Native and artificial reticuloplasmins co-accumulate in distinct domains of the endoplasmic reticulum and in post-endoplasmic reticulum compartments

We compared the subcellular distribution of native and artificial reticulop lasmins in endosperm, callus, and leaf tissues of transgenic rice (Oryza sa tiva) to determine the distribution of these proteins among endoplasmic ret iculum (ER) and post-ER compartments. The native reticuloplasmin was calret iculin. The artificial reticuloplasmin was a recombinant single-chain antib ody (scFv), expressed with an N-terminal signal peptide and the C-terminal KDEL sequence for retrieval to the ER (scFvT84.66-KDEL). We found that both molecules were distributed in the same manner. In endosperm, each accumula ted in ER-derived prolamine protein bodies, but also in glutelin protein st orage vacuoles, even though glutelins are known to pass through the Golgi a pparatus en route to these organelles. This finding may suggest that simila r mechanisms are involved in the sorting of reticuloplasmins and rice seed storage proteins. However, the presence of reticuloplasmins in protein stor age vacuoles could also be due to simple dispersal into these compartments during protein storage vacuole biogenesis, before glutelin deposition. In c allus and leaf mesophyll cells, both reticuloplasmins accumulated in riboso me-coated vesicles probably derived directly from the rough ER.