Involvement of the Golgi Apparatus in the Synthesis and Secretion of Hydroxyproline-rich Cell Wall Glycoproteins

Pulse labeling of carrot root phloem parenchyma (Daucus carota L. cv. Nantes) tissue with (14)C-proline followed by fractionation of the cytoplasmic organelles on sucrose gradients was used to determine the identity of the membranous organelles involved in the secretion of the hydroxyproline-rich glycoproteins of the cell wall. Identification of the organelles was done through electron-microscopical observations and through the localization of marker enzymes on the sucrose gradients. Enrichment of the organelles involved in secretion was determined by measuring the percentage of the incorporated radioactivity present as (14)C-hydroxyproline. The Golgi apparatus (dictyosome) was found to be a major site of glycoprotein transport. This identification was based on the observed enrichment of dictyosomes paralleling the purification of newly synthesized cell-wall glycoproteins. A marker enzyme for the Golgi apparatus, inosinediphosphatase, banded with the newly synthesized cell wall glycoproteins on sequential isopycnic and rate zonal sucrose gradients. Marker enzymes for the endoplasmic reticulum and the plasma membrane were clearly separated from the dictyosome-rich fraction. UDP-arabinose arabinosyl transferase, an enzyme involved in the glycosylation of the peptide moiety of this glycoprotein, also banded with the dictyosomes on both kinds of gradients. The results suggest an important role of the Golgi apparatus in the biosynthesis and the secretion of the cell wall glycoproteins of higher plants.