Genomic insights into the fungal lignocellulolytic system of Myceliophthora thermophila

Abstract

The microbial conversion of solid cellulosic biomass to liquid biofuels may provide a renewable energy source for transportation fuels. Cellulolytic fungi represent a promising group of organisms, as they have evolved complex systems for adaptation to their natural habitat. The filamentous fungus Myceliophthora thermophila constitutes an exceptionally powerful cellulolytic microorganism that synthesizes a complete set of enzymes necessary for the breakdown of plant cell wall. The genome of this fungus has been recently sequenced and annotated, allowing systematic examination and identification of enzymes required for the degradation of lignocellulosic biomass. The genomic analysis revealed the existence of an expanded enzymatic repertoire including numerous cellulases, hemicellulases, and enzymes with auxiliary activities, covering the most of the recognized CAZy families. Most of them were predicted to possess a secretion signal and undergo through post-translational glycosylation modifications. These data offer a better understanding of activities embedded in fungal lignocellulose decomposition mechanisms and suggest that M. thermophila could be made usable as an industrial production host for cellulolytic and hemicellulolytic enzymes.

Keywords: CAZy; Myceliophthora thermophila; biofuels; lignocellulolytic enzymes; plant biomass.

Figure 1

Distribution of cellulolytic enzymes of M. thermophila throughout eight GH families. Other activities refer to β-xylosidase (GH3), β-1,6-galactanase, β-1,3-glucanase, endo-1,4-beta-mannosidase or putative proteins with unknown function (GH5). GH74 represents xyloglucan specific 1,4-endoglucanase/xyloglucanase.

Figure 2

Distribution of hemicellulolytic enzymes of M. thermophila throughout nine GH families. Other activities refer to β-glycosidase (GH3), xylanase with endo-exo mode of action and xylobiohyrolase (GH30), and galactan 1,3-beta-galactosidase (GH43).

Figure 3

Distribution of hemicellulolytic enzymes of M. thermophila throughout nine CE families. Family CE4 is comprised of putative proteins with polysaccharide deacetylase activity, CE5 of cutinases and CE8, 12 of pectin esterases. ND (not determined) refers to sequences encoding putative proteins with unknown activity which are not classified to a specific family.

Figure 4

Distribution of enzymes of M. thermophila with auxiliary activities, classified to AA3/8, AA9 families and multicopper oxidases. M. thermophila distinguishes itself from other cellulolytic fungi, exhibiting an impressing number of LPMOs accessory enzymes belonging to AA9 family (previously described as GH61).

Figure 5

Theoretical molecular weight of secreted enzymes of M. thermophila classified in several GHs and CEs families, plotted against theoretical pI. The average molecular weight was calculated at 51.05 ± 16.2 kDa (range between 21 and 97 kDa) for cellulolytic enzymes, 35.5 ± 19.5 kDa (range between 22 and 89 kDa) for hemicellulases (GHs/CEs), and 28.51 ± 4.1 kDa (range between 23 and 39 kDa) for the fraction of esterases.