| Research Abstract |
The seeds of higher plants accumulate large quantities of storage protein. During seed maturation, storage protein precursors synthesized on rough endoplasmic reticulum are sorted to protein storage vacuoles, where they are converted into the mature forms and accumulated. We found that novel vesicles, PAC (precursor-accumulating) vesicles, mediate the mass transport of storage proteins to PSVs in maturing pumpkin seeds, Isolated PAC vesicles contain a large amount of the precursors of major storage proteins. On the membrane of the PAC vesicles, we found PV72, a type I integral membrane protein composed of a large lumenal domain and a transmembrane domain followed by a short cytosolic tail that contains a potential tyrosine-based motif. PV72 is specifically and transiently accumulated in maturing pumpkin seeds in association with the synthesis of storage proteins, but is not expressed in vegetative tissues. PV72 bound to peptides derived from pumpkin 2S albumin and also to the precursor of pumpkin 2S albumin in the presence of Ca^<2+> These observations imply that PV72 functions as a vacuolar sorting receptor for storage proteins in pumpkin seeds. We demonstrated that a PV72 homologue, AtVSR1/AtELP, of Arabidopsis functions as a sorting receptor for storage proteins. The alvsr1 mutant mis-sorts storage proteins by secreting them from cells, resulting in an enlarged and electron-dense extracellular space in the seeds. Our findings demonstrate a receptor-mediated transport of seed storage proteins to protein storage vacuoles in higher plants.
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