Nucleotide Sequence of a cDNA That Encodes a Rice Prolamin

Formation mechanism of the internal structure of type I protein bodies in rice endosperm: relationship between the localization of prolamin species and the expression of individual genes

Rice prolamins, a group of seed storage proteins, are synthesized on the rough endoplasmic reticulum (ER) and form type I protein bodies (PB-Is) in endosperm cells. Rice prolamins are encoded by a multigene family. In this study, the spatial accumulation patterns of various prolamin species in rice endosperm cells were investigated to determine the mechanism of formation of the internal structure of PB-Is. Immunofluorescence microscopic analysis of mature endosperm cells showed that the 10 kDa prolamin is mainly localized in the core of the PB-Is, the 13b prolamin is localized in the inner layer surrounding the core and the outermost layer, and the 13a and 16 kDa prolamins are localized in the middle layer. Real-time RT-PCR analysis showed that expression of the mRNA for 10 kDa prolamin precedes expression of 13a, 13b-1 and 16 kDa prolamin in the developing stages. mRNA expression for 13b-2 prolamin occurred after that of the other prolamin species. Immunoelectron microscopy of developing seeds showed that the 10 kDa prolamin polypeptide initially accumulates in the ER, and then 13b, 13a, 16 kDa and 13b prolamins are stacked in layers within the ER. Studies with transgenic rice seeds expressing prolamin-GFP fusion proteins under the control of native and constitutive promoters indicated that the temporal expression pattern of prolamin genes influenced the localization of prolamin proteins within the PB-Is. These findings indicate that the control of gene expression of prolamin species contributes to the internal structure of PB-Is.

Reducing rice seed storage protein accumulation leads to changes in nutrient quality and storage organelle formation

Rice (Oryza sativa) seed storage proteins (SSPs) are synthesized and deposited in storage organelles in the endosperm during seed maturation as a nitrogen source for germinating seedlings. We have generated glutelin, globulin, and prolamin knockdown lines and have examined their effects on seed quality. A reduction of one or a few SSP(s) was compensated for by increases in other SSPs at both the mRNA and protein levels. Especially, reduction of glutelins or sulfur-rich 10-kD prolamin levels was preferentially compensated by sulfur-poor or other sulfur-rich prolamins, respectively, indicating that sulfur-containing amino acids are involved in regulating SSP composition. Furthermore, a reduction in the levels of 13-kD prolamin resulted in enhancement of the total lysine content by 56% when compared with the wild type. This observation can be mainly accounted for by the increase in lysine-rich proteins. Although reducing the level of glutelins slightly decreased protein storage vacuoles (PSVs), the simultaneous reduction of glutelin and globulin levels altered the inner structure of PSVs, implicating globulin in framing PSV formation. Knock down of 13-kD prolamins not only reduced the size of endoplasmic reticulum-derived protein bodies (PBs) but also altered the rugged peripheral structure. In contrast, PBs became slightly smaller or unchanged by severe suppression of 10- or 16-kD prolamins, respectively, indicating that individual prolamins have distinct functions in the formation of PBs. Extreme increases or decreases in sulfur-poor prolamins resulted in the production of small PBs, suggesting that the ratio of individual prolamins is crucial for proper aggregation and folding of prolamins.