Dependence of ricin toxicity on translocation of the toxin A-chain from the endoplasmic reticulum to the cytosol

Ricin acts by translocating to the cytosol the enzymatically active toxin A -chain, which inactivates ribosomes. Retrograde intracellular transport and translocation of ricin was studied under conditions that alter the sensiti vity of cells to the toxin. For this purpose tyrosine sulfation of mutant A -chain in the Golgi apparatus, glycosylation in the endoplasmic reticulum ( ER) and appearance of A-chain in the cytosolic fraction was monitored. intr oduction of an ER retrieval signal, a C-terminal KDEL sequence, into the A- chain increased the toxicity and resulted in more efficient glycosylation, indicating enhanced transport from Golgi to ER. Calcium depletion inhibited neither sulfation nor glycosylation but inhibited translocation and toxici ty, suggesting that the toxin is translocated to the cytosol by the pathway used by misfolded proteins that are targeted to the proteasomes for degrad ation. Slightly acidified medium had a similar effect. The proteasome inhib itor, lactacystin, sensitized cells to ricin and increased the amount of ri cin A-chain in the cytosol, Anti-Sec61 alpha precipitated sulfated and glyc osylated ricin A-chain, suggesting that retrograde toxin translocation invo lves Sec61p, The data indicate that retrograde translocation across the ER membrane is required for intoxication.