Intracellular Transport and Secretion of Proteins in Plants

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This research includes experiments aimed at understanding the intracellular transport and secretion of enzymes from aleurone cells of barley (Hordeum vulgare L. cv Himalaya). Because of their unique properties, isolated aleurone protoplasts have been exploited for studies of protein secretion. The response of protoplasts isolated from barley aleurone to gibberellic acid and calcium is qualitatively similar to that of isolated aleurone layers. Using protoplasts this lab has shown that precursor forms of the low isoelectric point (low-pI) group of a-amylase isozymes are covalently modified to the secreted forms of the enzyme. This modification results in the lowering of isozyme pI without significantly affecting the molecular weight of the protein. Results to date show that these isozymes are not modified by glycosylation, phosphorylation or sulfation. Rather the data indicate a form of posttranslational modification of secreted proteins involving side-chain acylation. Radiolabel from ı14C!acetate is specifically incorporated into secreted forms of low-pI a-amylase. The precursor of the secreted protein is not labeled, so acetate is not incorporated into a-amylase via its metabolism into amino acids or sugars. Amino-terminal acetylation of high-pI a-amylase is also ruled out since these proteins are end- blocked. Experiments are designed to investigate the precise nature of a-amylase acylation and the possible function of this covalent modification. Experiments with Xenopus oocytes that provide information on the role of protein acylation in secretion are also being conducted. The secretion of macromolecules to the cell exterior is a fundamental process of higher plant and animal cells. This process is particularly important in plant seeds when stored food reserves are mobilized during germination. Our knowledge of intracellular transport of the enzymes involved is incomplete. This research addresses this knowledge gap.***//