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J. Biol. Chem., Vol. 282, Issue 17, 12951-12962, April 27, 2007

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A Barley Xyloglucan Xyloglucosyl Transferase Covalently Links Xyloglucan, Cellulosic Substrates, and (1,3;1,4)--D-Glucans^*

Maria Hrmova¹, Vladimir Farkas, Jelle Lahnstein, and Geoffrey B. Fincher²

From the School of Agriculture, Food and Wine, and Australian Centre for Plant Functional Genomics, University of Adelaide, Waite Campus, Glen Osmond, South Australia 5064, Australia and the Institute of Chemistry, Centre of Excellence GLYCOBIOS, Slovak Academy of Sciences, 84538 Bratislava, Slovak Republic

Molecular interactions between wall polysaccharides, which include cellulose and a range of noncellulosic polysaccharides such as xyloglucans and (1,3;1,4)--D-glucans, are fundamental to cell wall properties. These interactions have been assumed to be noncovalent in nature in most cases. Here we show that a highly purified barley xyloglucan xyloglucosyl transferase HvXET5 (EC 2.4.1.207 [EC] ), a member of the GH16 group of glycoside hydrolases, catalyzes the in vitro formation of covalent linkages between xyloglucans and cellulosic substrates and between xyloglucans and (1,3;1,4)--D-glucans. The rate of covalent bond formation catalyzed by HvXET5 with hydroxyethylcellulose (HEC) is comparable with that on tamarind xyloglucan, whereas that with (1,3; 1,4)--D-glucan is significant but slower. Matrix-assisted laser desorption ionization time-of-flight mass spectrometric analyses showed that oligosaccharides released from the fluorescent HEC:xyloglucan conjugate by a specific (1,4)--Dglucan endohydrolase consisted of xyloglucan substrate with one, two, or three glucosyl residues attached. Ancillary peaks contained hydroxyethyl substituents (m/z 45) and confirmed that the parent material consisted of HEC covalently linked with xyloglucan. Similarly, partial hydrolysis of the (1,3;1,4)--D-glucan:xyloglucan conjugate by a specific (1,3;1,4)--D-glucan endohydrolase revealed the presence of a series of fluorescent oligosaccharides that consisted of the fluorescent xyloglucan acceptor substrate linked covalently with 2-6 glucosyl residues. These findings raise the possibility that xyloglucan endo-transglucosylases could link different polysaccharides in vivo and hence influence cell wall strength, flexibility, and porosity.

Received for publication, December 15, 2006 , and in revised form, February 27, 2007.

^* This work was supported by grants from the Australian Research Council (to G. B. F. and M. H.), the Grains Research and Development Corp. and the South Australian State Government (to G. B. F.), and by Grants 2/6133/26 and CE II/2/2005 from the Slovak Academy of Sciences (to V. F.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement"in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence may be addressed. Tel.: 61-8-8303-7280; Fax: 61-8-8303-7102; E-mail: maria.hrmova{at}adelaide.edu.au.

² To whom correspondence maybe addressed. Tel.: 61-8-8303-7296; Fax: 61-8-8303-7102; E-mail: geoff.fincher{at}adelaide.edu.au.

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