RETROGRADE TRANSPORT FROM THE GOLGI-COMPLEX TO THE ER OF BOTH SHIGA TOXIN AND THE NONTOXIC SHIGA B-FRAGMENT IS REGULATED BY BUTYRIC-ACID AND CAMP

Endocytosed Shiga toxin is transported from the Golgi complex to the e ndoplasmic reticulum in butyric acid-treated A431 cells. We here exami ne the extent of this retrograde transport and its regulation. The sho rt B fragment of Shiga toxin is sufficient for transport to the ER. Th e B fragment of cholera toxin, which also binds to glycolipids, is tra nsported to all the Golgi cisterns, but cannot be localized in the ER even after butyric acid treatment. Under all conditions the toxic prot ein ricin was found predominantly in the trans-Golgi network. There is no transport of endocytosed fluid to the Golgi apparatus or to the ER even after butyric acid treatment and in the presence of Shiga toxin, indicating that transport to the ER, through the trans-Golgi network and the cisterns of the Golgi apparatus, involves several sorting stat ions. Since Shiga toxin receptors (Gb3) in butyric acid-treated A431 c ells seem to have a ceramide moiety with longer fatty acids than in un treated cells, the possibility exists that fatty acid composition of t he receptor is important for sorting to the ER. Both retrograde transp ort and intoxication with Shiga toxin can also be induced by cAMP, sup porting the idea that retrograde transport from the Golgi to the ER is required for intoxication. The data suggest that transport to the ER in cells in situ may depend on fatty acid composition and is regulated by physiological signals.