A role for the prion's better half.

Seldom has so little been known about a protein at the heart of a Nobel prize. This year's Nobel Prize in Physiology or Medicine lent respectability to the controversial theory that mad cow disease and several similar neurodegenerative disorders in people stem from the conversion of a harmless protein called PrP into an infectious, cell-killing agent known as a prion. Scientists still have few clues concerning PrP's normal purpose on the surface of cells. Mice lacking their PrP gene live to old age and appear healthy.

David R. Brown of the University of Cambridge in England argues that PrP's main role is to bind copper atoms. Moreover, this task may help cells resist the toxicity of highly reactive molecules called oxygen radicals.

Other researchers, notes Brown, had shown that a part of PrP can bind copper. He and his colleagues have now found that brain cells of mice lacking the protein die more readily after exposure to copper than normal brain cells do. Previously, they had discovered that brain cells lacking PrP succumb more easily to oxygen radicals. Superoxide dismutase, an enzyme that protects cells from oxygen radicals, requires copper for its activity, prompting Brown to suggest that PrP helps make copper available to the enzyme. Since copper can alter communication between brain cells, a role for PrP in copper metabolism may also explain subtle neurological differences observed in mice lacking PrP.