Fig. 1

Solution NMR structure of PcaLOOL12 (9CE9). A The primary amino acid sequence of PcaLOOL12 with the β-strand and α-helix elements of secondary structure colored blue and yellow, respectively. Disulfide bonds are observed between C26–C50 and C73–C76 with the position of these four cysteine residues identified with red residue numbers above the sequence. The P. carnosa genome contains 12 loosenin genes divided into two subgroups based on the position of an insertion relative to the predicted polysaccharide binding groove (PcaLOOL12 belongs in subgroup B) [15]. The red box is the characteristic 12–18 residue insertion observed in subgroup B loosenins and the asterisk after G44 indicates the position of the characteristic 8–9 residue insertion observed in subgroup A loosenins. The rectangles below the sequence denote conserved residues in the P. carnosa loosenin family predicted to form a polysaccharide binding groove, with the blue rectangles denoting residues associated with expansin activity (EXLX1) in this groove. The first two N-terminal residues are “scars” left over after cleavage of the thioredoxin moiety of the original construct. The six β-strands form a double-psi β-barrel (DPBB) structural fold. B A cartoon superposition of the ensemble of 20 solution structures calculated for PcaLOOL12. C A cartoon representation of a single structure in the ensemble rotated 180 º in the y-axis to better illustrate the location of the two disulfide bonds (side chains colored red) and the three β-strands on the back side of the protein. D A topology diagram illustrating the relative orientation of the elements of secondary structure with the α-helices colored yellow and the β-strands colored blue (psi-1) or purple (psi-2). The β-strands and α-helices are drawn to scale but not the connecting loops and turns. E Isolation of the two psi-motifs in the DPBB structural fold. F A view similar to (C) of the solvent-accessible surface of PcaLOOL12 with the residues predicted to form a polysaccharide binding groove colored cyan or blue (A). The three labeled blue residues are associated with activity in expansins. G A view identical to (F) highlighting the electrostatic potentials at the solvent-accessible surface of PcaLOOL12 (–5 to + 5 kT/e) with the negative and positive regions colored red and blue, respectively