A Transmembrane Form of the Prion Protein Contains an Uncleaved Signal Peptide and Is Retained in the Endoplasmic Reticululm

Abstract

Although there is considerable evidence that PrPSc is the infectious form of the prion protein, it has recently been proposed that a transmembrane variant called CtmPrP is the direct cause of prion-associated neurodegeneration. We report here, using a mutant form of PrP that is synthesized exclusively with theCtmPrP topology, that CtmPrP is retained in the endoplasmic reticulum and is degraded by the proteasome. We also demonstrate that CtmPrP 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 their synthesis in the endoplasmic reticulum, and they also suggest possible cellular pathways by whichCtmPrP may cause disease.

Prion Peptide 106–126 Modulates the Aggregation of Cellular Prion Protein and Induces the Synthesis of Potentially Neurotoxic Transmembrane PrP

“…3) The amount of 20-kDa and not full-length PrP increases in the presence of a proteasomal inhibitor, 2 consistent with its origin from Ctm PrP, which is normally degraded by the proteasomes (35). 4) The 20-kDa fragment is GPI-linked (as reported for Ctm PrP) and is transported to the plasma membrane within 1 h of synthesis.…”

Proteasome inhibitors promote the sequestration of PrPSc into aggresomes within the cytosol of prion-infected CAD neuronal cells

“…mAbs Sha31 (aa 145-152;Féraudet et al, 2005) and ICSM33 (aa 91-110; KhaliliShirazi et al, 2007) were used to detect PrP Sc by Western blotting or immunocytochemistry. A polyclonal antibody (pAb) generated against aa 14-27 of mouse PrP (Stewart et al, 2001) was used to detect the N-terminal signal peptide. Rabbit pAbs anti-giantin and anti-vimentin were purchased from Abcam.…”

“…The unglycosylated PrP form accumulating in PrP-overexpressing cell models under proteasome inhibition has been proposed by some authors to be immature molecules that have escaped translocation into the ER, as they apparently retained their signal peptide (Stewart et al, 2001). To address this important issue, we used antibodies directed against the N-terminal signal peptide.…”

New insights into cellular prion protein (PrPc) functions: The “ying and yang” of a relevant protein

“…In addition, aberrant N-terminal truncated forms of PrP c tethered to the cell membrane alter the response to oxidative stress and become proteaseresistant (Zeng et al, 2003). Moreover, processing of the putative N-terminal transmembrane fragment could be determinant in neurodegenerative diseases, since mice expressing a PrP variant (mutation that alters topological orientation) promote neuronal degeneration (Hegde et al, 1998;Stewart et al, 2001). The overexpression of an N-terminal truncated PrP c lacking the OR region in an immortalized PrP c -deficient cell line promotes cell death through the caspase-3 pathway.…”

“…Residues 110 to 135 confer hydrophobic properties to PrP c , and mutations in this part alter endoplasmic reticulum regulation and promote cell degeneration (Hegde et al, 1998;Stewart et al, 2001) while the Stop Transfer Effector sequence (residues 103 to 111) directs PrP c chains to the transmembrane (Yost et al, 1990). Furthermore, the transmembrane part of PrP c holds specific residues that regulate chain orientation when the protein is anchored to the cell membrane (Ott et al, 2007) promote neurotoxicity in neuronal cell cultures (Forloni et al, 1993) because of their self-aggregative properties.…”