Fig. 1. The "protein only" model. A, in the normal cell PrPC is synthesized, transported to the cell surface and recycled. B, the protein only model assumes that the prion is identical with PrPSc (or PrP*). Exogenous prions cause the conversion of the normal cellular protein PrPC into PrPSc, either at the cell surface or after internalization. C, PrPSc accumulates intracellularly in late endosomes or lysosomes, and the cell surface is depleted of PrPC. PrPSc is also released into the extracellular space.
From: C. Weissmann (1999) Molecular genetics of transmissible spongiform encephalopathies. J. Biol. Chem. 274: 3-6 (html)(pdf)