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

Proprotein precursors of vacuolar components are transportedfrom endoplasmic reticulum to the dense vesicles, and then targetedto the vacuoles, where they are processed proteolytically totheir mature forms by a vacuolar processing enzyme. Immunoelectronmicroscopy of the maturing endosperm of castor bean (Ricinnscommunis) revealed that the vacuolar processing enzyme is selectivelylocalized in the dense vesicles as well as in the vacuolar matrix.This indicates that the vacuolar processing enzyme is transportedto vacuoles via dense vesicles as does IIS globulin, a majorseed protein. During seed maturation of castor bean, an increasein the activity of the vacuolar processing enzyme in the endospermpreceded increases in amounts of total protein. The enzymaticactivity reached a maximum at the late stage of seed maturationand then decreased during seed germination concomitantly withthe degradation of seed storage proteins. We examined the distributionof the enzyme in different tissues of various plants. The processingenzyme was found in cotyledons of castor bean, pumpkin and soybean,as well as in endosperm, and low-level processing activity wasalso detected in roots, hypocotyls and leaves of castor bean,pumpkin, soybean, mung bean and spinach. These results suggestthat the proprotein-processing machinery is widely distributedin vacuoles of various plant tissues. (Received July 11, 1993; Accepted August 17, 1993)