Structural and biochemical evidence that scrapie-associated fibrils assemble in vivo

Scrapie-associated fibrils (SAF) are a ubiquitous pathological feature of brains affected by scrapie and the other scrapie-like agents. They are composed of PrP, a heterogeneous glycoprotein which is also present in normal brain but not as SAF. The PrP protein associated with SAF is partially resistant to proteinase K, whereas the soluble form is not. It has been proposed that SAF do not exist as such in vivo, but rather self-assemble from subunit structures liberated from membranes by detergent extraction during purification. We have purified SAF by a method that does not employ proteinase K. We show that the PrP protein from infected but not uninfected brain is partially resistant to protease digestion before and after detergent extraction. Likewise, SAF can be sheared by sonication before or after detergent extraction. In addition, SAF from mice infected with different strains of scrapie have different sedimentation properties. Since SAF-dependent properties exist before detergent extraction, then so must SAF. They are therefore not a detergent-induced artefact but most probably assemble in vivo.