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Prions' dirty little secret

Fifteen years ago, scientists at the National Institutes of Health reported that malformed prions—proteins that can trigger lethal illnesses including mad cow disease—remain on soil surfaces for at least 3 years. Now, scientists report why rain doesn't flush away the prions: The proteins bind almost irreversibly to clay.

In fact, clay can "retain up to its own mass of ... prion proteins," says Peggy Rigou of the National Institute of Agronomic Research (INRA) in Jouy-en-Josas, France.

Her team added sheep prions to pure clay, sandy soil, and loam. Positively charged parts of the protein molecules bound to the negatively charged surface of the clay that was present in all the soil samples. Extensive washing failed to dislodge the prions. However, when the chemists treated the mixtures to make the proteins negatively charged and then ran an electric current through each mixture, the prions migrated off the clay particles.

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