Organelles involved in the synthesis and transport of hydroxyproline-containing glycoproteins in carrot root discs.

Isopycnic sucrose density gradients of homogenates from carrot root tissue were analyzed radiochromatographically, radiochemically, and photometrically for the presence of hydroxyproline residues. Significant amounts were found in endoplasmic reticulum (ER), Golgi apparatus (GA) and plasma membrane (PM) fractions as designated by the presence of marker enzymes for these membranes. Some hydroxyproline-containing macromolecules could be detected in the soluble cytoplasm (cytosol) but this was interpreted as an artefact due to homogenization. Hydroxyproline-rich polymers can be released from a mixed ER/PM fraction by freezing and thawing in water. The PM-associated hydroxyproline polymer is suggested to be an arabinogalactan protein rather than cell wall extensin. Nevertheless, the polypeptides of both glycoproteins are considered as being synthesized at the ER and transported via the GA to the PM.