Structural basis for the action of xyloglucan endotransglycosylases/hydrolases: Insights from homology modeling

Abstract

Xyloglucan endotransglycosylase/hydrolases (XTH) are believed to play an important role in modifying the cell wall structure through two different, but related actions: the cleavage of a cross-linking xyloglucan polymer (xyloglucan endohydrolase (XEH) activity) and transfer of a newly generated end to another sugar polymer (xyloglucan endotransglycosylase (XET) activity). These enzymes normally show predominantly either XET or XEH. Question remains concerning the origin of the XET or XEH activity of poplar (Populus trichocarpa) XTH proteins as well as the 3-dimensional structural features that might contribute to the different activities for different phylogenetic groups. Previous investigations have demonstrated that the key structural features controlling the activity may involve the loop 1 and 2 regions of the XTH enzymes. Here we report homology models for 40 poplar (Populus trichocarpa) XTH proteins and analyze their loop 1 and 2 regions. These analyses allow us to generate some hypotheses concerning the possible activities of these enzymes. The results show that the protein models for subfamilies (SFs) I/II/IV match well with that of the known XET enzyme (Ptt-XET16-34 from a Populus hybrid), suggesting that they might mainly function as the XET. It is found that the SF III-A models match well with that of the known XEH TmXTH1. Therefore, they may function as XEH as well. Although the SF III-B members are found to have a truncated loop 2, comparison of the homology models with the existing TmXTH1 structure seems to suggest that they might have relatively high possibility to function as XEH instead of XET.