The KDEL retrieval system is exploited by Pseudomonas exotoxin A, but not by Shiga-like toxin-1, during retrograde transport from the Golgi complex to the endoplasmic reticulum

To investigate the role of the KDEL receptor in the retrieval of protein to xins to the mammalian cell endoplasmic reticulum (ER), lysozyme variants co ntaining AARL or KDEL C-terminal tags, or the human KDEL receptor, have bee n expressed in toxin-treated COS 7 and HeLa cells. Expression of the lysozy me variants and the KDEL receptor was confirmed by immunofluorescence, When such cells were challenged with diphtheria toxin (DT) or Escherichia coli Shiga-like toxin 1 (SLT-1), there was no observable difference in their sen sitivities as compared to cells which did not express these exogenous prote ins. By contrast, the cytotoxicity of Pseudomonas exotoxin A (PE) is reduce d by expressing lysozyme-KDEL, which causes a redistribution of the KDEL re ceptor from the Golgi complex to the ER, and cells are sensitised to this t oxin when they express additional KDEL receptors, These data suggest that, in contrast to SLT-1, PE can exploit the KDEL receptor in order to reach th e ER lumen where it is believed that membrane transfer to the cytosol occur s. This contention was confirmed by microinjecting into Vero cells antibodi es raised against the cytoplasmically exposed tail of the KDEL receptor. Im munofluorescence confirmed that these antibodies prevented the retrograde t ransport of the KDEL receptor from the Golgi complex to the ER, and this in turn reduced the cytotoxicity of PE, but not that of SLT-1, to these cells .