A Second Extensin-Like Hydroxyproline-Rich Glycoprotein from Carrot Cell Walls

The insoluble extensin matrix of dicot cell walls has been studied most fruitfully by examining the salt-extractable precursors to this matrix. Multiple extensin-like hydroxyproline-rich glycoproteins (HRGPs) have been isolated, or their existence inferred, from tomato, potato, bean, soybean, melon, carrot, and other plants. We and others previously have studied a carrot extensin which we call extensin-1. Here we report on the properties of extensin-2, a second salt-extractable hydroxyproline-rich glycoprotein from carrot. Like extensin-1, extensin-2 contains large amounts of hydroxyproline, serine, histidine, and lysine. In contrast, its tyrosine content is only about one-third that of extensin-1. Arabinose and galactose are the most abundant neutral sugars in both proteins, and nearly identical buoyant densities in CsCl suggest a similar proportion of carbohydrate in each. The size of extensin-2 is about half the size of extensin-1 based on: (a) the measured lengths of shadowed molecules (about 40 versus 84 nanometers); (b) the migration of extensin-2 in acid-urea gels relative to monomers, dimers, and trimers of extensin-1; and (c) the Stokes' radii of these molecules as determined by gel filtration chromatography. Electron microscopy of shadowed extensin-2 molecules indicates that they contain kinks, which may indicate the presence of intramolecular isodityrosine cross-links, but intermolecular cross-links, either with other extensin-2 molecules or extensin-1 molecules, are observed rarely if ever.