Biosynthesis of Arabinogalactan-Protein in Lolium multiflorum (Ryegrass) Endosperm Cells. II. In vitro Incorporation of Galactosyl Residues From UDPgalactose Into Polymeric Products

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Abstract
When mixed-membrane fractions from suspension-cultured Lolium multiflorum endosperm cells are incubated in vitro with UDP-[14C]galactose, 66% ethanol-insoluble products of apparent molecular weight greater than 60 000 are labelled in both galactosyl and glucosyl residues, suggesting that an active UDPgalactose 4-epimerase is present on the membrane fraction. The epimerase can be inhibited with ADPribose, to produce polymeric material in which [14C]galactosyl residues pre-dominate. While some of these residues appear to be associated with glycoproteins, affinity chromatography of the products on mouse myeloma protein J539-Sepharose provides evidence that β-galactans containing 1,6-linkages are amongst the products. Monosaccharide analyses and methylation data indicate that the mixed-membrane preparations contain associated polysaccharide of structure analogous to the 1,3;1,4-β-glucans, arabinoxylans and arabino-3,6-galactans normally found in cell walls or secreted into the medium.