Analysis of Protein Transport to the Vacuole in Plants as a Basis for the Production of Usefull Proteins in Higher Plants

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The vacuole is the largest organelle in many vegetative cells of plants, and it plays a function similar to animal lysosome to digest various macromolecules to replenish their constituents. In contrast, vacuoles in storage tissues of plants accumulate a large amount of proteins, in particular storage proteins, and they are called protein storage vacuoles. In order to aim large scale accumulation of usefull proteins in genetically engineered plants, it is important to understand how storage proteins are selectively transported to the protein storage vacuoles.We have previously analyzed the mechanisms involved in selective transport of sweetpotato sporamin, in transformed tobacco cells, as a model storage protein, and as a model expression system, respectively. We have shown that the NPIR sequencein the N-terminal propeptide of the precursor to sporamin is the vacuolar sorting signal, and that the mechanism of the vacuolar transport of proteins mediated by NPIR-signal is different from that mediated by the hydrophobic sorting signal located in the C-terminal propeptides of several other vacuolar proteins. In this study, we further extended our study, and obtained the following results ;(1) The NPIR-signal functions as a vacuolar sorting signal even when it is located at the C-terminus of the passenger protein. (2) The vacuolar transport of soluble proteins requires an activity similar to V-type H^+-ATPase which is localized in the pre-vacuolar compartment. (3) We isolated a cDNA of tobacco encoding a protein homologous to the N-ethylmaleimide-sensitive fusion protein (NSF) for the first time from the plant kingdom.