Electrostatic analysis of bacterial expansins

Abstract

Expansins are a family of proteins with plant cell wall remodeling-activity, which bind cell wall components through hydrophobic and electrostatic interactions. A shallow area on the surface of the protein serves as the polysaccharide binding site (PBS) and it is composed of conserved residues. However, electric charge differences on the opposite face of the PBS produce basic, neutral, or acidic proteins. An analysis of forty-four bacterial expansins, homologues of BsEXLX1, revealed two main groups defined by: (a) the presence or absence of disulfide bonds; and (b) by the proteins isoelectric point (pI). We determined the location of the residues responsible for the pI on the structure of representative expansins. Our results suggest that the electric charge at the opposite site of the PBS may help in substrate differentiation among expansins from different species; in addition, electrostatic polarization between the front and the back of the molecule could affect expansin activity on cellulose.

Bacterial expansins and related proteins from the world of microbes

“…Sequence alignments, phylogenetic tools, as well as protein domain and fold recognition programs confirmed that all identified proteins contain both expansin domains and showed that the predicted protein structures resemble both B. subtilis EXLX1 and plant expansins. Modeling of bacterial expansins and surface charge properties by Pastor et al (2014) likewise support structural homology between plant and bacterial expansins. This study identified three clades among the bacterial expansins.…”

“…Pastor et al (2014) calculated the pI for a selection of 44 phylogenetically diverse bacterial expansins and found that half were acidic proteins (pIs of ~5), nearly half were basic proteins (pIs of ~9) and ~10% had neutral pIs. Protein pI generally correlated with the type of bacterial cell wall of the species, with some exceptions.…”

“…Cellulose binding was essential for a productive interaction (wall loosening) whereas the electrostatic interactions were nonproductive. Pastor et al (2014) speculated that electrostatic gradients on the expansin surface may facilitate its wall loosening action by promoting reorientation of H-bonds in cellulose. This idea remains to be carefully tested, but the altered activities of ‘RKK’ and other mutants seem difficult to explain by this theory.…”

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

“…Plant and microbial expansins share a greater degree of structural similarity than their low sequence identity (approximately 15%) might suggest (Yennawar et al ., ; Kerff et al ., ). They are composed of two tightly juxtaposed domains that are stabilized by ionic interactions and in some cases by disulphide bonds (Kerff et al ., ; Yennawar et al ., ; Pastor et al ., ). On the surface, a shallow groove comprised of conserved residues spans the two domains and forms the polysaccharide binding site, where a glucan molecule of nine saccharides contacts the substrate, stabilized by three aromatic residues on D2 and through polar interactions on D1.…”

“…On the other hand, acidic expansins such as Exl1 from P. carotovorum or Exlx2 from H. chejuensis seem to be repelled by pectin and hemicellulose and binding increases after their depletion (when cellulose is the main polysaccharide remaining), further supporting the notion that cellulose is the target of bacterial expansins (Lee et al ., 2010; Olarte‐Lozano et al ., ). This difference allows separation of bacterial expansins in two groups: acidic, when their pI < 5; or basic, when pI > 8 (Pastor et al ., ). The reason for this is mysterious considering that productive binding occurs through the polysaccharide binding surface that is conserved among all groups of expansins.…”