RNAs that code for the major rice storage proteins are localized to specific subdomains of the cortical endoplasmic reticulum [ER) in developing endosperm. Prolamine RNAs are localized to the ER and delimit the prolamine intracisternal inclusion granules (PB-ER), whereas glutelin RNAs are targeted to the cisternal ER. To study the transport of prolamine RNAs to the surface of the prolamine protein bodies in living endosperm cells, we adapted a two-gene system consisting of green fluorescent protein (GFP) fused to the viral RNA binding protein MS2 and a hybrid prolamine RNA containing tandem MS2 RNA binding sites. Using laser scanning confocal microscopy, we show that the GFP-labeled prolamine RNAs are transported as particles that move at an average speed of 0.3 to 0.4 mum/s. These prolamine RNA transport particles generally move unidirectionally in a stop-and-go manner, although nonlinear bidirectional, restricted, and nearly random movement patterns also were observed. Transport is dependent on intact microfilaments, because particle movement is inhibited rapidly by the actin filament-disrupting drugs cytochalasin D and latrunculin B. Direct evidence was obtained that these prolamine RNA-containing particles are transported to the prolamine protein bodies. The significance of these results with regard to protein synthesis in plants is discussed.
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