FIG. 6.

Comparisons of amino acid sequences and three-dimensional (3D) structures of the GH43 proteins harboring the XX domain. (A) Amino acid sequence alignment of SXA, XynB3, and FSUAXH1. Sequences were aligned with ClustalW, and the output files were entered into the BoxShade version 3.21 program, with the fraction of sequences that must agree for shading set at 1.0. The conserved amino acids are shaded black, while similar amino acids are shaded gray. The Y484 residue of FSUAXH1, which was demonstrated as being critical to binding to arabinoxylan in this study, is marked by an asterisk. Phylogenetic positions of the three proteins (cluster A) are also shown in Fig. 5A. (B) 3D structures of GH43 proteins with an XX domain. The 3D structures of XynB3 and SXA were obtained from the Protein Data Bank (http://www.pdb.org/pdb/home/home.do). The 3D homology model of FSUAXH1 TM2 was created based on the crystal structure of the β-xylosidase from Bacillus halodurans C-125 (PDB file, 1YRZ). Regions corresponding to the loop in XynB3 (494 to 507) that may impact substrate specificity are highlighted in magenta. The side chain of the Y484 residue in FSUAXH1 TM2 is indicated with an arrow.