Response to Apostol and Surewicz: Structural Underpinnings of Prion Protein Conversion

This is a response to a letter by Apostol and Surewicz (1)

We welcome discussion on probably the most important remaining question of the prion protein conversion. The engineered disulfides in our paper (2) represent very strong structural constraints, which allowed us to investigate conservation of the relative orientation of secondary structure elements in the converted PrP. Results are very robust and clearly distinguish two types of disulfides that either allow or prevent conversion, depending on the position within the subdomains of PrP. Our results are indeed incompatible with many of the proposed PrP conversion models (3–5), which will have to incorporate those important constraints. We can't agree that we did not use biophysical methods, because we determined that amyloid fibrils of PrP mutants are indistinguishable from wild-type fibrils regarding secondary structure, transmission electron microscope (TEM) fibril morphology, binding pattern of five epitope-defined antibodies as well as efficient seeding of wild-type mPrP conversion (not shown in paper). Interpretation of experiments such as hydrogen exchange is not straightforward because in addition to the secondary structure, several different mechanisms may lead to increased protection, e.g. packing of fibrils. We would like to emphasize that our model is the most plausible also from the biochemical point of view, as it is difficult to conceive that a completely different tertiary structure would be able to optimize the cooperative interactions as well as or even better than the native state. Finally we also demonstrated that the pattern of PrP conversion propensity is conserved in prion propagation in cell lines, which underscores the physiological relevance of our findings.