Dec. 9, 1999 La Jolla, Calif. -- The living cell is the prototypical shape-shifter. At any given moment, it will reorganize itself to move, grow, replicate and even die.
To achieve its Gumby-like existence, the cell's internal protein scaffolding, its cytoskeleton, is split apart and put back together ... over and over again.
Until recently, little was known about how this basic life process was accomplished on the molecular level. But now scientists at The Salk Institute have shown in three-dimensional detail how remodeling begins in one of the most prevalent of these cytoskeletal proteins, called actin.
The study, published in the current issue of the journal Science, reveals part of the structure of molecular scissors called gelsolin, caught in the act of severing and capping actin.
"It's a dramatic structural reorganization," said Senyon Choe, associate professor of structural biology at Salk and the study's principal investigator. "It's first unexpected, and it's also nice to see."
Aside from revealing in atomic detail this fundamental step in the life of a cell, knowledge of the structure of this active region of gelsolin could have implications for the rational design of future drugs.
For example, it's been suggested that gelsolin might be used as an inhaler to break up the thick actin-based mucous in the lungs of patients with cystic fibrosis. This condition is the most common cause of chronic lung disease in children and young adults, and the most common fatal hereditary disorder affecting Caucasians in the United States.
Gelsolin also is widely found in the blood stream, where it dissolves actin left over from dead cells. Without gelsolin, the actin filaments would soon accumulate in blood vessels, causing life-threatening clots.
"We know from experience that knowledge of structures provides a reliable foundation to plan experiments that lead to drug development down the road," said Robert C. Robinson, a postdoctoral researcher in Choe's lab and the study's first author. "Similar information cannot be obtained from other methods."
Using X-ray crystallography techniques, the scientists determined how gelsolin works to sever actin. It all begins when calcium ions bind to free-floating gelsolin. In its normal state, the protein is compactly held together by a molecular sheet that runs through the core of three of gelsolin's six moving parts, known as G4, G5 and G6. Upon binding with calcium, the sheet is severed between G4 and G6. The molecule is torn apart, exposing the actin-binding site on G4. This allows the protein to sever the actin filaments, much like the two arms of a pair of scissors opening up to cut a piece of string.
During this process, G6 undergoes a radical transformation, rotating about 90 degrees in both a horizontal and vertical plane.
"To structural biologists, this is a remarkable conformational change," said Choe. "The protein is configured as if it were a tightly wound spring. When it's exposed to calcium, the spring is released, and it causes all these rearrangements."
Also participating in the study, funded by the National Institutes of Health, were Vincent P. Le of the Salk Institute; Marisen Mejiliano and Helen L. Yin, at the University of Texas Southwestern Medical Center; and Leslie D. Burtnick, of the University of British Columbia.
The Salk Institute for Biological Studies, located in La Jolla, Calif., is an independent nonprofit institution dedicated to fundamental discoveries in the life sciences, the improvement of human health and conditions, and the training of future generations of researchers. The Institute was founded in 1960 by Jonas Salk, M.D., with a gift of land from the City of San Diego and the financial support of the March of Dimes Birth Defects Foundation. Website: http://www.salk.edu.
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