Sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis of the starchy endosperm protein of rice (Oryza sativa L. Japonica cv Koshihikari) during seed development confirmed that storage protein begins to accumulate about 5 days after flowering. Two polypeptide groups, 22 to 23 and 37 to 39 kilodaltons, the components of glutelin, the major storage protein in rice seed, appeared 5 days after flowering. A 26-kilodalton polypeptide, the globulin component, also appeared 5 days after flowering. Smaller polypeptides (10-to 16-kilodaltons) including prolamin components, appeared about 10 days after flowering. In contrast, the levels of the 76-and 57-kilodalton polypeptides were fairly constant throughout seed development. Transmission electron microscopy and fractionation by sucrose density gradient centrifugation of the starchy endosperms at various stages of development showed that protein body type II, the accumulation site of glutelin and globulin, was formed faster than protein body type I, the accumulation site of prolamin.The 57-kilodalton polypeptide but not the glutelin subunits was labeled in a 2-hour treatment with I14CIleucine given between 4 and 12 days after flowering to developing ears. In vivo pulse-chase labeling studies showed the 57-kllodalton polypeptide to be a precursor of the 22 to 23 and 37 to 39 kilodalton subunits. The 57-kilodalton polypeptide was salt-soluble, but the mature glutelin subunits were almost salt insoluble.In vitro protein synthesis also showed that the mRNAs directly coding the 22 to 23 and 37 to 39 kilodalton components were absent in developing seeds and that the 57-kilodalton polypeptide was the major product. Thus, it was concluded that the two subunits of rice glutelin are formed through post-translational cleavage of the 57-kilodalton polypeptide.The major storage proteins of most cereal grains are glutelin and prolamin. In rice grains, however, glutelin is the major protein of the starchy endosperm, constituting at least 80%o of the total protein, prolamin accounting for less than 5% (12). Rice glutelin has a mol wt of 6 x I05 according to Tecson et al. (27), but Takeda et a!. (25) demonstrated that it has a heterogeneous mol wt. The increase in glutelin in the developing rice grain coincides with the appearance of PB' in the starchy endosperm 7 to 8 DAF (7,29), and glutelin is found exclusively in PB (7,26,30). In a previous paper (26), we reported two types of PB in the starchy endosperm of rice grains and described their isolation. One (type I, PB-I) is spherical with a concentric ring structure, whereas the other (type II, PB-II) is stained homogeneously by osmium tetroxide and does not have this structure. PB-I contains prolamin, and PB-II is rich in glutelin and globulin. The glutelin in PB-II is composed of two principal subunits, the 22-to 23-and 37-to 39-kD complexes, and the prolamin in PB-I is composed mainly of 13-kD polypeptide.
