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A more recent version of this article appeared on January 1, 2004
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Submitted on June 11, 2003
Revised on August 16, 2003
Accepted on September 8, 2003
-factor during posttranslational protein transport into the ER
1 Howard Hughes Medical Institute and Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115, U.S.A., Whitehead Institute, 9 Cambridge Center,
Cambridge, Massachusetts 02142, USA
2 School of Biological Sciences, 2.205 Stopford Building, University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom
3 Howard Hughes Medical Institute and Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115, U.S.A.
* Corresponding author. E-mail address: tom rapoport{at}hms.harvard.edu.
Posttranslational translocation of prepro-
-factor (pp
F)
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 photocrosslinking 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 TM segments of Sec61p that line
the actual translocation pore, a late translocation intermediate of
pp
F 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 pp
F had similar interactions with
Sec61p. These data suggest that the channel pore is lined by several TM
segments, which have no significant affinity for the translocating
polypeptide chain.
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