Purification and Characterization of a Salt-extractable Hydroxyproline-rich Glycoprotein from Aerated Carrot Discs

The salt-extractable hydroxyproline-rich glycoprotein (HRGP) of the cell wall of aerated carrot root discs has been studied by polyacrylamide gel electrophoresis. The predominant proline-labeled protein extractable from the cell wall is rich in hydroxyproline as shown by its specific loss of (3)H from proline labeled in position 4 and its shift in electrophoretic mobility after labeling in the presence of an inhibitor of hydroxyproline synthesis. Unlabeled HRGP can be identified by staining gels for carbohydrate. The HRGP has been purified by ion exchange chromatography and CsCl gradient centrifugation. The HRGP consists of about 50% protein and 50% carbohydrate with an overall molecular weight of 86,000. The amino acid composition of the protein portion consists of 50% hydroxyproline, 19% basic amino acids, 12% serine, and 10% tyrosine. This glycoprotein accumulates in a salt-extractable pool in the cell wall beginning between 10 and 20 hours of aeration and may also become incorporated into the nonextractable portion of the cell wall.