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Journal of Experimental Botany 2009 60(13):3687-3696; doi:10.1093/jxb/erp208
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© The Author [2009]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

RESEARCH PAPER

Creating S-type characteristics in the F-type enzyme fructan:fructan 1-fructosyltransferase of Triticum aestivum L.

Lindsey Schroeven1, Willem Lammens1, Akira Kawakami2, Midori Yoshida2, André Van Laere1 and Wim Van den Ende1,*

1Laboratorium voor Moleculaire Plantenfysiologie, Faculteit Wetenschappen, Departement Biologie, KU Leuven, Kasteelpark Arenberg 31, bus 2434, B-3001 Heverlee, Belgium
2National Agricultural Research Center for Hokkaido Region, Hitsujigaoka, Sapporo 062-855, Japan

* To whom correspondence should be addressed: E-mail: wim.vandenende{at}bio.kuleuven.be

Invertases cleave sucrose in glucose and fructose, using water as an acceptor. Fructosyltransferases catalyse the transfer of a fructosyl residue between sucrose and/or fructan molecules. Plant fructosyltransferases (FTs) evolved from vacuolar invertases by small mutational changes, leading to differences in substrate specificity. The S-type of enzymes (invertases, sucrose:sucrose 1-fructosyltransferases or 1-SSTs, and sucrose:fructan 6-fructosyltransferases or 6-SFTs) prefer sucrose as the donor substrate while F-type enzymes (fructan:fructan 1-fructosyltransferases or 1-FFTs and fructan:fructan 6G-fructosyltransferases or 6G-FFTs) preferentially use fructan as the donor substrate. Recently, a functional Asp/Arg or Asp/Lys couple in the Hypervariable Loop (HVL) was suggested to be essential to keep Asp in a favourable orientation for binding sucrose as the donor substrate in S-type enzymes. However, the F-type enzyme 1-FFT of Triticum aestivum (Ta1-FFT) also contains the Asp/Arg couple in the HVL, although it prefers fructan as the donor substrate. In this paper, mutagenesis studies on Ta1-FFT are presented. In Ta1-SST, Tyr282 (the Asp281 homologue) seems to be essential in creating a tight H-bond Network (HBN) in which the Arg-residue of the Asp/Arg couple is held in a fixed position. This tight HBN is disrupted in Ta1-FFT, leading to a more flexible Arg-residue and a dysfunctional Asp/Arg couple. A single D281Y mutation in Ta1-FFT restored the tight HBN and introduced typical S-type characteristics. Conclusively, in wheat FTs Asp281 (and its homologues) is involved in donor substrate specificity.

Key words: Fructan:fructan 1-fructosyltransferase (1-FFT), site-directed mutagenesis, substrate specificity, sucrose:sucrose 1-fructosyltransferase (1-SST), Triticum aestivum, wheat

Received 23 April 2009; Revised 5 June 2009 Accepted 5 June 2009


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