Very little is known about prions, proteins that in certain forms cause bovine spongiform encephalopathy (BSE), or "mad cow disease." But since infectious prions have made the jump from cows to humans¿in whom they cause Creutzfeldt-Jakob disease¿almost 100 people have died, and a mass slaughter of cattle is under way in Europe. Now two scientists from the University of California at San Francisco have looked into how dangerous prions may have made the leap from cows to people in the first place. Their findings were published in today's issue of Nature.

Unlike bacteria or viruses, prions appear to become infectious merely by assuming an altered shape. They then transmit their characteristics via protein-protein interactions, making otherwise normal prion proteins also adopt an abnormal shape. These abnormal proteins become insoluble, which¿in mammals, at least¿leads them to clump together and create brain-cell-killing plaques.

In order to understand how prions bridged the biological gap between different species, Jonathan Weissman and graduate student Peter Chien stitched together segments of prions from two different species of yeast. They found that the resulting hybrid could become infectious in both original kinds of yeast¿exactly the sort of ability needed to cross over into a different organism.

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