An interaction between ricin and calreticulin that may have implications for toxin trafficking

Here we demonstrate that ricin is able to interact with the molecular chape rone calreticulin both in vitro and in vivo. The interaction occurred with ricin holotoxin, but not with free:ricin A chain; and it was prevented in t he presence of lactose, suggesting that it was mediated by the lectin activ ity of the ricin B chain. This lectin is galactose-specific, and metabolic labeling with [H-3]galactose or treating galactose oxidase-modified calreti culin with sodium [H-3]borohydride indicated that Vero cell calreticulin po ssesses a terminally galactosylated oligosaccharide. Brefeldin A treatment indicated that the intracellular interaction occurred initially in a post-G olgi stack compartment,:possibly the trans-Golgi network, whereas the reduc tive separation of ricin subunits occurred in an earlier part of the secret ory pathway, most probably the endoplasmic reticulum (ER), Intoxicating Ver o cells with ricin whose A chain had been modified to include either a tyro sine sulfation site or the sulfation site plus available N-glycosylation si tes, in the presence of (Na2OSO4)-S-35, confirmed that calreticulin interac ted with endocytosed ricin that had already undergone retrograde transport to both the Golgi and the ER, Although we cannot exclude the possibility th at the interaction between ricin and calreticulin is an indirect one, the d ata presented are consistent with the idea that calreticulin may function a s a recycling carrier for retrograde transport of ricin from the Golgi to t he ER.