Ultrastructural studies on scrapie prion protein crystals obtained from reverse micellar solutions.

The structural transition from the cellular prion protein (PrPC) that is rich in alpha-helices to the pathological form (PrPSc) that has a high beta-sheet content seems to be the fundamental event underlying the prion diseases. Determination of the structure of PrPSc and the N-terminally truncated PrP 27-30 has been complicated by their insolubility. Here we report the solubilization of PrP 27-30 through a system of reverse micelles that yields monomeric and dimeric PrP. Although solubilization of PrP 27-30 was not accompanied by any recognizable change in secondary structure as measured by FTIR spectroscopy, it did result in a loss of prion infectivity. The formation of small two- and three-dimensional crystals upon exposure to uranyl salts argues that soluble PrP 27-30 possesses considerable tertiary structure. The crystals of PrP 27-30 grown from reverse micellar solutions suggest a novel crystallization mechanism that might be applicable for other membrane proteins. A variety of different crystal lattices diffracted up to 1.85 nm by electron microscopy. Despite the lack of measurable biological activity, the structure of PrP 27-30 in these crystals may provide insight into the structural transition that occurs during PrPSc formation.