A transmembrane form of the prion protein contains an uncleaved signal peptide and is retained in the endoplasmic reticululm

Although there is considerable evidence that PrPSc is the infectious form o f the prion protein, it has recently been proposed that a transmembrane var iant called (PrP)-Pr-Ctm is the direct cause of prion-associated neurodegen eration. We report here, using a mutant form of PrP that is synthesized exc lusively with the (PrP)-Pr-Ctm topology, that (PrP)-Pr-Ctm is retained in t he endoplasmic reticulum and is degraded by the proteasome. We also demonst rate that (PrP)-Pr-Ctm contains an uncleaved, N-terminal signal peptide as well as a C-terminal glycolipid anchor. These results provide insight into general mechanisms that control the topology of membrane proteins during th eir synthesis in the endoplasmic reticulum, and they also suggest possible cellular pathways by which (PrP)-Pr-Ctm may cause disease.