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The water-soluble polymeric components of wheat endosperm have been extracted by two different procedures and their chemical composition studied in detail. Water extracts of wheat flour that had first been treated with hot 80% ethanol contained only 2% protein, but if the ethanol treatment was omitted up to 20% of the extracted polymeric fraction was low-molecular-weight non-dialysable protein material. Density-gradient ultracentrifugation in caesium chloride solutions indicated that most of this protein was free, whereas the 2% protein in the water extract of ethanol-treated flottr was firmly bound to a polysaccharide. This bound protein (a peptide) was characterized by high levels of hydroxyproline (16-20% of the amino acids present).Fractional precipitation of the water-soluble components with ammonium sulphate enabled the separation of a high-molecular-weight arabinoxylan (xylose : arabinose = 1·6 -1 . 9) and a lowmolecular-weight arabinogalactan (galactose :. arabinose =approx. 1· 5), the latter. being associated with the hydroxyproline-rich peptide.

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1. An arabinogalactan-peptide from wheat endosperm was studied by using physicochemical techniques and some aspects of its chemical structure were determined. 2. The arabinogalactan-peptide is a non-associating, polydisperse macromolecule ([unk]=22000) which exhibits only minor non-ideal effects in aqueous solution. 3. Examination of the products of partial acid hydrolysis of the polysaccharide component showed that arabinose is present in the alpha-l-arabinofuranosyl configuration, and i.r.-absorption spectroscopy and optical-rotation studies suggest that the d-galactopyranose residues are linked by glycosidic linkages in the beta-anomeric configuration. 4. The arabinogalactan is linked to a peptide which represents 8% (w/w) of the arabinogalactan-peptide and which may be present as a molecular core. Partial degradation of the polymer by successive treatment with oxalic acid and NaOH showed that the linkage between polysaccharide and peptide involves galactose and hydroxyproline residues and is glycosidic in nature. A tentative model is proposed for the structure of the wheat endosperm arabinogalactan-peptide. 5. The subcellular location and function of the arabinogalactan-peptide is discussed in relation to previous work with related molecules.

An arabinogalactan-protein (AGP) purified from the filtrate of liquid-suspension-cultured Italian-ryegrass (Lolium multiflorum) endosperm cells by affinity chromatography on myeloma protein J539-Sepharose was deglycosylated with trifluoromethanesulphonic acid to remove polysaccharide chains that are covalently associated with hydroxyproline residues in the peptide component of the proteoglycan. The protein core, which accounts for less than 10% (w/w) of the intact proteoglycan, was purified by h.p.l.c. It has an apparent Mr of 35,000, but reacts very poorly with both Coomassie Brilliant Blue R and silver stains. Amino-acid-sequence analysis of the N-terminus of the h.p.l.c.-purified protein core and of tryptic peptides generated from the unpurified protein reveals a high content of hydroxyproline and alanine. These are sometimes arranged in short (Ala-Hyp) repeat sequences of up to six residues. Polyclonal antibodies raised against the protein core do not cross-react with native AGP, the synthetic peptide (Ala-Hyp)4, poly-L-hydroxyproline or poly-L-proline. The results suggest that the polysaccharide chains in the native AGP render the protein core of the proteoglycan inaccessible to the antibodies and that the immunodominant epitopes include domains of the protein other than those rich in Ala-Hyp repeating units.