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Originally published as MBoC in Press, 10.1091/mbc.E08-01-0087 on May 28, 2008

Vol. 19, Issue 8, 3463-3476, August 2008

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Retrotranslocation of Prion Proteins from the Endoplasmic Reticulum by Preventing GPI Signal Transamidation

Aarthi Ashok, and Ramanujan S. Hegde

Cell Biology and Metabolism Program, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892

Submitted January 28, 2008; Revised May 5, 2008; Accepted May 15, 2008
Monitoring Editor: Jonathan S. Weissman

Neurodegeneration in diseases caused by altered metabolism of mammalian prion protein (PrP) can be averted by reducing PrP expression. To identify novel pathways for PrP down-regulation, we analyzed cells that had adapted to the negative selection pressure of stable overexpression of a disease-causing PrP mutant. A mutant cell line was isolated that selectively and quantitatively routes wild-type and various mutant PrPs for ER retrotranslocation and proteasomal degradation. Biochemical analyses of the mutant cells revealed that a defect in glycosylphosphatidylinositol (GPI) anchor synthesis leads to an unprocessed GPI-anchoring signal sequence that directs both ER retention and efficient retrotranslocation of PrP. An unprocessed GPI signal was sufficient to impart ER retention, but not retrotranslocation, to a heterologous protein, revealing an unexpected role for the mature domain in the metabolism of misprocessed GPI-anchored proteins. Our results provide new insights into the quality control pathways for unprocessed GPI-anchored proteins and identify transamidation of the GPI signal sequence as a step in PrP biosynthesis that is absolutely required for its surface expression. As each GPI signal sequence is unique, these results also identify signal recognition by the GPI-transamidase as a potential step for selective small molecule perturbation of PrP expression.

This was published online ahead of print in MBC in Press (http://www.molbiolcell.org/cgi/doi/10.1091/mbc.E08-01-0087) on May 28, 2008.

Address correspondence to: Ramanujan S. Hegde (hegder{at}mail.nih.gov)




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