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CBM50 members are also known as LysM domains. They bind to the N-acetylglucosamine residues in bacterial peptidoglycans and in chitin. For example CBM50 of Lactococcus lactisN-acetylglucosaminidase AcmA was shown to bind to the glycan chain of bacterial peptidoglycans, a β-1,4 linked heteropolymer of alternating N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc) [1]. A CBM50 module from Pteris ryukyuensis chitinase-A (PrChi-A) was demonstrated to bind to chitin, a β-1,4-linked homopolymer of GlcNAc [2]. From isothermal titration calorimetry, the CBM50 module from PrChi-A was found to bind to (GlcNAc)n (n=4,5) with the binding stoichiometry of 1:1, whereas no significant binding heat was observed for the binding to (GlcNAc)2 [3]. The binding site of the CBM50 module can accommodate at least three saccharide units.
Structural Features
CBM50 modules are about 50 amino acids long. The three-dimensional structures of several CBM50 modules attached to carbohydrate-active enzymes have been deposited in the Protein Data Bank (example PDB entries: 1e0g [4], 2mkx and 4pxv). The CBM50 modules have a βααβ fold with the two helices packing against one side of the two-stranded antiparallel β-sheet. Although no crystal structure of the CBM50 module in complex with the ligand has been determined yet, Ohnuma et al. first identified the chitin oligosaccharide binding site of the CBM50 module from PrChi-A based on the NMR titration experiments [3]. The chitin oligosaccharide binding site was estimated to be located in a shallow groove formed by the N-terminal part of helix 1, the loop between strand 1 and helix 1, the C-terminal part of helix 2, and the loop between helix 2 and strand 2.
Functionalities
CBM50 modules are generally found in bacterial lysins including muramidase [5], N-acetylglucosaminidase [1], γ-D-glutamate-meso-diaminopimelate muropeptidase [6] and N-acetylmuramoyl-L-alanine amidase [7]. The CBM50 modules in lysins are shown to bind to bacterial peptidoglycan and involved in cell division by localizing these enzymes to the divisional site [8]. CBM50 modules were also found in family GH18 chitinases [2, 9], and contribute to the antifungal activity of the enzymes through their binding ability to chitinous component of the fungal cell wall. CBM50 modules are found not only in carbohydrate-active enzymes but also in LysM-containing plant cell surface receptors for chitin oligosaccharides and their derivatives [10, 11] and fungal effectors [12]. The receptor proteins are involved in plant-microbe interactions upon symbiosis or infection.
Family Firsts
First Identified
CBM50s are also known as LysM domains. The LysM domain was first identified in lysozyme from Bacillus phage f29 [13]. LysM domains were first classified as a CBM in 2008 after demonstrating chitin oligosaccharide binding by an N-terminal LysM domain from Pteris ryukyuensis chitinase-A [3].
First Structural Characterization
The first three-dimensional structure of CBM50 module was determined for the LysM domain from E. coli membrane-bond lytic murein transglycosylase D (MltD) (PDB entry: 1e0g) by NMR spectroscopy [4].
