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

Abstract

Pulse labeling of carrot root phloem parenchyma (Daucus carota L. cv. Nantes) tissue with '4C-proline followed by fractionation of the cytoplasmic organelles on sucrose gradients was used to determine the identity of the membranous organelies involved in the secretion of the hydroxyproline-rich gly. coproteins 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 '4C-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. UDParabinose 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.The primary walls of plant cells contain a structural hydroxyproline-rich glycoprotein called "extensin" (for review see Lamport [12]). This protein is synthesized in the cytoplasm as a proline-rich polypeptide and the proline residues are hydroxylated by a soluble peptidyl proline hydroxylase (20). The 'This investigation was supported by the Atomic Energy Commission under Contract AT(04-3)-34 PA 159.

Biosynthesis of Arabinogalactan-Protein in Lolium multiflorum (Ryegrass) Endosperm Cells

“…The involvement of the Golgi apparatus in the synthesis and secretion of hydroxyproline-rich macromolecules has been demonstrated in carrots by providing tissue with [14CJproline and locating radioactive hydroxyproline in sucrose gradients of homogenates (8). Using a similar approach Wienecke et al (30) detected labeled hydroxyproline not only in dictyosomes, but also in ER and plasma membrane.…”

Effects of Indoleacetic Acid on Dictyosomes of Apical and Expanding Cells of Oat Coleoptiles

“…On the whole, within the entire incubation period, the number of dictyosomes does not differ significantly (P > 0.25, Table I) between the two treatments in both cell types. However, in the auxin-treated segments, the dictyosome number shows an initial decrease, followed by an increase, and another decrease, in both the outer epidermis and the parenchyma as compared to the control (Table I, (2,5,7,10,11,13,15,17,19,20). The present study adds further support to the ascribed function of the dictyosomes.…”

Experimental modelling of exine-like structures

“…The same sequence is known for plant cells (Gardiner & Chrispeels, 1975;Nagahashi & Beevers, 1978;Lerouge et al, 1998). Glycoproteins are one of the main components of the exo-cellular matrix (= glycocalyx) in animal cells, as well as of the primexine matrix (= glycocalyx) of the developing sporoderm in plants (Pettitt & Jermy, 1974;Rowley & Dahl, 1977;Pettitt, 1979).…”