title: Structure-function analysis of a broad specificity Populus trichocarpa endo-β-glucanase reveals an evolutionary link between bacterial licheninases and plant XTH gene products.
abstract: The large xyloglucan endotransglycosylase/hydrolase (XTH) gene family continues to be the focus of much attention in studies of plant cell wall morphogenesis due to the unique catalytic functions of the enzymes it encodes. The XTH gene products compose a subfamily of glycoside hydrolase family 16 (GH16), which also comprises a broad range of microbial endoglucanases and endogalactanases, as well as yeast cell wall chitin/β-glucan transglycosylases. Previous whole-family phylogenetic analyses have suggested that the closest relatives to the XTH gene products are the bacterial licheninases (EC 3.2.1.73), which specifically hydrolyze linear mixed linkage β(1→3)/β(1→4)-glucans. In addition to their specificity for the highly branched xyloglucan polysaccharide, XTH gene products are distinguished from the licheninases and other GH16 enzyme subfamilies by significant active site loop alterations and a large C-terminal extension. Given these differences, the molecular evolution of the XTH gene products in GH16 has remained enigmatic. Here, we present the biochemical and structural analysis of a unique, mixed function endoglucanase from black cottonwood (Populus trichocarpa), which reveals a small, newly recognized subfamily of GH16 members intermediate between the bacterial licheninases and plant XTH gene products. We postulate that this clade comprises an important link in the evolution of the large plant XTH gene families from a putative microbial ancestor. As such, this analysis provides new insights into the diversification of GH16 and further unites the apparently disparate members of this important family of proteins.
authors.affiliation: Division of Glycoscience, School of Biotechnology, Royal Institute of Technology (KTH), SE-106 91 Stockholm, Sweden.
authors.forename: Jens M
authors.initials: JM
authors.lastname: Eklöf
authors.forename: Shaheen
authors.initials: S
authors.lastname: Shojania
authors.forename: Mark
authors.initials: M
authors.lastname: Okon
authors.forename: Lawrence P
authors.initials: LP
authors.lastname: McIntosh
authors.forename: Harry
authors.initials: H
authors.lastname: Brumer
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism; D020134:Catalytic Domain; D019143:Evolution, Molecular; D006026:Glycoside Hydrolases / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D016695:Glycosyltransferases / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D010802:Phylogeny; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D032107:Populus / Q000201:enzymology / Q000235:genetics*; D017433:Protein Structure, Secondary
chemical_list: D001426:Bacterial Proteins; D010940:Plant Proteins; D016695:Glycosyltransferases; C473049:xyloglucan - xyloglucosyltransferase; D006026:Glycoside Hydrolases; C028020:licheninase
keywords: Carbohydrate Processing; Cellulase; Enzyme Kinetics; Enzyme Structure; Glycoside Hydrolases; Licheninase; Plant Cell Wall; Polysaccharide; Xyloglucan; Xyloglucan Endotransglycosylase/Hydrolase (XTH)