Evidence that the transport of ricin to the cytoplasm is independent of both Rab6A and COPI

• Published in: Journal of cell science Vol. 116; no. Pt 17; pp. 3503 - 3510
• Main Authors: Chen, Alice, AbuJarour, Ramzey J, Draper, Rockford K
• Format: Journal Article
• Language: English
• Published: England 01-09-2003
• Subjects: Animals; Biological Transport; Cells, Cultured; Cercopithecus aethiops; CHO Cells; Cholera Toxin - metabolism; Cholera Toxin - toxicity; Coat Protein Complex I - metabolism; Cricetinae; Cricetulus; Endoplasmic Reticulum - metabolism; Golgi Apparatus - metabolism; Microscopy, Fluorescence; Models, Molecular; Protein Sorting Signals - physiology; rab GTP-Binding Proteins - metabolism; Ricin - metabolism; Ricin - toxicity; Shiga Toxin - metabolism; Shiga Toxin - toxicity; Vero Cells
• ISSN: 0021-9533; 1477-9137
• DOI: 10.1242/jcs.00641

Cholera toxin, Shiga toxin and ricin are examples of protein toxins that require retrograde transport from the Golgi complex into the endoplasmic reticulum (ER) to express their cytotoxic activities and different toxins appear to use different pathways of retrograde transport. Cholera toxin contains the mammalian retrograde targeting signal KDEL and is believed to exploit the coat protein I (COPI) and KDEL receptor-dependent pathway to go from the Golgi complex to the ER. Shiga toxin, however, has no KDEL sequence to specify its inclusion in COPI-coated retrograde vesicles and is believed to use a recently discovered COPI-independent and Rab6A-dependent retrograde pathway to enter the ER. Ricin, like Shiga toxin, does not contain a KDEL sequence and is therefore a candidate to use the COPI-independent and Rab6A-dependent pathway of retrograde transport to access the ER. We measured the effect of the GDP-restricted mutant of Rab6A (Rab6A-T27N) on the cytotoxic activity of ricin and found that expressing Rab6A-T27N in cells did not inhibit the cytotoxicity of ricin, suggesting that ricin enters the cytoplasm by a retrograde pathway that does not involve Rab6A. Moreover, ricin still intoxicated cells when Rab6A and COPI were simultaneously inhibited, implying that ricin requires neither Rab6A nor COPI to intoxicate cells.