Heavy Metal Research Is Music To Biologists' Ears

In an important advance in understanding the molecular underpinnings of this disease, scientists supported by the National Institute of General Medical Sciences (NIGMS) have deciphered the three-dimensional structure of a yeast copper "chaperone" protein, a molecule that transports copper to the SOD enzyme. Although copper is necessary for life, it is a potentially toxic "heavy metal" that--in the wrong cellular locale--can damage other molecules, and in some cases can even cause disease. As the name suggests, the copper chaperone protein protects copper from unwanted cellular interactions and safely delivers it to its destination.

The work also identifies a potential target for developing drugs to treat Lou Gehrig's disease, a uniformly fatal condition.

The research report appears in the August 1999 issue of the journal Nature Structural Biology.

A little over six years ago, scientists first linked Lou Gehrig's disease to SOD by showing that some people with the disease had "misspellings" in the gene that encodes SOD. The finding prompted a series of studies aimed at determining how copper gets inserted into this prevalent cellular enzyme. In 1997, Dr. Valeria Culotta of The Johns Hopkins University, collaborating with Dr. Thomas O'Halloran of Northwestern University, first discovered a protein that ferries copper molecules throughout a yeast cell; they coined it a copper chaperone. (Further research has shown that the yeast chaperone is very similar to its counterpart in humans.) Around the same time, Dr. Culotta and her collaborator Dr. Jonathan Gitlin of Washington University in St. Louis unearthed another yeast copper chaperone and found that it was required for SOD to work properly.

Recently, in the April 30, 1999 issue of the journal Science, Drs. Culotta and O'Halloran reported that this second chaperone protein directly supplies a molecule of copper to SOD and showed how the chaperone is a necessary ingredient for the free radical-destroying protein's activity.

"No one thought SOD would need a chaperone," said Dr. Culotta. A long-standing paradox, she noted, was that although in a test tube SOD soaks up copper molecules like a sponge, "[SOD] can't find copper inside a living cell." She and her collaborators now recognize that this is because the potentially dangerous metal is so well hidden, bound up by several protective proteins.

"Only vanishingly small amounts of free metal are normally available in cells, and elevated levels may only be present in disease," said Dr. Peter Preusch, a biochemist at NIGMS. He described the new work as being "of profound significance."

According to Dr. O'Halloran, the recent findings render the copper chaperone a "suspect in the mystery of Lou Gehrig's disease--but suspects are just that. This may be a case of guilt by association."

In a continuing quest to reveal the secrets of how copper traffics throughout a cell, recently Dr. O'Halloran teamed up with a Northwestern colleague, Dr. Amy Rosenzweig, to unravel the structure, or three-dimensional shape, of the copper chaperone protein. Dr. Rosenzweig is well-versed in determining the structures of proteins, using a technique called X-ray crystallography. In this technique, scientists bombard a tiny crystal of protein with high-energy X-rays, then piece together the protein's shape by tracing the directions in which the energy is scattered. According to Dr. Rosenzweig, knowing what a protein looks like can say a lot about how it works.

The "picture" the researchers obtained was worth the proverbial "thousand words." Drs. O'Halloran and Rosenzweig's data offered new insights into the way the copper chaperone might interact with the SOD protein.

Apparent in the X-ray crystallographic data is that the copper chaperone occurs "in double," or as a protein dimer consisting of two identical units. In turn, each of the units is itself composed of separate structural components, called "domains." Coincidentally, SOD also exists as a dimer inside living cells.

Separate test-tube experiments fortify the structure-based predictions. Reporting in the August 20 issue of the Journal of Biological Chemistry, Drs. Culotta and O'Halloran demonstrate how they meticulously picked apart the copper chaperone protein into its separate domains. Each distinctly shaped domain of the protein, the researchers show, plays a separate role in grabbing copper and routing it safely to the SOD enzyme.

Synthesizing all these results, the researchers believe they now have a better handle on how the metal transfer occurs: An SOD-resembling domain in the chaperone transiently associates with half of the SOD dimer, and then passes off the crucial copper cargo. In ALS, researchers suspect, the defective SOD--when energized with the copper it needs to function--runs amuck and causes cellular damage.

That information may someday help patients. With the new knowledge in hand, the team predicts they will soon be able to catch the chaperone-SOD duo in the act of trading off copper--a feat offering drug developers a crystal-clear glimpse of how to cripple this molecular embrace in patients with Lou Gehrig's disease.