Protein biosynthetic capacity in the endosperm tissue of ripening castor bean seeds

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Abstract
Endosperm tissue was excised from Ricinus communis plants at different stages during seed maturation. The various stages were characterized on the basis of total RNA, protein and lipid content. Polyadenylated RNA was recovered from the total RNA by affinity chromatography on oligo(dT)cellulose. With the exception of that isolated from dry seeds, this poly(A+) RNA actively programmed protein synthesis in cell-free systems containing either wheat germ S30 extracts or nuclease treated rabbit reticulocyte lysates at each developmental stage examined. Translational products were separated electrophoretically and were visualized by fluorography. The capacity to synthesize protein was also estimated during ‘in vivo’ labelling studies. Developmental changes in the capacity of maturing endosperm tissue to synthesize a characteristic protein, R. communis agglutinin, were followed by immunoprecipitating this protein from the total ‘in vitro’ products synthesized at various stages. Endoplasmic reticulum membranes were isolated from maturing endosperm tissue by sucrose density gradient centrifugation. The role of the endoplasmic reticulum (ER) in protein glycosylation was indicated by (a) localizing the enzymes catalysing the incorporation of N-acetylglucosamine and mannose into mono- and oligosaccharide lipid and into glycoprotein, (b) localizing particulate 3H-labelled glycoprotein amongst cellular fractions prepared from endosperm tissue which had been incubated with [3H]N-acetylglucosamine.