Cells infected with prions contain both prion protein isoforms cellular prion protein (PrP C ) and scrapie prion protein (PrP Sc ). PrPSc is formed posttranslationally through the pathological refolding of PrP C . In scrapieinfected ScN2a cells, the metabolism of both PrP isoforms involves cholesterol-dependent pathways. We show here that both PrP C and PrP Sc are attached to Triton X-100-insoluble, low-density complexes or "rafts." These complexes are sensitive to saponin and thus probably contain cholesterol. This finding suggests that the transformation PrP C 3 PrP Sc occurs within rafts. It also reveals the existence of rafts in late compartments of the endocytic pathway, where most PrP Sc resides. When Triton X-100 lysates of cells were incubated at 37°C prior to density analysis, PrP C was still found in buoyant complexes, although it now failed to sediment at high speed. This property was shared by another glycophosphatidyl inositol protein, Thy-1, and also by the raft resident GM1. In one ScN2a clone and in the brain of a Syrian hamster with scrapie, Triton X-100 extraction at 37°C permitted resolution of PrP C and PrP Sc into two distinct peaks of different densities. This suggests that there are two populations of PrP-containing rafts and may permit isolation of PrP C -specific rafts from those containing PrP Sc . Our findings reinforce the contention that rafts are involved in various aspects of PrP metabolism and in the "life cycle" of prions.Prions are unique proteinaceous pathogens that cause a series of fatal encephalopathies such as Creutzfeldt-Jakob disease of humans, scrapie of sheep, and bovine spongiform encephalopathy (1). Prions seem to propagate in the host by posttranslationally (2, 3) refolding a normal host protein, the cellular prion protein (PrP C ), 1 to an aberrant conformation (4, 5). The only known component of prions is the misfolded isoform of PrP C , the scrapie prion protein (PrP Sc ) (6, 7). Current evidence argues that direct interaction of PrP Sc with PrP C is a prerequisite for the transformation PrP C ϩ PrP Sc 3 2PrP Sc (8,9). PrP C is a phosphoinositol glycolipid (GPI)-anchored glycoprotein present on the surface of neurons and other cells (10,11). The PrP isoforms appear to be chemically identical (12) but differ in their conformation (4); PrP C contains ϳ40% ␣-helix and is devoid of ␤-sheet, whereas PrP Sc has more than 40% ␤-sheet (4, 13-16). The two PrP isoforms differ considerably in their properties; PrP C is readily soluble in most detergents and is completely degraded by proteases, whereas PrP Sc is insoluble in detergents, possesses a protease-resistant core termed PrP27-30, and polymerizes into amyloidic structures called prion rods (17,18). Since no isoform-specific PrP antibody has yet been developed, the disparate properties of PrP C and PrP Sc serve as the sole ways to differentiate experimentally between these proteins. The subcellular sites where PrP Sc is formed, and the trafficking pathways leading to these sites, remain largely unknown. Scrapie-infecte...

Axenically grown Bdellovibrio stolpii (i.e., grown independently of the host) was examined for superoxide dismutase, catalase, and peroxidase activities. Kinetics of enzyme synthesis were determined for aerobically grown cultures and for cultures exposed to 100% oxygen. Enzymatic activities varied with the age of the culture. Normally grown cultures exhibited maximum activity during the first 10 h of growth and again as the stationary phase was approached, beginning at about 48 h. Polyacrylamide gel electropherograms of cell-free extracts revealed that B. stolpii contained one major band (1) and two minor bands (II, III) of superoxide dismutase activity. Each of these enzymes was inactivated by H2O2, indicating that they were iron-containing enzymes. Manganese-containing superoxide dismutase was not detected in B. stolpii. Increased oxygenation did not appreciably stimulate enzyme synthesis, for only superoxide dismutase was induced, reaching maximum activity at 10 h and then rapidly falling to normal levels. Superoxide dismutase appears to be the main enzymatic defense against oxygen toxicity in B. stolpii. Induction of superoxide dismutase with 100% oxygen was manifested as an increase in the intensities of the two minor bands of activity, suggesting that isozyme I is constitutive, whereas isozymes II and III are inducible. The induction of isozymes II and III by 100% oxygen was prevented by an inhibitor of protein biosynthesis, chloramphenicol.