Messenger RNA targeting of rice seed storage proteins to specific ER subdomains

Rice seeds, a rich reserve of starch and protein, are a major food source i n many countries. Unlike the seeds of other plants, which typically accumul ate one major type of storage protein, rice seeds use two major classes, pr olamines and globulin-like glutelins. Both storage proteins are synthesized on the endoplasmic reticulum (ER) and translocated to the ER lumen, but ar e then sorted into separate intracellular compartments(1,2). Prolamines are retained in the ER lumen as protein bodies whereas glutelins are transport ed and stored in protein storage vacuoles. Mechanisms responsible for the r etention of prolamines within the ER lumen and their assembly into intracis ternal inclusion granules are unknown, but the involvement of RNA localizat ion has been suggested(3). Here we show that the storage protein RNAs are l ocalized to distinct ER membranes and that prolamine RNAs are targeted to t he prolamine protein bodies by a mechanism based on RNA signal(s), a proces s that also requires a translation initiation codon. Our results indicate t hat the ER may be composed of subdomains that specialize in the synthesis o f proteins directed to different compartments of the plant endomembrane sys tem.