Structural basis for entropy-driven cellulose binding by a type-A cellulose-binding module (CBM) and bacterial expansin

Abstract

Components of modular cellulases, type-A cellulose-binding modules (CBMs) bind to crystalline cellulose and enhance enzyme effectiveness, but structural details of the interaction are uncertain. We analyzed cellulose binding by EXLX1, a bacterial expansin with ability to loosen plant cell walls and whose domain D2 has type-A CBM characteristics. EXLX1 strongly binds to crystalline cellulose via D2, whereas its affinity for soluble cellooligosaccharides is weak. Calorimetry indicated cellulose binding was largely entropically driven. We solved the crystal structures of EXLX1 complexed with cellulose-like oligosaccharides to find that EXLX1 binds the ligands through hydrophobic interactions of three linearly arranged aromatic residues in D2. The crystal structures revealed a unique form of ligand-mediated dimerization, with the oligosaccharide sandwiched between two D2 domains in opposite polarity. This report clarifies the molecular target of expansin and the specific molecular interactions of a type-A CBM with cellulose.plant cell wall | X-ray crystallography

Discrete and Structurally Unique Proteins (Tāpirins) Mediate Attachment of Extremely Thermophilic Caldicellulosiruptor Species to Cellulose

“…6). Both ta pirins bound to various forms of cellulose with affinities similar to those reported for CBMs (60,62,63,72), fungal swollenin (64), and bacterial expansin (61). Taken together, the ta pirin proteins bind to cellulose with high affinity over a range of temperatures from 25°C (trans vivo) to 70°C (in vitro), confirming their hypothesized function as a new class of cellulose-binding proteins.…”

“…7D) over nine ␤-strands flat on the surface and spaced ϳ5-10 Å apart. That arrangement, possibly together with the hydrophobic residues of the loop Ser-443-Ala-472, could act as an effective binding platform for cellodextrins similar to the flat binding faces of some carbohydrate-binding modules (3,61). When all ta pirin amino acid sequences were aligned against each other, aromatic residues at these seven positions are conserved across both classes of ta pirins from strongly cellulolytic members of the genus Caldicellulosiruptor.…”

Identification of SPOR Domain Amino Acids Important for Septal Localization, Peptidoglycan Binding, and a Disulfide Bond in the Cell Division Protein FtsN

“…The strongest support for this hypothesis is that FtsN SPOR binds the glycan backbone of PG in vitro and fails to localize in a triple amidase mutant in vivo (15,18). Stacking of aromatic amino acids, especially tryptophan, with the sugar rings of oligosaccharides is known to be important for many proteincarbohydrate binding interactions (49)(50)(51)(52). Both FtsN SPOR and DamX SPOR have a ␤-sheet tryptophan that plays a key role in septal localization: W283 in FtsN and W416 in DamX.…”

The Target of β-Expansin EXPB1 in Maize Cell Walls from Binding and Solid-State NMR Studies

“…Analysis of BsEXLX1, the expansin from Bacillus subtilis (Kerff et al, 2008;Georgelis et al, 2015), has been particularly insightful for assessing the complex binding properties and functional targeting of this protein within the CW. Such progress was enabled by expression of BsEXLX1 in Escherichia coli, facilitating creation of site-directed mutants (Georgelis et al, 2011(Georgelis et al, , 2012 and 13 C, 15 N labeling for solid-state NMR (SSNMR) analysis of EXLX1-CW interactions in muro (Wang et al, 2013). Compared with BsEXLX1, plant expansins have stronger wall-loosening activity and different selectivity, so their actions still require further study.…”