Minisymposium 23: Protein targeting

Abs # 44004: Transport and processing of seed storage proteins in Arabidopsis thaliana

Presenter:	Shimada, Tomoo , tshimada@gr.bot.kyoto-u.ac.jp
Authors	Shimada, Tomoo  (A)   Yamada, Kenji  (B)   Fuji, Kentaro  (A)   Tamura, Kentaro  (A)   Kondo, Maki  (B)   Nishimura, Mikio  (B)   Hara-Nishimura, Ikuko  (A)
Affiliations:	(A): Graduate School of Science, Kyoto University (B): Department of Cell Biology, National Institute for Basic Biology

Seeds of higher plants accumulate large quantities of storage proteins in protein storage vacuoles (PSVs). During seed maturation, storage protein precursors are synthesized on rER as larger precursors, are transported into protein storage vacuoles by vesicle-mediated machinery, and then are processed to make mature forms. Here we show molecular mechanisms for the transport and processing of seed storage proteins in Arabidopsis thaliana. Previously, we identified a type I integral membrane protein, designated PV72, as a putative receptor responsible for the sorting of storage proteins in maturing pumpkin seeds. Here, we show crucial evidence that AtVSR1/AtELP, one of PV72 homologs in Arabidopsis, functions as a sorting receptor for seed storage proteins in Aradidopsis seeds1). The atvsr1 seeds abnormally accumulate the precursors of 12S globulin and 2S albumin, together with the mature forms of these proteins. The atvsr1 mutant mis-sorts storage proteins by secreting them from cells, resulting in an enlarged and electron-dense extracellular space in the seeds. These findings demonstrate a receptor-mediated transport of seed storage proteins to PSVs. Previously, we identified VPE (vacuolar processing enzyme) that is responsible for the processing of storage protein precursors in PSVs. Here, we show in vivo evidence that VPEs function as a processing enzyme for seed storage proteins in Arabidopsis). Arabidopsis mutants that have a defect in the βVPE gene accumulate detectable amounts of the precursors of the storage proteins in their seeds. We generated various mutants lacking different VPE isoforms: αVPE, βVPE, and/or γVPE. More than 90% of VPE activity is abolished in the βvpe-3 seeds, and no VPE activity is detected in the αvpe-1/βvpe-3/γvpe-1 seeds. The triple mutant seeds accumulate no properly processed mature storage proteins. Instead, large amounts of storage protein precursors are found in the seeds of this mutant. Overall results suggest that seed-type βVPE is most essential for the processing of storage proteins, and that the vegetative-type VPEs, αVPE and γVPE, complement βVPE activity in this processing. 1) T. Shimada et al. (2003) Proc. Natl. Acad. Sci. USA, 100, 16095-16100. 2) T. Shimada et al. (2003) J. Biol. Chem. 278, 32292-32299.