The rice RMR1 defines a novel organelle as a prevacuolar compartment for the protein storage vacuole pathway

Receptor-mediated protein sorting is one of the mechanisms for transporting soluble proteins to the protein storage vacuoles (PSVs) in plant cells. Members of vacuolar sorting receptor (VSR) family proteins and receptor homology region-transmembrane domain-RING-H2 (RMR) family proteins have been shown to function in mediating the transport of storage proteins to PSVs in plants. However, no prevacuolar compartment (PVC) for the PSV pathway has been identified. In this study, I used a rice RMR protein (OsRMR1) as a probe to study the PSV pathway in rice. Using confocal immunofluorescent and immunogold electron microscopy (EM) with specific OsRMR1 antibodies, I have identified a novel organelle as a PVC for the PSV pathway, because OsRMR1 antibodies labeled the Golgi apparatus, trans-Golgi network (TGN) and the novel organelle in both rice cultured cells and developing rice seeds, as well as the protein body Type II (PBII) in developing rice seeds. This novel organelle is morphologically distinct from the lytic PVC or multivesicular body (MVB).;Further in vivo and in vitro studies using the truncated OsRMR1 proteins from the culture media of transgenic BY-2 cells demonstrated that OsRMR1 functioned as a receptor in transporting vicilin-like storage proteins via specific interaction with their vacuolar sorting determinants. Taken together, the OsRMR1 is a sorting receptor for the PSV pathway that defines a novel organelle as PVC for PSV in rice.