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 act ivity of the B-chain recognizing terminal galactose residues. Ricin-A chain is then translocated to the cytosol from various endocytic struc tures. Cell death is the result of catalytic inactivation of protein s ynthesis. Using I-125-ricin overlays, we examined the distribution of ricin binding-proteins within highly purified preparations of plasma m embrane vesicles, endosomes and lysosomes from lymphocytes. All compar tments of the endocytic pathway had distinct profiles; some ricin-bind ing proteins were present throughout the pathway; others were restrict ed to the plasma membrane and endosomes. The major endosomal protein r ecognized by I-125-ricin, a 166 kDa glycoprotein, was endosome-specifi c. When endosomal proteins were solubilized before chromatography onto ricin-agarose this protein was also by far the major specifically-bou nd glycoprotein. This 166 kDa glycoprotein might be involved in ricin translocation from this compartment. (C) 1997 Academic Press Limited.