Non-enzymatic action of expansins (2020)

From their simulations of endoglucanase Cel45A, Bharadwaj et al. (1Bharadwaj V.S. Knott B.C. Ståhlberg J. Beckham G.T. Crowley M.F. The hydrolysis mechanism of a GH45 cellulase and its potential relation to lytic transglycosylase and expansin function.J. Biol. Chem. 2020; 295 (32054684): 4477-448710.1074/jbc.RA119.011406Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar) propose that structurally related expansins and MltA may cut glycan backbones without generating reducing ends. This is tenable for MltA, a peptidoglycan lytic transglycosylase whose action produces nonreducing 1,6-anhydro products, but is untenable for expansins. Expansins loosen plant cell walls and induce wall expansion. Contrary to the assertion by Bharadwaj et al., the conclusion that expansins are not lytic is not merely based on lack of new reducing ends but is supported by multiple (negative) tests for polysaccharide cleavage that do not rely on detection of reducing ends. At least eight studies with three divergent groups of expansins document this point.•For instance, α-expansin did not reduce the viscosity of various wall polysaccharide solutions, an endolytic assay that does not rely on measuring reducing ends (e.g. see Ref. 2McQueen-Mason S.J. Cosgrove D.J. Expansin mode of action on cell walls: analysis of wall hydrolysis, stress relaxation, and binding.Plant Physiol. 1995; 107 (11536663): 87-10010.1104/pp.107.1.87Crossref PubMed Scopus (364) Google Scholar and other studies).•Walls treated with α-expansin did not release saccharide fragments, measured by pulsed amperometric detection, which can detect nonreducing saccharides (3McQueen-Mason S. Durachko D.M. Cosgrove D.J. Two endogenous proteins that induce cell wall extension in plants.Plant Cell. 1992; 4 (11538167): 1425-143310.1105/tpc.4.11.1425Crossref PubMed Scopus (723) Google Scholar).•In the case of β-expansins, protein treatments did not cleave the backbones of a wide range of dye-coupled cross-linked wall polysaccharides; nor did they cleave backbones of polysaccharides extracted from plant cell walls, measured by gel permeation chromatography (4Tabuchi A. Li L.C. Cosgrove D.J. Matrix solubilization and cell wall weakening by β-expansin (group-1 allergen) from maize pollen.Plant J. 2011; 68 (21749508): 546-55910.1111/j.1365-313X.2011.04705.xCrossref PubMed Scopus (48) Google Scholar).•For five microbial expansins, tests with a range of dye-coupled cross-linked polysaccharides likewise did not detect lytic activity (e.g. see Ref. 5Kerff F. Amoroso A. Herman R. Sauvage E. Petrella S. Filée P. Charlier P. Joris B. Tabuchi A. Nikolaidis N. Cosgrove D.J. Crystal structure and activity of Bacillus subtilis YoaJ (EXLX1), a bacterial expansin that promotes root colonization.Proc. Natl. Acad. Sci. U.S.A. 2008; 105 (18971341): 16876-1688110.1073/pnas.0809382105Crossref PubMed Scopus (136) Google Scholar). Thus, extensive published evidence argues against lytic action by expansins, as proposed by Bharadwaj (1Bharadwaj V.S. Knott B.C. Ståhlberg J. Beckham G.T. Crowley M.F. The hydrolysis mechanism of a GH45 cellulase and its potential relation to lytic transglycosylase and expansin function.J. Biol. Chem. 2020; 295 (32054684): 4477-448710.1074/jbc.RA119.011406Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar), and attempts to identify 1,6-anhydro products seem unlikely to succeed. Reply to Cosgrove: Non-enzymatic action of expansinsJournal of Biological ChemistryVol. 295Issue 19PreviewIn our computational study, we use molecular simulations to substantiate a hypothetical mechanism for glycosidic bond cleavage in the presence of a single catalytic acid at the active site of the mutant D10N HiCel45A. In addition to discussing this plausible mechanism from the context of structurally related MltA lytic transglycosylase and subfamily C GH45s, we also suggest the implications of the plausible mechanism for our current understanding of the action of expansins and lytic transglycosylases. Full-Text PDF Open Access