title: Crystal structures of a poplar xyloglucan endotransglycosylase reveal details of transglycosylation acceptor binding.
abstract: 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 structure-function studies, the crystal structure of Populus tremula x tremuloides XET16A (PttXET16A), heterologously expressed in Pichia pastoris, has been determined at 1.8-A resolution. Even though the overall structure of PttXET16A is a curved beta-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 beta-strand and a short alpha-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.
authors.affiliation: Department of Cell and Molecular Biology, Uppsala University, BMC, S-75124 Uppsala, Sweden.
authors.forename: Patrik
authors.initials: P
authors.lastname: Johansson
authors.forename: Harry
authors.initials: H
authors.lastname: Brumer
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Baumann
authors.forename: Asa M
authors.initials: AM
authors.lastname: Kallas
authors.forename: Hongbin
authors.initials: H
authors.lastname: Henriksson
authors.forename: Stuart E
authors.initials: SE
authors.lastname: Denman
authors.forename: Tuula T
authors.initials: TT
authors.lastname: Teeri
authors.forename: T Alwyn
authors.initials: TA
authors.lastname: Jones
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002240:Carbohydrate Sequence; D018360:Crystallography, X-Ray; D005936:Glucans / Q000737:chemistry; D006031:Glycosylation; D016695:Glycosyltransferases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009844:Oligosaccharides / Q000737:chemistry; D010843:Pichia / Q000235:genetics; D032107:Populus / Q000201:enzymology* / Q000235:genetics; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D014990:Xylans / Q000737:chemistry
chemical_list: D005936:Glucans; D009844:Oligosaccharides; D011994:Recombinant Proteins; D014990:Xylans; C029353:xyloglucan; D016695:Glycosyltransferases; C473049:xyloglucan - xyloglucosyltransferase