Carbohydrate-binding module (CBMs) are
protein modules from microbial glycoside hydrolases that have the capacity
to bind specifically to polysaccharides.
A CBM is defined as contiguous amino acid
sequence within a carbohydrate-active enzyme with a discreet fold having
carbohydrate-binding activity that is distinct from other non-catalytic
sugar binding proteins such as lectins and sugar transport proteins. CBM
protein modules can range in length from 40 to 150 amino acids.
Currently 67 different families
of CBMs are recognized in the CAZy (Carbohydrate-Active enZYmes)
CBMs were previously classified as
cellulose-binding domains (CBDs) based on the initial discovery
of modules that bound cellulose. However, modules are being found that bind
polymers other than cellulose yet otherwise meet the CBM criteria which has
led to the use of a more inclusive terminology. These other polymer groups
include xylans, mannans and non-cellulosic glucans.
A review of the molecular basis of CBM
recognition of glycans is Gilbert et al., 2013.
Our research on CBMs is
in collaboration with
Harry Gilbert at
Recombinant protein engineering of CBMs to produce soluble forms
with polyhistidine tags has led to the development of flexible molecular
probes for cell wall polymers that can be used in procedures equivalent
to those used for monoclonal antibodies (McCartney
et al., 2004). The differential capacities of a series of
xylan-binding CBMs to recognize xylans in secondary cell walls in
sections of a range of plant materials has indicated that CBMs
from diverse families have distinct abilities to bind to xylans in the context of cell wall composites (McCartney
et al., 2006). We have also used CBMs alongside xylan-directed
monoclonal antibodies to study the recognition and degradation of
xylan polymers in both primary and secondary cell walls (Hervé
et al., 2009).
binding to TS tobacco stem
CBMs that are components of cell wall
polysaccharide hydrolases are proposed to potentiate the action of
the attached catalytic modules by increasing access to appropriate
sites within compacted cell wall composites. The recent discovery
that family 35 CBMs can bind to the product of pectate lyase
action and thus direct catalytic modules to regions of cell walls
where pectin degradation has occurred (Montanier
et al. 2009) is a nice example of a mechanism whereby CBMs can
contribute to the concerted degradation of cell walls by sets of
enzymes. Using diverse fusions of catalytic modules with CBMs we
have demonstrated the role of CBMs in the access to glycans and
their degradation in the context of intact cell walls (Hervé
et al., 2010).
CBMs that are
directed to crystalline cellulose or to amorphous cellulose in
vitro also show capacities to discriminate between cell walls
of distinct cell types and of different species indicating high
levels of variation in cellulose microstructures within cell walls
(Blake et al., 2006).
CBM9-2:FITC (purple) binding to epidermis
& collenchyma inner cell walls in TS celery petiole / DIC image