FUCTIONAL DIFFERENTIATION OF PLANT VACUOLES

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Novel vesicles that accumulate large amounts of proprotein precursors of storage proteins were purified from maturing pumpkin seeds. These vesicles were designated precursor-accumulating (PAC) vesicles and *ave diameters of 200 to 400 nm. They contain an electron-dense core of storage proteins surrounded by an electron-translucent layer, and some vesicles also contained small vesicle-like structures. Numerous electron-dense aggregates of storage proteins within the endoplasmic reticulum were found to be develop into the electron-dense cores of the PAC vesicles and then leave the endoplasmic reticulum. The unique PAC vesicles might mediate a transport pathway for insoluble aggregates of storage proteins directly to protein storage vacuoles. We also found that PAC vesicle can be induced in vegetative cells by ectopic expression of the protein that is destined to be compartmentalized into the PAC vesicles.Inactive precursors that are accumulated in the PAC vesicles should be converted int … More o mature forms. Vacuolar processing enzyme (VPE) has been shown to be responsible for maturation of various seed proteins in plant vacuoles. Arabidopsis has three VPE homologues; βVPE is specific to seeds and αVPE and γVPE are specific to vegetative organs. We found that the vegetative VPE has a vacuolar processing activity and is localized in the lytic vacuoles. The mRNA levels of both vegetative VPEs were increased in the primary leaves during senescence in parallel with the increase of the mRNA level of a senescence- associated gene (SAG2). The vegetative VPE might regulate the activation of some functional vacuolar proteins that are known to respond to these treatments.To investigate a VPE system in protein-storage vacuoles, we isolated the PAC vesicles and characterized a l00-kDa component (PVIOO) of the vesicles. PVIOO was composed of a hydrophobic signal peptide and the following three domains: an 11-kDa Cysrich domain with four CxxxC motifs (C, Cys), a 34-kDa Arg/Glu-rich domain composed of six homologous repeats, and a 50-kDa vicilin-like domain. Two Cysrich peptides, three Arg/Glu-rich peptides and the vicilin-like protein were produced by cleaving Asn-Gin bonds of PV100 and that all these proteins had a pyroglutamate at their NH2 terminus. VPE was responsible for cleaving Asn-Gin bonds of a single precursor, PVIOO, to produce multiple seed proteins. It is likely that the Asn-Gin stretches not only provide cleavage sites for VPE but also produce aminopeptidase-resistant proteins. Cys-rich peptide function as a trypsin inhibitor and Arg/Glu-rich peptides function as cytotoxic peptides. Our findings suggested that PVIOO is converted into different functional proteins in the vacuoles of seed cells. Less