title: Control of xyloglucan endotransglucosylase activity by salts and anionic polymers.
abstract: Crude extracts of cauliflower florets had high xyloglucan endotransglucosylase (XET) activity, but this was largely lost after partial purification and de-salting. Activity was restored (promoted up to 40-fold) by any of a wide variety of inorganic and organic salts. Optimum concentrations for Na+, K+ and NH4+ salts were typically approximately 300 mM. The chlorides of Ca2+, Mg2+, Al3+ and La3+ were optimally active at lower concentrations (e.g. 0.1 mM LaCl3), but became inhibitory at higher concentrations (e.g. 5 mM LaCl3). Some anionic polysaccharides at 0.04-0.2% w/v (e.g. gum arabic, pectin and hypochlorite-oxidised xyloglucan) promoted the XET activity of de-salted enzyme, especially if a sub-optimal concentration of NaCl was also present; others (e.g. homogalacturonan, 4- O-methyl-glucuronoxylan and alginate) were inhibitory. Similar ionic effects were noted on the XET activity of the Arabidopsis protein XTH24 (heterologously expressed by insect cells); in this case carboxymethylcellulose was also stimulatory. To look for endogenous modulators of XET activity, we prepared a cold-water extract of cauliflower florets; after boiling and centrifugation, the supernatant [boiled cauliflower preparation (BCP)] promoted the XET activity of de-salted cauliflower enzyme and of XTH24. About half the activator present in BCP was an ethanol-precipitable, anionic polymer of apparent Mr <5,000. After acid hydrolysis the polymer yielded much arabinose and galactose, and small amounts of galacturonic and glucuronic acids amino acids were also present. The polymer may thus contain arabinogalactan-proteins. We suggest that acidic polymers and/or other apoplastic ions are naturally occurring regulators of XET action in vivo, and may thus control cell wall assembly, loosening, and growth.
authors.affiliation: The Edinburgh Cell Wall Group, ICMB, The University of Edinburgh, Daniel Rutherford Building, The King's Buildings, EH9 3JH, UK.
authors.forename: Takumi
authors.initials: T
authors.lastname: Takeda
authors.forename: Stephen C
authors.initials: SC
authors.lastname: Fry
mesh_terms: D000838:Anions / Q000737:chemistry; D017360:Arabidopsis / Q000201:enzymology / Q000235:genetics; D029681:Arabidopsis Proteins / Q000235:genetics; D001937:Brassica / Q000201:enzymology* / Q000254:growth & development; D002473:Cell Wall / Q000201:enzymology; D002850:Chromatography, Gel; D002852:Chromatography, Ion Exchange; D018506:Gene Expression Regulation, Plant; D016695:Glycosyltransferases / Q000235:genetics / Q000378:metabolism*; D012492:Salts / Q000494:pharmacology*
chemical_list: D000838:Anions; D029681:Arabidopsis Proteins; D012492:Salts; D016695:Glycosyltransferases; C110295:TCH4 protein, Arabidopsis; C473049:xyloglucan - xyloglucosyltransferase