All mammals produce a version of the prion protein, but scientists don't know what it normally does. In an altered form, the prion protein becomes an infectious agent that causes "mad cow disease" and its counterparts in other animals, including humans. Researchers now suspect the normal prion protein plays a role in the transport or regulation of copper in the body's tissues.
"We all have the prion protein in us, but no one knows exactly what its usual function is. In very rare instances it undergoes a change in conformation, and then it causes disease," said Glenn Millhauser, a professor of chemistry and biochemistry at the University of California, Santa Cruz.
Millhauser's group is working to understand both the normal and disease-causing forms of the prion protein.
"The prion protein is the only infectious protein that causes disease in vertebrates. Every time you look at it, you see something surprising," Millhauser said.
Proteins, which are linear chains of smaller molecules called amino acids, fold into complex three-dimensional shapes to carry out their functions. The prion protein sometimes folds into the wrong shape. Misfolded copies of the protein then act as templates for normal prion proteins to refold incorrectly. These misfolded proteins build up in the brain, causing memory loss, lack of coordination, dementia, and eventually death. The human prion disease is called Creutzfeldt-Jakob disease, a variant of which can be acquired by eating beef from a cow with bovine spongiform encephalopathy (mad cow disease).
Although it is still a mystery what the protein normally does, scientists are getting closer to the answer. To discover a protein's function, researchers usually create a mutation in the corresponding gene and observe what goes wrong in lab animals. But animals without a functional prion protein don't have severe medical problems.
Scientists got their first clue when they discovered that the prion protein binds copper, a mineral essential to living systems in small amounts but toxic at high concentrations. Copper has many functions, including helping the body absorb iron and aiding in nerve and brain function. In humans, too little copper can cause anemia and depression; too much can result in headaches, kidney damage, and psychological problems. Animals without functional prion proteins experience tissue damage that researchers believe is linked to a copper imbalance, but the effects are minimal over the life span of most laboratory animals.
Millhauser has been working with colleagues at the Albert Einstein College of Medicine, the Medical College of Wisconsin, UC Davis, and UC San Francisco to understand how the prion protein interacts with copper ions. Their research has revealed tantalizing new clues to the possible function of normal prion proteins and the factors involved in misfolding. The group's latest results were published in a March 2002 issue of the journal Biochemistry.
"Exploring copper regulation could lead to a new level of understanding of how these prion diseases develop and how they cause degeneration of the nervous system," Millhauser said.
The prion protein binds copper in a domain that contains a series of eight amino acids repeated four or more times. Other researchers had shown that each "octarepeat" binds one copper ion. Millhauser's group found that only five amino acids out of the eight are necessary for copper binding.
Working with UCSC chemist William Scott, Millhauser also determined the three-dimensional structure of this short sequence bound to copper using a technique called x-ray crystallography. He found that the copper binds to the backbone of the protein in two places and to an amino acid side chain.
The group confirmed their results with electron paramagnetic resonance (EPR), a technique that measures copper's absorption of microwaves while in a strong magnetic field. The absorption strength indicates which atoms are bound to the copper. The EPR experiments showed that the binding observed in the x-ray crystallography is the same when the amino acid sequence is dissolved, as it would be in a cell, and when the amino acids are part of the whole protein.
Metal binding to a protein backbone is extremely sensitive to changes in acidity. If the prion protein's environment becomes acidic, the protein will release copper bound to it. Thus, the prion protein may be part of a system to transport copper in and out of cells using differences in acidity.
Alternatively, the prion protein could sense the copper concentration outside cells and send a signal to a separate transport system. The amino acid glutamine is part of the repeat sequence in all prion proteins, but it is not involved in copper binding. Millhauser thinks the glutamine might send a signal to the cell when the protein binds copper, perhaps by binding to glutamines in other prion proteins.
He also believes the glutamine could be important in the misfolding of prion proteins. If glutamines from different prion proteins do bind, then other parts of the protein can come into contact. If one of the proteins is misfolded, it could act as a template for the other protein to become misfolded. Millhauser and Scott are now using x-ray crystallography to examine copper binding to the entire protein and are performing experiments to better understand the misfolding process.
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Editor's note: Reporters may contact Glenn Millhauser at (831) 459-2176 or glennm@chemistry.ucsc.edu.
