helping to integrate the cell's external and internal environments.

The Bull's Eye

Arnaout's team, which included Jian-Ping Xiong, HMS instructor of medicine at MGH, and Thilo Stehle, HMS assistant professor of pediatrics at MGH, was specifically interested in integrin alpha-V beta-3, an attractive drug development target for several potential applications. The integrin is expressed on many cell types and has a role in allowing new blood vessels to form at tumor sites and in allowing the epithelial cells of breast tumors to spread. It also serves as an entry point for adenovirus as well as the viruses that cause foot-and-mouth disease and HIV. And it is expressed in the bone-resorbing cells implicated in osteoporosis. After the team succeeded in crystallizing the protein, it still faced the task of getting a handle on it. When a beam of x-rays passes through a crystal, the resulting diffraction pattern is like a coded message giving a complex set of coordinates. "What is also needed is to get the strongest x-ray source you can lay your hands on because you need as many diffractions as possible, especially for a huge protein," Arnaout said.

A Molecular Flip-flop

The crystal, surprisingly, did not assume the shape it did in earlier studies using a different technique; each of the two legs was severely bent at a single joint. "When we saw this structure folded, it was the first indication that you had a focal point in the structure—which we called the "genu," because it's really like a knee—that is, highly flexible," Arnaout explained. It was known that integrins shape-shift when switching from an inactive to an active state, and this flexible joint offers a possible mechanism for this change.

Thilo Stehle, first author Jian-Ping Xiong, M. Amin Arnaout, and David Scott

A team of HMS researchers has for the first time determined the crystal structure of one of the many integrin proteins found on the surface of cells. From left are Thilo Stehle, first author Jian-Ping Xiong, M. Amin Arnaout, and David Scott. Stehle and Xiong are modeling 3-D glasses for seeing their structure in depth. Photo by Steve Gilbert

The crystal structure shows the integrin in its active state. If a bent shape indicates activity, perhaps the binding site is not the only place to target a drug. "Maybe we should block the ability to flex," Arnaout said, which may keep the protein inactive.

—Courtney Humphries

Copyright 2001 by the President and Fellows of Harvard College