Effects of Gibberellic Acid and of Tunicamycin on Glycosyl‐Transferase Activities and on α‐Amylase Secretion in Barley

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Abstract
A crude membrane fraction was prepared from isolated aleurone layers, the secretory tissue of barley grains. The layers were pre-incubated in the presence or absence of the phytohormone GA3. The membranes catalyzed the transfer of [14C]mannose from GDP-[14C]mannose and of N-[14C]acetylglucosamine from UDP-N-[14C]acetylglucosamine to endogenous and exogenous dolichyl monophosphate. When GA3 was present during the pretreatment the activity of the transferases was increased by a factor of 2-3. A significantly increased activity was observable within 4 h after the addition of GA3, whereas the GA3-induced secretion of the glycoprotein .alpha.-amylase started only after 12 h. Tunicamycin inhibited the secretion of .alpha.-amylase by 60-80%. Intracellularly, however, no .alpha.-amylase accumulated. Tunicamycin did not inhibit the rate of total protein synthesis by more than 10%. The possibility is discussed that the synthesis of the protein portion of glycoproteins is specifically inhibited, when glycosylation is prevented.