RICIN-BINDING PROTEINS ALONG THE ENDOCYTIC PATHWAY: THE MAJOR ENDOSOMAL RICIN-BINDING PROTEIN IS ENDOSOME-SPECIFIC

Ricin is internalized after binding at the cell surface via lectin activity of the B-chain recognizing terminal galactose residues. Ricin-A chain is then translocated to the cytosol from various endocytic structures. Cell death is the result of catalytic inactivation of protein synthesis. Using ¹²⁵I-ricin overlays, we examined the distribution of ricin binding-proteins within highly purified preparations of plasma membrane vesicles, endosomes and lysosomes from lymphocytes. All compartments of the endocytic pathway had distinct profiles; some ricin-binding proteins were present throughout the pathway; others were restricted to the plasma membrane and endosomes. The major endosomal protein recognized by ¹²⁵I-ricin, a 166kDa glycoprotein, was endosome-specific. When endosomal proteins were solubilized before chromatography onto ricin-agarose this protein was also by far the major specifically-bound glycoprotein. This 166 kDa glycoprotein might be involved in ricin translocation from this compartment.