Shiga toxin B-subunit binds to the chaperone BiP and the nucleolar protein B23.
BACKGROUND
In many cell lines, such as HeLa cells, STxB (Shiga toxin B-subunit) is transported from the plasma membrane to the ER (endoplasmic reticulum), via early/recycling endosomes and the Golgi apparatus, bypassing the late endocytic pathway. In human monocyte-derived macrophages and dendritic cells that are not sensitive to Shiga toxin-induced protein biosynthesis inhibition, STxB is not detectably targeted to the retrograde route and is degraded in late endosomes/lysosomes.


RESULTS
We have identified B-subunit interacting proteins in HeLa cells and macrophages. In HeLa cells, the ER-localized chaperone BiP (binding protein) was co-immunoprecipitated with the B-subunit. This interaction was not observed in macrophages, consistent with our previous trafficking results. In both cell types, the B-subunit also interacted with the nucleolar protein B23. Consistently, the B-subunit could be detected on nucleoli, suggesting that it could serve to bring the holotoxin to the site of synthesis of its molecular target, rRNA. The nucleolar localization data are critically discussed.


CONCLUSIONS
The interaction of STxB with BiP, involved in the retrotranslocation process to the cytosol and nucleolar B23, as described in this study, might be of relevance for explaining the efficiency of even low doses of Shiga toxin to inactivate cellular ribosomes, and for the use of STxB as a vector for targeting antigens to cytosolic proteasomes of the MHC I-restricted antigen presentation pathway.
Falguières
Thomas
T
Laboratoire Trafic et Signalisation, Unité Mixte de Recherche 144 Curie/CNRS, Institut Curie, 26 rue d'Ulm, F-75248 Paris Cedex 05, France.
Johannes
Ludger
L
D021581:Active Transport, Cell Nucleus; D000255:Adenosine Triphosphate / Q000378:metabolism; D002466:Cell Nucleolus / Q000378:metabolism*; D002478:Cells, Cultured; D004721:Endoplasmic Reticulum / Q000378:metabolism; D000091342:Endoplasmic Reticulum Chaperone BiP; D006360:Heat-Shock Proteins / Q000378:metabolism*; D006801:Humans; D008264:Macrophages / Q000378:metabolism*; D018832:Molecular Chaperones / Q000378:metabolism*; D011485:Protein Binding; D021122:Protein Subunits / Q000378:metabolism; D022621:Shiga Toxin / Q000378:metabolism*
D000091342:Endoplasmic Reticulum Chaperone BiP; D006360:Heat-Shock Proteins; D018832:Molecular Chaperones; D021122:Protein Subunits; D022621:Shiga Toxin; D000255:Adenosine Triphosphate