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Carbohydrate Binding Module Family 49
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Plant endoglucanases belong to glycosyl hydrolase family 9 (GH9), which contains enzymes capable of breaking β-1,4 glycosidic bonds within a glycan chain . The GH9 family in plants has been divided into three sub-families on the basis of variations in protein sequences [1, 2]. The GH9C sub-family proteins are comprised of a single N-terminal transmembrane helix, a GH9 catalytic domain, and a a C-terminal carbohydrate binding module (CBM49). Urbanowicz et al. showed that the CBM49 module of SlCel9C1 binds crystalline cellulose .
CBM49 domains are about 100–110 amino acids long. BLAST searches and predictive protein modeling indicate that these domains are most similar to CBM2. A refined model of the SlCel9C1 CBM domain, based on the template from CBM2 of C. fimi xylanase 10A (1EXG), closely matched the features of the β-barrel fold of the parent structure (i.e. only a few short insertions/deletions are present in the final alignment).
- Fold: Predicted β-barrel fold
- Type: Predicted Type A
- Features of ligand binding: No structural information is available. However, site-directed mutagenesis of SlCel9C1-CBM49 showed that Trp522, Trp559, and Trp573 contribute to the interaction of CBM49 with crystalline cellulose.
CBM49 modules are found appended to plant GH9 endoglucanases, and are currently thought to be unique to plants . The three members of plant GH9 class C have not been well studied in Arabidopsis, however, the tomato and rice orthologs of At1g64390 (AtGH9C2) have been examined. For example, Urbanowicz et al. provided evidence for the binding of the tomato SlCel9C1 CBM to crystalline cellulose, as well as hydrolysis of artificial cellulosic polymers, and a variety of plant cell wall polysaccharides by the catalytic domain . A similar study was performed on the orthologous rice endoglucanase, further confirming that the catalytic domain is capable of hydrolyzing a suite of polysaccharides . Yoshida and Komae also provided evidence that the CBM49 module is post-translationally cleaved in the apoplast . Glass et al. have shown that over-expression of PtGH9C2 and down-regulation of AtGH9C2 results in plants with modified degrees of cell wall crystallinity, which was inversely correlated with changes in plant height and rosette diameter . Genetic modification of GH9C enzymes in planta suggest that CBM49s function to target plant GH9 enzymes to crystalline cellulose prior to proteolytic cleavage, thereby regulating cross-linking with hemicellulosic polysaccharides, and preserving the crystallinity of the newly synthesized cellulose microfibrils, limiting cell expansion .
- First Identified
- The cellulose binding function of CBM49 was first demonstrated in Solanum lycopersicum SlCel9C in 2007 .
- First Structural Characterization
- There is no available structure for CBM49 at this time.
- Urbanowicz BR, Bennett AB, Del Campillo E, Catalá C, Hayashi T, Henrissat B, Höfte H, McQueen-Mason SJ, Patterson SE, Shoseyov O, Teeri TT, and Rose JK. (2007) Structural organization and a standardized nomenclature for plant endo-1,4-beta-glucanases (cellulases) of glycosyl hydrolase family 9. Plant Physiol. 144, 1693-6. DOI:10.1104/pp.107.102574 |
- Libertini E, Li Y, and McQueen-Mason SJ. (2004) Phylogenetic analysis of the plant endo-beta-1,4-glucanase gene family. J Mol Evol. 58, 506-15. DOI:10.1007/s00239-003-2571-x |
- Urbanowicz BR, Catalá C, Irwin D, Wilson DB, Ripoll DR, and Rose JK. (2007) A tomato endo-beta-1,4-glucanase, SlCel9C1, represents a distinct subclass with a new family of carbohydrate binding modules (CBM49). J Biol Chem. 282, 12066-74. DOI:10.1074/jbc.M607925200 |
- Yoshida K and Komae K. (2006) A rice family 9 glycoside hydrolase isozyme with broad substrate specificity for hemicelluloses in type II cell walls. Plant Cell Physiol. 47, 1541-54. DOI:10.1093/pcp/pcl020 |
- Yoshida K, Imaizumi N, Kaneko S, Kawagoe Y, Tagiri A, Tanaka H, Nishitani K, and Komae K. (2006) Carbohydrate-binding module of a rice endo-beta-1,4-glycanase, OsCel9A, expressed in auxin-induced lateral root primordia, is post-translationally truncated. Plant Cell Physiol. 47, 1555-71. DOI:10.1093/pcp/pcl021 |
- Glass M, Barkwill S, Unda F, and Mansfield SD. (2015) Endo-β-1,4-glucanases impact plant cell wall development by influencing cellulose crystallization. J Integr Plant Biol. 57, 396-410. DOI:10.1111/jipb.12353 |