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© The Rockefeller University Press, 0021-9525/2000//1203 $5.00
The Journal of Cell Biology, Volume 148, Number 6, , 2000 1203-1212

Original Article

Cholera Toxin Is Exported from Microsomes by the Sec61p Complex



Anton Schmitza, Helga Herrgena, Alexandra Winkelera, and Volker Herzoga

a Institut für Zellbiologie, Rheinische Friedrich-Wilhelms-Universität Bonn, 53121 Bonn, Germany
Institut für Zellbiologie, Rheinische Friedrich-Wilhelms-Universität Bonn, Ulrich-Haberland-Strasse 61a, 53121 Bonn, Germany.49-228-73530249-228-735306

anton-schmitz{at}uni-bonn.de

After endocytosis cholera toxin is transported to the endoplasmic reticulum (ER), from where its A1 subunit (CTA1) is assumed to be transferred to the cytosol by an as-yet unknown mechanism. Here, export of CTA1 from the ER to the cytosol was investigated in a cell-free assay using either microsomes loaded with CTA1 by in vitro translation or reconstituted microsomes containing CTA1 purified from V. cholerae. Export of CTA1 from the microsomes was time- and adenosine triphosphate–dependent and required lumenal ER proteins. By coimmunoprecipitation CTA1 was shown to be associated during export with the Sec61p complex, which mediates import of proteins into the ER. Export of CTA1 was inhibited when the Sec61p complexes were blocked by nascent polypeptides arrested during import, demonstrating that the export of CTA1 depended on translocation-competent Sec61p complexes. Export of CTA1 from the reconstituted microsomes indicated the de novo insertion of the toxin into the Sec61p complex from the lumenal side. Our results suggest that Sec61p complex–mediated protein export from the ER is not restricted to ER-associated protein degradation but is also used by bacterial toxins, enabling their entry into the cytosol of the target cell.

Key Words: cholera toxin • endoplasmic reticulum • Sec61p complex • translocation • endoplasmic reticulum–associated protein degradation

© 2000 The Rockefeller University Press

Abbreviations used in this paper: {alpha}1-AT, {alpha}1-antitrypsin; BiP, heavy chain binding protein; CPY*, mutant form of carboxypeptidase Y; CTA1, cholera toxin A1 subunit; DHFR, dihydrofolate reductase; RNC, ribosome nascent chain complex; TX, Triton X.


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