Proteins in walls of wheat aleurone cells

The protein content (1% w/w) of purified walls from aleurone layers is twice that of walls from the starchy endosperm but their amino acid compositions are comparable. Aprotic solvent extraction and specific enzymatic and chemical degradation of wall polysaccharides released some proteins, and treatment with specific proteases released peptides. The aleurone wall residue (18% of the original wall) after(1-->3,1-->4)-beta-glucan and xylan hydrolase digestion contains 4.5% (w/w) protein associated with cellulosic glucan, glucomannan and highly-substituted arabinoxylan and remains autofluorescent. Wall residues after treatment with I M sodium hydroxide also remain autofluorescent indicating they retain a significant proportion of their original 1.84% (w/w) of hydroxycinnamic acids and that these are not exclusively attached to the wall polymers by ester linkages. Little protein was extracted from the walls using non-degradative solvents. However, significant quantities were recovered from whole walls and xylanase-treated wall residues by SDS/mercaptoethanol extraction and electroelution. The various protein fractions isolated were characterised by their amino acid compositions and, in some cases, by amino acid sequencing. Three classes of proteins were identified in wall fractions or from proteolysis fragments: glycine-rich proteins (37-86%), proline-rich proteins (11-39%) and proteins with up to 23% serine. Protein-polysaccharide cross-linking through tyrosine-hydroxycinnamic acid dimerisation may be responsible for the alkali-resistant autofluorescence and the insolubility of the protein may be due, in part, to cross-linking through tyrosine-tyrosine bridges. These associations may also contribute to the relative resistance of the inner aleurone wall layer to enzymic dissolution during germination. (C) 2002 Elsevier Science Ltd.