The Site of Synthesis and Accumulation of Rice Storage Proteins

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Electron microscopy showed that the two types of protein bodies (PB) in starchy endosperms of rice were formed differently during the period of storage protein accumulation. Two routes for the transport of storage protein from the site of synthesis at the rough endoplasmic reticulum (RER) to the site of accumulation were also proposed. PB-I, bound by a single membrane to which ribosomes were attached, was thought to develop inside the cisternae of RER, while the PB-II membrane was thought to originate from the vacuole. In the wheat germ cell-free translation system, storage protein-related polypeptides of developing rice endosperms, including a precursor of glutelin and putative precursors of prolamin, were directed by membrane-bound polysomes but not by free-polysomes. Immunoassay of the total translation products directed by a PB fraction showed that 46% were storage protein-related polypeptides. Rice storage proteins (prolamin) that accumulate in PB-1 appear to be synthesized by membrane-bound polysomes attached to PB-1 or RER and to pass through the membrane into the lumen where they aggregate and are deposited. The proteins (glutelin and globulin) that accumulate in PB-II, however, seem to be synthesized by mcmbrane-bound polysomes as a large precursor and to become sequestered into the cisternal space of RER, from where they are transferred to the vacuolar precursor of PB-II .
日本植物生理学会の論文