Functional Compartmentation of the Golgi Apparatus of Plant Cells

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Abstract
The Golgi apparatus of plant cells is engaged in both the processing of glycoproteins and the synthesis of complex polysaccharides. To investigate the compartmentalization of these functions within individual Golgi stacks, we have analyzed the ultrastructure and the immunolabeling patterns of high-pressure frozen and freeze-substituted suspension-cultured sycamore maple (Acer pseudoplatanus L.) cells. As a result of the improved structural preservation, three morphological types of Golgi cisternae, designated cis, medial, and trans, as well as the trans Golgi network, could be identified. The number of cis cisternae per Golgi stack was found to be fairly constant at approximately 1, whereas the number of medial and trans cisternae per stack was variable and accounted for the varying number of cisternae (3-10) among the many Golgi stacks examined. By using a battery of seven antibodies whose specific sugar epitopes on secreted polysaccharides and glycoproteins are known, we have been able to determine in which types of cisternae specific sugars are added to N-linked glycans, and to xyloglucan and polygalacturonic acid/rhamnogalacturonan-I, two complex polysaccharides. The findings are as follows. The beta-1,4-linked d-glucosyl backbone of xyloglucan is synthesized in trans cisternae, and the terminal fucosyl residues on the trisaccharide side chains of xyloglucan are partly added in the trans cisternae, and partly in the trans Golgi network. In contrast, the polygalacturonic/rhamnogalacturonan-I backbone is assembled in cis and medial cisternae, methylesterification of the carboxyl groups of the galacturonic acid residues in the polygalacturonic acid domains occurs mostly in medial cisternae, and arabinose-containing side chains of the polygalacturonic acid domains are added to the nascent polygalacturonic acid/rhamnogalacturonan-I molecules in the trans cisternae. Double labeling experiments demonstrate that xyloglucan and polygalacturonic acid/rhamnogalacturonan-I can be synthesized concomitantly within the same Golgi stack. Finally, we show that the xylosyl residue-linked beta-1,2 to the beta-linked mannose of the core of N-linked glycans is added in medial cisternae. Taken together, our results indicate that in sycamore maple suspension-cultured cells, different types of Golgi cisternae contain different sets of glycosyl transferases, that the functional organization of the biosynthetic pathways of complex polysaccharides is consistent with these molecules being processed in a cis to trans direction like the N-linked glycans, and that the complex polysaccharide xyloglucan is assembled exclusively in trans Golgi cisternae and the trans Golgi network.