The biosynthesis of the lectins and the other major storage proteins, the 11S globulins and the 2S albumins, which are found in protein bodies has been studied in developing castor bean endosperm cells. Newly synthesized proteins were radiolabelled by incubating intact endosperm tissue with [35S]methionine. The intracellular distribution of radiolabelled proteins was determined after fractionating endosperm homogenates by sucrose density gradient centrifugation. Pulse-chase experiments revealed that all the major protein body components are initially segregated in precursor form into the lumen of the endoplasmic reticulum. The lectin precursors appeared as a group of 64 000-68 000-Mr glycosylated polypeptides, the 11S globulins as a group of 46 000-55 000-Mr polypeptides and the 2S albumins as a single 32 500-Mr polypeptide. These precursors were transferred from the endoplasmic reticulum to a population of transporting vesicles. The subsequent disappearance of the precursors from this vesicle fraction was accompanied by the accumulation of mature polypeptides in the protein body matrix (lectins and 2S albumins) or in the insoluble protein body crystalloid complexes (11S globulins). The castor bean proteins studied all exist as heterodimers in the protein bodies. After intracellular transport an endoproteolytic step is required to release each subunit of the heterodimer from the appropriate single polypeptide precursor.
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