In vitro endoproteolytic cleavage of castor bean lectin presursors

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The toxic (A) and cell binding (B) chains of the castor bean lectins ricin and Ricinus communis agglutinin are synthesized together in the form of single polypeptide precursors. The precursors are transported in vivo to the protein bodies where an endoproteolytic step generates the individual lectin subunits. We have demonstrated an acid endopeptidase activity from developing castor bean endosperm or from protein bodies isolated from mature seeds which cleaves the precursors to release the subunits in vitro.