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Vol. 13, Issue 11, 3775-3786, November 2002

Cotranslational Partitioning of Nascent Prion Protein into Multiple Populations at the Translocation Channel

Soo Jung Kim, and Ramanujan S. Hegde^*

Laboratory of Cellular Oncology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892

The decisive events that direct a single polypeptide such as the prion protein (PrP) to be synthesized at the endoplasmic reticulum in both fully translocated and transmembrane forms are poorly understood. In this study, we demonstrate that the topological heterogeneity of PrP is determined cotranslationally, while at the translocation channel. By evaluating sequential intermediates during PrP topogenesis, we find that signal sequence-mediated initiation of translocation results in an interaction between nascent PrP and endoplasmic reticulum chaperones, committing the N terminus to the lumen. Synthesis of the transmembrane domain before completion of this step allows it to direct the generation of ^CtmPrP, a transmembrane form with its N terminus in the cytosol. Thus, segregation of nascent PrP into different topological configurations is critically dependent on the precise timing of signal-mediated initiation of N-terminus translocation. Consequently, this step could be experimentally tuned to modify PrP topogenesis, including complete reversal of the elevated ^CtmPrP caused by disease-associated mutations in the transmembrane domain. These results delineate the sequence of events involved in PrP biogenesis, explain the mechanism of action of ^CtmPrP-favoring mutations associated with neurodegenerative disease, and more generally, reveal that translocation substrates can be cotranslationally partitioned into multiple populations at the translocon.

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^* Present address: Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, 18 Library Dr., Bldg. 18, Room 101, Bethesda, MD 20892.

Corresponding author. E-mail address: hegder{at}mail.nih.gov.

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Molecular Biology of the Cell
Vol. 13, 3775-3786, November 2002
Copyright © 2002 by The American Society for Cell Biology

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