Control of Storage Protein Biosynthesis by mRNA Targeting

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The major metabolic activities of plant seeds are directed toward the synthesis of carbon (in the form of lipids and starch) and nitrogen (in the form of storage proteins) reserves. Although appearing to be independent processes, carbon and nitrogen metabolism are inter-related as evidenced by the near parallel responses exhibited by defined genetic mutants and transgenic plants altered in gene expression in one of these two processes. The mechanisms responsible for this coordinated expression of carbon and nitrogen reserves are not known. This grant application proposes a series of studies on one aspect of nitrogen metabolism, i.e. the control of storage protein biosynthesis in developing rice endosperm. Rice is an excellent system to study storage protein biosynthesis as it synthesizes and accumulates both major classes of storage proteins, prolamines and glutelins, the latter an 11S globulin. Moreover, rice stores these proteins in different subcellular compartments. Prolamines are deposited directly as protein intracisternal granules within the endoplasmic reticulum (ER) lumen, whereas glutelins are transported through the endomembrane system to a protein storage vacuole. Previous studies from this laboratory have demonstrated that the RNAs encoding these two classes of storage proteins are not localized in a stochastic fashion on the ER. Instead, prolamine mRNAs are highly enriched on the ER membranes that bind the prolamine protein bodies (PBs) whereas glutelin mRNAs are predominant on the cisternal ER. The localization of prolamine mRNAs to the prolamine PBs occurs by directed RNA transport from the nucleus. To better understand this process, studies will be undertaken to identify the nucleotide sequences that direct RNA transport and localization and to identify and characterize proteins, which are responsible for the transport and anchoring of prolamine and glutelin RNAs to the prolamine PBs and cisternal ER, respectively. As RNA transport occurs by the formation and movement of particles, efforts will be made to isolate these RNA transport particles and determine what other RNA species may be co-transported. An integrated approach will be taken to achieve these goals, and will incorporate the use of biochemical, cellular, molecular and ultrastructural techniques. This study will provide new information on how RNAs and, in turn, proteins are sorted in plant cells and how the ER can differentiate into unique domains with specific functions. Moreover, it will lead to new insights into the mechanisms that may control utilization of carbon and nitrogen and their conversion into storage reserves.