Protein accumulation in the maize endosperm: role of polyribosomes and the cytoskeleton

Abstract We compared fresh weight, dry weight, amounts of several proteins and polyribosome content (putative protein synthesizing potential) in three maize varieties: wild-type (WT), opaque-2 (o2), and sweet corn (SW). All three varieties increased in fresh weight until 28 d after pollination (DAP), but o2 lost large amounts of water during the later stages, thus rendering its dry weight content much higher. Protein content (mg seed –1 ) was increased similarly in all three varieties, but protein concentration (mg g-tissue –1 ) was highest in o2 and lowest in WT. WT and SW accumulated about three times as much zein, and three times the amount of protein bodies (PB) as did o2, but only about half the amount of the cytoskeleton proteins, actin, β;-tubulin, and EF1α. Protein synthesizing potential (measured as polysomes and polysomal mRNA) was high in WT, somewhat lower in SW, and much lower in o2. The accumulation of total proteins was similar in all three varieties, the accumulation of zeins was much higher in WT and SW than in o2, but the accumulation of cytoskeleton proteins was higher, and the polysome content was much lower, in o2. This apparent discrepancy (less polysomes, but equal protein accumulation in o2) can be explained by the fact that protein synthesis is dependent not only on the amount of polyribosomes, but also on the cytoskeleton, either in the construction of the cytoskeleton as a scaffold for polysomes·mRNA, or as a provider of factors (EF1α) essential for translation.