Understanding the structure and function of bacterial expansins: a prerequisite towards practical applications for the bioenergy and agricultural industries

Abstract

SummarySince the publication of a landmark article on the structure of EXLX1 from Bacillus subtilis in 2011, our knowledge of bacterial expansins has steadily increased and our view and understanding of these enigmatic proteins has advanced with relation to their structure, phylogenetic relationships and substrate interaction, although the molecular basis for their mechanism of action remains to be determined. Lignocellulosic material represents a source of fermentable sugars for the production of biofuels, and cell‐wall degrading activities are essential to efficiently release such sugars from their polymeric structures. Because expansins from fungi and bacteria seem to be required to properly colonize or cause disease to plant tissues, and because they share some characteristics with their plant counterparts for loosening the cell wall they have been seen as a promising tool to overcome the recalcitrance of these materials. However, microbial expansins activity is at best, very low compared with plant expansins activity. This revision analyses recent work on bacterial expansins structure, function and biological role, emphasizing our need to focus on their mechanism of action as a means to design better strategies for their use, in both in the energy and agricultural industries.

Overexpression of paralogues of the wheat expansin gene TaEXPA8 improves low‐temperature tolerance in Arabidopsis

“…Expansins play important roles in diverse biological processes related to cell wall modification, including seed germination (Yan et al, ), root growth and development (Yu et al, ), stem growth and internode elongation (Kuluev et al, ), leaf initiation and expansion (Kuluev et al, ), and fruit growth and ripening (Ishimaru et al, ). Expansins are also associated with biotic and abiotic stress tolerance (Yu et al, ; Zheng et al, ; Martinez‐Anaya ). For example, the overexpression of PpEXP1 increased heat stress tolerance in transgenic tobacco plants (Xu et al, ).…”

On the distinct binding modes of expansin and carbohydrate-binding module proteins on crystalline and nanofibrous cellulose: implications for cellulose degradation by designer cellulosomes

“…In addition to CBMs with a potential disruptive function on crystalline cellulose 24,25 , other groups of proteins were proposed to perform non-catalytic disruption of recalcitrant cellulose fibres 10,11,23,26,27 . Expansins, expansin-like proteins, and swollenins are proposed to bind to cellulose and non-hydrolytically disrupt the hydrogen-bonding networks in cellulose [28][29][30] and increase the efficiency of catalytic hydrolysis in a synergistic manner [31][32][33] . Plant expansins, which are suggested to be amongst the most effective promoters of hydrolysis, belong to two main groups: α-expansins and β-expansins that bind to xyloglucanenriched cellulose and to cell walls of high arabinoxylan content, respectively 26,30,34,35 .…”

“…Expansins, expansin-like proteins, and swollenins are proposed to bind to cellulose and non-hydrolytically disrupt the hydrogen-bonding networks in cellulose [28][29][30] and increase the efficiency of catalytic hydrolysis in a synergistic manner [31][32][33] . Plant expansins, which are suggested to be amongst the most effective promoters of hydrolysis, belong to two main groups: α-expansins and β-expansins that bind to xyloglucanenriched cellulose and to cell walls of high arabinoxylan content, respectively 26,30,34,35 . On the other hand, bacterial expansins have the ability to bind to whole (native) cell walls [36][37][38] and so are especially promising for industrial applications 39 .…”

“…The D1 domain lacks catalytic activity, even though it has been found to be structurally similar to family 45 glycoside hydrolases 43 , because it lacks key residues that are critical for catalytic activity. Moreover, not much is known about the D1 domain action on the substrate 26,39,42 . On the other hand, the D2 domain is a CBM with a flat aromatic-rich surface that can recognise crystalline cellulose 42 , and these interactions are well documented 39,40,42 .…”

“…Moreover, not much is known about the D1 domain action on the substrate 26,39,42 . On the other hand, the D2 domain is a CBM with a flat aromatic-rich surface that can recognise crystalline cellulose 42 , and these interactions are well documented 39,40,42 . This domain has been assigned to the family 63 in the CAZy database 39 .…”