CHIMERIC TOXIN COMPOSED OF VISCUMIN A-CHAIN AND RICIN B-CHAIN

Disulfide exchange was used to obtain a chimeric toxin composed of the binding subunit of ricin and the catalytic subunit of viscumin. Immun oenzyme analysis performed with the aid of immobilized asyalofetuin de monstrated that the chimeric protein did not differ in its galactose-b inding activity from native ricin. When the cytotoxic activity of toxi ns was analyzed using the Jurcat cell Line, it appeared that the activ ity of the chimeric protein is four times higher than that of native v iscumin but lower than that of ricin. Both toxins have approximately t he same number of binding sites at the surface of target cells. In the presence of 10 mM NH4Cl, the intralysosomal pH increases and intracel lular proteases are inhibited. In such a case the activities of chimer ic toxin and ricin were indistinguishable. Hence the ricin B-subunit p ossesses an important protective function, which prevents the A-subuni t from degradation during its transport from the cell surface to the s ite of its transmembrane translocation to the cytosol.