The biosynthesis, degradation, and function of cell wall β-xylosylated xyloglucan mirrors that of arabinoxyloglucan

Abstract

O_LIXyloglucan is an abundant polysaccharide in many primary cell walls and in the human diet. Decoration of its -xylosyl side chains with further sugars is critical for plant growth, even though the sugars themselves vary considerably between species. Plants in the Ericales order--prevalent in human diets--exhibit {beta}1,2-linked xylosyl decorations. The biosynthetic enzymes responsible for adding these xylosyl decorations, as well as the hydrolases that remove them in the human gut, are unidentified. C_LIO_LIGT47 xyloglucan glycosyltransferase candidates were expressed in Arabidopsis and endo-xyloglucanase products from transgenic wall material were analysed by electrophoresis, mass spectrometry, and NMR. The activities of gut bacterial hydrolases BoGH43A and BoGH43B on synthetic glycosides and xyloglucan oligosaccharides were measured by colorimetry and electrophoresis. C_LIO_LICcXBT1 is a xyloglucan {beta}-xylosyltransferase from coffee that can modify Arabidopsis xyloglucan and restore the growth of galactosyltransferase mutants. Related VmXST1 is a weakly active xyloglucan -arabinofuranosyltransferase from cranberry. BoGH43A hydrolyses both -arabinofuranosylated and {beta}-xylosylated oligosaccharides. C_LIO_LICcXBT1s presence in coffee and BoGH43As promiscuity suggest that {beta}-xylosylated xyloglucan is not only more widespread than thought, but might also nourish beneficial gut bacteria. The evolutionary instability of transferase specificity and lack of hydrolase specificity hint that, to enzymes, xylosides and arabinofuranosides are closely resemblant. C_LI