Molecular dynamics of the Bacillus subtilis expansin EXLX1: interaction with substrates and structural basis of the lack of activity of mutants

Expansins are disruptive proteins that loosen growing plant cell walls and can enhance the enzymatic hydrolysis of cellulose. The canonical expansin structure consists of one domain responsible for substrate binding (D2) and another domain (D1) of unknown function, but essential for activity. Although the effects of expansins on cell walls and cellulose fibrils are known, the molecular mechanism underlying their biophysical function is poorly understood. Here, we use molecular dynamics simulations to gain insights into the mechanism of action of the Bacillus subtilis expansin BsEXLX1. We show that BsEXLX1 can slide on the hydrophobic surface of crystalline cellulose via the flat aromatic surface of its binding domain D2, comprised mainly of residues Trp125 and Trp126. Also, we observe that BsEXLX1 can hydrogen bond a free glucan chain in a twisted conformation and that the twisting is chiefly induced by means of residue Asp82 located on D1, which has been shown to be essential for expansin activity. These results suggest that BsEXLX1 could move on the surface of cellulose and disrupt hydrogen bonds by twisting glucan chains. Simulations of the inactive BsEXLX1 mutants Asp82Asn and Tyr73Ala indicate structural alterations around the twisting center in the domain D1, which suggest a molecular basis for the lack of activity of these mutants and corroborate the idea that BsEXLX1 works by inducing twists on glucan chains. Moreover, simulations of the double mutant Asp82Asn/Tyr73Leu predict the recovery of the lost activity of BsEXLX1-Asp82Asn. Our results provide a dynamical view of the expansin-substrate interactions at the molecular scale and help shed light on the expansin mechanism.