First published online March 12, 2004; 10.1105/tpc.020065
The Plant Cell 16:874-886 (2004)
© 2004 American Society of Plant Biologists
Crystal Structures of a Poplar Xyloglucan Endotransglycosylase Reveal Details of Transglycosylation Acceptor Binding
Patrik Johanssona,
Harry Brumer, IIIb,
Martin J. Baumannb,
Åsa M. Kallasb,
Hongbin Henrikssonb,
Stuart E. Denmanb,1,
Tuula T. Teerib and
T. Alwyn Jonesa,2
a Department of Cell and Molecular Biology, Uppsala University, BMC, S-75124 Uppsala, Sweden
b Department of Biotechnology, Royal Institute of Technology, AlbaNova University Center, S-106 91 Stockholm, Sweden
2 To whom correspondence should be addressed. E-mail alwyn{at}xray.bmc.uu.se; fax 46-18-536971.
Xyloglucan endotransglycosylases (XETs) cleave and religate xyloglucan polymers in plant cell walls via a transglycosylation mechanism. Thus, XET is a key enzyme in all plant processes that require cell wall remodeling. To provide a basis for detailed structurefunction studies, the crystal structure of Populus tremula x tremuloides XET16A (PttXET16A), heterologously expressed in Pichia pastoris, has been determined at 1.8-Å resolution. Even though the overall structure of PttXET16A is a curved ß-sandwich similar to other enzymes in the glycoside hydrolase family GH16, parts of its substrate binding cleft are more reminiscent of the distantly related family GH7. In addition, XET has a C-terminal extension that packs against the conserved core, providing an additional ß-strand and a short -helix. The structure of XET in complex with a xyloglucan nonasaccharide, XLLG, reveals a very favorable acceptor binding site, which is a necessary but not sufficient prerequisite for transglycosylation. Biochemical data imply that the enzyme requires sugar residues in both acceptor and donor sites to properly orient the glycosidic bond relative to the catalytic residues.
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