A Short Peptide in Rice Glutelin Directs Trafficking of Protein into the Protein Storage Vacuoles of the Endosperm Cells

Several protein vacuolar sorting determinants (VSDs) have been identified in higher plants. Glutelin as a major storage protein in rice endosperm cells is transported to a protein storage vacuole (PSV). How glutelin sort to PSV and the mechanism of the intracellular trafficking has remained unknown. Here, a sequence-specific vacuolar sorting determinant (ssVSD) is identified by serial deletions of rice glutelin and its role in the protein-sorting process analyzed by transgenic approaches and transient assays. The ssVSD consists of six residues (QRLKHN) within the β-subunit of glutelin is sufficient to direct the glutelin to the protein body II in the rice endosperm cells. We found that protein-sorting via the ssVSD takes place by a ～680-kDa sorting complex containing the receptor Oryza sativa receptor-like membrane Ring-H2 3 (OsRMR3). Further study indicated that OsRMR3 and the ssVSD are essential for glutelin trafficking. Furthermore, site-directed mutagenesis showed that the leucine residues in the ssVSD are critical for protein sorting.