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Vol. 15, Issue 1, 1-10, January 2004

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Interactions between Sec Complex and Prepro--Factor during Posttranslational Protein Transport into the Endoplasmic Reticulum

Kathrin Plath ^* , Barrie M. Wilkinson , Colin J. Stirling , and Tom A. Rapoport ^* 

^* Howard Hughes Medical Institute and Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115; School of Biological Sciences, University of Manchester, Manchester M13 9PT, United Kingdom

Submitted June 11, 2003; Revised August 16, 2003; Accepted September 8, 2003
Monitoring Editor: Randy Schekman

Posttranslational translocation of prepro--factor (ppF) across the yeast endoplasmic reticulum membrane begins with the binding of the signal sequence to the Sec complex, a membrane component consisting of the trimeric Sec61p complex and the tetrameric Sec62p/63p complex. We show by photo-cross-linking that the signal sequence is bound directly to a site where it contacts simultaneously Sec61p and Sec62p, suggesting that there is a single signal sequence recognition step. We found no evidence for the simultaneous contact of the signal sequence with two Sec61p molecules. To identify transmembrane segments of Sec61p that line the actual translocation pore, a late translocation intermediate of ppF was generated with photoreactive probes incorporated into the mature portion of the polypeptide. Cross-linking to multiple regions of Sec61p was observed. In contrast to the signal sequence, neighboring positions of the mature portion of ppF had similar interactions with Sec61p. These data suggest that the channel pore is lined by several transmembrane segments, which have no significant affinity for the translocating polypeptide chain.

Present address: Whitehead Institute, 9 Cambridge Center, Cambridge, MA 02142.

Corresponding author. E-mail address: tom_rapoport{at}hms.harvard.edu.

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