Studies on the release of barley aleurone cell proteins: Kinetics of labelling

Protein release from gibberellic acid-treated aleurone cells of barley (Hordeum vulgare L.) was followed in pulse-chase experiments with radioactively labelled amino acids. After a 10-min pulse of [(3)H]leucine or [(3)H]tryptophan, label was incorporated into trichloroacetic-acid (TCA)-insoluble material; some of this was released into the incubation medium during a chase with carrier amino acid. This relase of TCA-insoluble material into the incubation medium had no appreciable lag period. Precipitation with rabbit-anti-α-amylase antibody of the radioactivity released from aleurone layers into the medium during chasing indicates that as much as 70% of the radioactivity present is α-amylase. Aleurone cell homogenates were fractionated by differential centrifugation after pulsing with labelled amino acids. Radioactivity in TCA-insoluble materials was distributed equally among all sediment fractions indicating that no specific accumulation of label occurred. Tissue was also fractionated after labelling with a pulse of [(3)H]tyrosine and [(14)C]-tryptophan, and the distribution of radioactivity in various fractions also showed that no preferential sedimentation of label occurred. Altogether, no experimental evidence could be found to support the hypothesis that proteins are released from aleurone cells via discrete secretory organelles.