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Methods: Material was cold-water-extracted from 25 plant species; proteins were precipitated by heat-denaturation, then CHP was ethanol-precipitated. For XAF assays, CHP (or sub-fractions thereof) was applied to washed Arabidopsis thaliana cell walls, and enzymes thus solubilised were assayed radiochemically for XET activity. In some experiments, the CHP was pre-treated with trifluoroacetic acid (TFA), alkali (NaOH) or glycanases.
· CHP specifically desorbed wall-bound XTHs, but not β-glucosidases, phosphatases or peroxidases.
· · CHP preparations from 25 angiosperms all possessed XAF activity but had no consistent monosaccharide composition.
· Of eleven individual plant polymers tested, only gum arabic and tamarind xyloglucan were XAF-active, albeit less so than CHP.
· On gel-permeation chromatography, XAF-active cauliflower CHP eluted with molecular weight ~7,000–140,000, though no specific sugar residue(s) co-eluted exactly with XAF activity.
· Cauliflower XAF activity survived cold alkali and warm dilute TFA (which break ester and glycofuranosyl linkages respectively), but was inactivated by hot 2M TFA (which breaks glycopyranosyl linkages).
· Cauliflower XAF activity was remarkably stable to diverse glycanases and glycosidases.
Conclusions: XAFs are naturally occurring heat-stable polymers that specifically desorb (thereby activating) wall-bound XTHs. Their XAF activity considerably exceeds that of gum arabic and tamarind xyloglucan, and they were not identifiable as any major plant polysaccharide. We propose that XAF is a specific, minor, plant polymer that regulates xyloglucan transglycosylation in vivo, and thus wall assembly and restructuring
- Cell wall
- XET (xyloglucan endotransglucosylase activity)
- XTH (xyloglucan endotransglucosylase/hydrolase)
- Arabidopsis thaliana
- Brassica oleracea (cauliflower)
- XAF (XET activating factor)
- functional properties
- sugar composition
- enzymic digestion
- plant polymer (heat-stable)
- wall-bound enzymes
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