Hydrolysis of protein in vacuoles isolated from higher plant tissue

Abstract

THE vacuoles of higher plant cells are known to be sites of deposition of salts and metabolites. A role as the lytic compartment has also been proposed¹, and Matile has argued that the vacuole participates in the metabolism of higher plant cells in the same way that lysosomes do in other organisms. Recently, isolation methods have been developed which allow a direct analysis of vacuolar constituents and we have shown previously that the vacuoles isolated from the endosperm of young castor bean seedlings contain at least 25% of the cellular protein, 62% of the sucrose and various hydrolytic enzymes (acid protease, carboxypeptidase, phosphodiesterase, RNAase, phytase and β-glucosidase) in amounts indicating a primarily (possibly exclusive) vacuolar localisation². In this tissue it is known that when the dry seed imbibes water the outer, water-soluble matrix of the protein bodies is dissolved and small vacuoles containing the protein crystalloid are formed. These coalesce early in germination to form the large central vacuole, and the remnants of the protein bodies disappear within a few days as growth proceeds³. We have examined vacuoles isolated from the endosperm of 4-d-old seedlings, when it is known that a net loss of protein is occurring. Here we compare the protein composition of the vacuoles with that of the original protein bodies, and show that hydrolysis of the endogenous protein continues in the isolated vacuoles which are free from other cellular components.