This week (Nature, Vol. 410, No. 6825, 08 Mar 2001) Peter Chien and Jonathan Weissman of the University of California–San Francisco explain how they used two species of yeast (Candida albicans and Saccharomyces cerevisae) and an artificial yeast prion protein to investigate how prions jump the species barrier — as happened in the recent BSE / new-variant CJD outbreak.
Yeast cells manufacture a protein, called sup35 that, under certain circumstances, displays prion like behaviour: it forms aggregates, converts its normal counterpart to the prion form, and thus can forms self propagating fibres. But the prion form of sup35 in C albicans can not convert sup35 from S cerevisae into prions and vice versa, due to an ill-understood species-barrier.
The researchers created a flexible prion chimaera — by fusing portions of a S. cerevisae prion and a C. albicans prion, which in principle could take on the shape of either of the two yeast prions. Once introduced in a specific yeast strain, C albicans for example, the chimaera could only take the shape of the prion specific for that strain (C albicans in this example).
Chien and Weissman’s results suggest that the shape of the yeast prion proteins dictate whether they are infectious and this may account for the strong species barrier found in most prion diseases. The fact however, that prions can exist in multiple forms or ‘strains’ in each species may mean that one strain of many may be of the right shape to cause infection across species barriers.
"It may be time to consider the disturbing possibility that certain bovine prion forms have an enhanced ability to cross the species barrier to humans," writes Susan Liebman of the University of Illinois in Chicago, in accompanying News and Views article.
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