Subcellular site of lectin synthesis in developing rice embryos

Embryos of developing rice (Oryza sativa L. cv. Koshihikari) caryopses which actively synthesize lectin were labelled with [S]cysteine for different times and newly synthesized rice lectin was isolated by affinity chromatography. Gel filtration of embryo extracts on Sepharose-4B indicated that a large portion of the labelled lectin was associated with the particulate fraction. Experiments with detergent indicated that this lectin was sequestered within organelles. When extracts of pulse-labelled embryos were fractionated on isopycnic sucrose gradients, this detergent-released lectin banded in the same density-region as the endoplasmic reticulum (ER) marker enzyme NADH-cytochrome c reductase. Both radioactivity in rice lectin and the enzyme activity shifted towards a higher density in the presence of 2 mM Mg acetate, indicating that the labelled lectin was associated with the rough ER. The ER-bound lectin could be chased from this organelle when tissue was incubated in unlabelled cysteine following a 1 h pulse of labelled cysteine. Radioactivity chased out of the ER with a half-life of 4 h and accumulated in the soluble fraction. In the ER the lectin was present as a polypeptide with mol. wt. 23 000, while in the soluble fraction it occurred as polypeptides with mol. wt. 18 000, 10 000 and 8000. The rice lectin in the ER is capable of binding carbohydrates since it binds readily to the affinity gels. It is associated into dimers with an approximate mol. wt. of 46 000. The results show that newly synthesized rice lectin is transiently sequestered within the ER before further transport and processing take place.