Ricin A chain can be chemically cross-linked to the mammalian ribosomal proteins L9 and L10e

Indirect immunofluorescence studies revealed that when fixed, permeabilized cultured human cells were incubated with ricin A chain, the toxin molecule localized in a staining pattern indicative of binding to the endoplasmic reticulum and to nucleoli. Chemical cross-linking experiments were performed to identify the cellular components that mediated the binding of ricin A chain. Conjugates were formed between 125I-labeled ricin A chain and two proteins present in preparations of total cell membranes and in samples of purified mammalian ribosomes. Specificity of the ricin A chain-ribosome interaction was demonstrated by inhibition of formation of the complexes by excess unlabeled ricin A chain, but not by excess unlabeled gelonin, another ribosome-inactivating protein. Complexes of ricin A chain cross-linked to the ribosomal proteins were purified and subjected to proteolytic digestion with trypsin. Amino acid sequencing of internal tryptic peptides enabled identification of the ricin A chain-binding proteins as L9 and L10e of the mammalian large ribosomal subunit.