A Vacuolar Processing Enzyme in Maturing and Germinating Seeds : Its Distribution and Associated Changes during Development

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Proprotein precursors of vacuolar components are transported from endoplasmic reticulum to the dense vesicles, and then targeted to the vacuoles, where they are processed proteolytically to their mature forms by a vacuolar processing enzyme. Immunoelectron microscopy of the maturing endosperm of castor bean (Ricinus communis) revealed that the vacuolar processing enzyme is selectively localized in the dense vesicles as well as in the vacuolar matrix. This indicates that the vacuolar processing enzyme is transported to vacuoles via dense vesicles as does 11S globulin, a major seed protein. During seed maturation of castor bean, an increase in the activity of the vacuolar processing enzyme in the endosperm preceded increases in amounts of total protein. The enzymatic activity reached a maximum at the late stage of seed maturation and then decreased during seed germination concomitantly with the degradation of seed storage proteins. We examined the distribution of the enzyme in different tissues of various plants. The processing enzyme was found in cotyledons of castor bean, pumpkin and soybean, as well as in endosperm, and low-level processing activity was also detected in roots, hypocotyls and leaves of castor bean, pumpkin, soybean, mung bean and spinach. These results suggest that the proprotein-processing machinery is widely distributed in vacuoles of various plant tissues.
日本植物生理学会の論文