title: Understanding the biological rationale for the diversity of cellulose-directed carbohydrate-binding modules in prokaryotic enzymes.
abstract: Plant cell walls are degraded by glycoside hydrolases that often contain noncatalytic carbohydrate-binding modules (CBMs), which potentiate degradation. There are currently 11 sequence-based cellulose-directed CBM families; however, the biological significance of the structural diversity displayed by these protein modules is uncertain. Here we interrogate the capacity of eight cellulose-binding CBMs to bind to cell walls. These modules target crystalline cellulose (type A) and are located in families 1, 2a, 3a, and 10 (CBM1, CBM2a, CBM3a, and CBM10, respectively); internal regions of amorphous cellulose (type B; CBM4-1, CBM17, CBM28); and the ends of cellulose chains (type C; CBM9-2). Type A CBMs bound particularly effectively to secondary cell walls, although they also recognized primary cell walls. Type A CBM2a and CBM10, derived from the same enzyme, displayed differential binding to cell walls depending upon cell type, tissue, and taxon of origin. Type B CBMs and the type C CBM displayed much weaker binding to cell walls than type A CBMs. CBM17 bound more extensively to cell walls than CBM4-1, even though these type B modules display similar binding to amorphous cellulose in vitro. The thickened primary cell walls of celery collenchyma showed significant binding by some type B modules, indicating that in these walls the cellulose chains do not form highly ordered crystalline structures. Pectate lyase treatment of sections resulted in an increased binding of cellulose-directed CBMs, demonstrating that decloaking cellulose microfibrils of pectic polymers can increase CBM access. The differential recognition of cell walls of diverse origin provides a biological rationale for the diversity of cellulose-directed CBMs that occur in cell wall hydrolases and conversely reveals the variety of cellulose microstructures in primary and secondary cell walls.
authors.affiliation: Centre for Plant Sciences, University of Leeds, Leeds LS2 9JT, United Kingdom.
authors.forename: Anthony W
authors.initials: AW
authors.lastname: Blake
authors.forename: Lesley
authors.initials: L
authors.lastname: McCartney
authors.forename: James E
authors.initials: JE
authors.lastname: Flint
authors.forename: David N
authors.initials: DN
authors.lastname: Bolam
authors.forename: Alisdair B
authors.initials: AB
authors.lastname: Boraston
authors.forename: Harry J
authors.initials: HJ
authors.lastname: Gilbert
authors.forename: J Paul
authors.initials: JP
authors.lastname: Knox
mesh_terms: D017360:Arabidopsis / Q000378:metabolism; D050260:Carbohydrate Metabolism; D002241:Carbohydrates / Q000737:chemistry*; D002473:Cell Wall / Q000378:metabolism*; D002482:Cellulose / Q000737:chemistry*; D019084:Fluorescent Antibody Technique, Indirect; D008954:Models, Biological; D008958:Models, Molecular; D010940:Plant Proteins / Q000737:chemistry; D011133:Polysaccharide-Lyases / Q000737:chemistry; D011485:Protein Binding; D011487:Protein Conformation; D017510:Protein Folding
chemical_list: D002241:Carbohydrates; D010940:Plant Proteins; D002482:Cellulose; D011133:Polysaccharide-Lyases; C023911:pectate lyase