Purification of protein body-I of rice seed and its polypeptide composition

Protein body type one (PB-I) was isolated and purified from developing rice grain by a combination of sucrose density gradient centrifugation and treatment with pepsin. SDS-PAGE analysis showed that isolated PB-I contains several polypeptide groups, the largest having an apparent molecular size of 13 kDa and two smaller ones of 10 kDa and 16 kDa. The 13-kDa group was found to be composed of two polypeptides of slightly different molecular sizes, 13a (larger component) and 13b (smaller component). Most of the 13a and 13b polypeptides were shown to be largely prolamins, although there were also some salt- and alcohol-insoluble polypeptides with an apparent molecular size of 13 kDa. It was concluded that PB-I is the accumulation site of rice prolamin. It was further estimated that the protein amount in PB-I accounted for about 20% of the total protein of rice endosperm.