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

Since 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.