In the course of cell wall growing, the cellulose-hemicellulose network enlarges through its creep driven by turgor-generated forces in the cell wall and catalyzed by a cell wall loosening protein; expansin. It has been suggested that plant expansins facilitate the cell wall creep without breakdown of wall polymer. Instead, expansins may disrupt the noncovalent binding of matrix polysaccharides to cellulose. A Bacillus subtilis expansin, named EXLX1, while it promotes plant infection or root surface colonization by bacteria, also possess cell wall extension activity. EXLX1 is a 23-kDa protein consisting of 2 compact domains, D1 and D2, which is remarkably similar to the structure of plant b-expansins (1). An open, planner putative polysaccharide biding surface spans the two EXLX1 domains, formed by Asp-82, Thr-12, and other polar residues in D1, and by three aromatic residue (Trp-125, Trp-126, and Tyr-157) in D2. The present paper reports a molecular dynamics simulation study of the EXLX1 protein bound to a native cellulose crystal model. The initial models of the expansin-cellulose crystal complex were built based on the crystal structure of the EXLX1-cellohexaose complex (4FER). The complex models were placed in a periodic water box and subjected to a NTP molecular dynamics (MD) at 300 K and 1 bar for hundreds of nanoseconds. Decomposition analysis of binding energies derived from the MD trajectories reveled that Asp-82 and other polar residues, and the three aromatic residues significantly contributed to the binding energy at the protein-cellulose interface, which was consistent with the results of the surface function analysis where mutation of Asp-82 or one of three aromatic residues to alanine eliminated wall loosening activity from EXLX1 (2). References (1) F. Kerff et al., (2008) Proc. Natl. Acad. Sci. USA, 105, 16876-16881. (2) N. Georgelis et al., (2011) J. Biol. Chem., 286, 16814-16823.

Dynamics behaviors of a bacterial expansin on cellulose crystal surface

251st National Meeting (2016)

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