Arabinogalactan protein in the extracellular space of Phaseolus vulgaris hypocotyls

Abstract Most hydroxyproline in the soluble fraction (cytosol, extracellular fluid and the contents of ruptured organelles) of homogenized bean hypocotyls originated from arabinogalactan protein. Using a vacuum infiltration-centrifugation technique, we extracted hydroxyproline-containing compounds from the extracellular space, accounting for about 25% of hydroxyproline in the soluble fraction. The bulk of this material was soluble in 5% trichloroacetic acid and could be precipitated with β-Gal-Yariv reagent. Isoelectrofocusing of the extracellular solution showed a major hydroxyproline peak at low pH, and minor peaks at pH 5 and 9, respectively. We conclude that arabinogalactan protein accounts for most of the salt-soluble, extracellular hydroxyproline-containing compounds.