Abstract
A novel expansin protein (ScExlx1) was found, cloned and expressed from the Basidiomycete fungus Schizophylum commune. This protein showed the canonical features of plant expansins. ScExlx1 showed the ability to form “bubbles” in cotton fibers, reduce the size of avicel particles and enhance reducing sugar liberation from cotton fibers pretreated with the protein and then treated with cellulases. ScExlx1 was able to bind cellulose, birchwood xylan and chitin and this property was not affected by different sodium chloride concentrations. A novel property of ScExlx1 is its capacity to enhance reducing sugars (N-acetyl glucosamine) liberation from pretreated chitin and further added with chitinase, which has not been reported for any expansin or expansin-like protein. To the best of our knowledge, this is the first report of a bona fide fungal expansin found in a basidiomycete and we could express the bioactive protein in Pichia pastoris.
Highlights
Expansins are non-enzymatic proteins that induce extensibility and stress relaxation of plant cell walls, acting as loosening agents [1,2]
A fungal strain of Schizophyllum commune RVAN10 isolated from the northeast region of Nuevo León State in Mexico was used for this work
Changes at positions 30 (Thr/Ser), 44 (His/Arg), 170 (Ile/Val), 216 (Ser/Thr) and 221 (Thr/Ser) were conservative; while other five changes occurred as follows: 25 (Met/Thr), non polar hydrophobic/polar uncharged, 41 (Lys/Gln), polar basic/polar uncharged, 128 and 182 polar uncharged/polar acid, and 223 (Pro/Ser) non polar hydrophobic/polar uncharged. This comparative analysis between ScExlx1 and the available S. commune putative expansin sequences showed no discrepancies between amino acids responsible for binding to polysaccharides neither the “active” site of the protein suggested by [26]
Summary
Expansins are non-enzymatic proteins that induce extensibility and stress relaxation of plant cell walls, acting as loosening agents [1,2]. D1 is distantly related to the catalytic domain of glycoside hydrolase family-45 (GH45) and D2 is distantly related to group-2 grass pollen allergens [2]. These proteins form a long shallow groove with highly conserved polar and aromatic residues suitably positioned along both domains, to potentially bind plant cell wall polysaccharides [3,6]. Cellulose-active proteins with distant homology to either D1 or D2 only have been reported
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