Biosynthesis, Intracellular Transport and In Vitro Processing of 11S Globulin Precursor Proteins of Developing Castor Bean Endosperm

In vivo labeling experiments to study the biosynthesis of 11S globulin in developing castor bean (Ricinus communis) endosperm demonstrated that the subunit polypeptides of the 11S globulin were synthesized as high molecular weight precursors with heterogeneous molecular weights. These proglobulin species were not synthesized concomitantly during seed maturation. The largest proglobulin was synthesized from 20 days after anthesis, whereas the smaller proglobulins were synthesized from 30 days after anthesis. Subcellular fractionation of the pulse-labeled endosperm showed that the [35S]methionine label was present in proglobulins in both the endoplasmic reticulum (ER) and dense vesicles shortly after the pulse labeling. The label in the proglobulin in ER decreased during the chase and appeared in mature globulins associated with crystalloids of vacuoles (protein bodies). Proglobulins in the ER fraction prepared from the pulse-labeled developing endosperm were processed in vitro into globulins by the matrix fraction of protein bodies isolated from the dry castor bean. Overall results indicate that precursor proglobulin molecules synthesized on rough ER are transported to vacuoles via dense vesicles, and are cleaved there by the matrix protease to yield mature globulin.