Identification and characterization of putative xylose and cellobiose transporters in Aspergillus nidulans

Abstract

BackgroundThe conversion of lignocellulosic biomass to biofuels (second-generation biofuel production) is an environmentally friendlier alternative to petroleum-based energy sources. Enzymatic deconstruction of lignocellulose, catalyzed by filamentous fungi such as Aspergillus nidulans, releases a mixture of mono- and polysaccharides, including hexose (glucose) and pentose (xylose) sugars, cellodextrins (cellobiose), and xylooligosaccharides (xylobiose). These sugars can subsequently be fermented by yeast cells to ethanol. One of the major drawbacks in this process lies in the inability of yeast, such as Saccharomyces cerevisiae, to successfully internalize sugars other than glucose. The aim of this study was, therefore, to screen the genome of A. nidulans, which encodes a multitude of sugar transporters, for transporters able to internalize non-glucose sugars and characterize them when introduced into S. cerevisiae.ResultsThis work identified two proteins in A. nidulans, CltA and CltB, with roles in cellobiose transport and cellulose signaling, respectively. CltA, when introduced into S. cerevisiae, conferred growth on low and high concentrations of cellobiose. Deletion of cltB resulted in reduced growth and extracellular cellulase activity in A. nidulans in the presence of cellobiose. CltB, when introduced into S. cerevisiae, was not able to confer growth on cellobiose, suggesting that this protein is a sensor rather than a transporter. However, we have shown that the introduction of additional functional copies of CltB increases the growth in the presence of low concentrations of cellobiose, strongly indicating CltB is able to transport cellobiose. Furthermore, a previously identified glucose transporter, HxtB, was also found to be a major xylose transporter in A. nidulans. In S. cerevisiae, HxtB conferred growth on xylose which was accompanied by ethanol production.ConclusionsThis work identified a cellobiose transporter, a xylose transporter, and a putative cellulose transceptor in A. nidulans. This is the first time that a sensor role for a protein in A. nidulans has been proposed. Both transporters are also able to transport glucose, highlighting the preference of A. nidulans for this carbon source. This work provides a basis for future studies which aim at characterizing and/or genetically engineering Aspergillus spp. transporters, which, in addition to glucose, can also internalize other carbon sources, to improve transport and fermentation of non-glucose sugars in S. cerevisiae.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0611-1) contains supplementary material, which is available to authorized users.

Transcriptome Profiling-Based Analysis of Carbohydrate-Active Enzymes in Aspergillus terreus Involved in Plant Biomass Degradation

“…Transformation of S. cerevisiae with cellobiose and xylose transporters from A. nidulans , for example, has highlighted the potential in the use of such genes from Aspergillus sp. for genetic engineering of yeasts for complete fermentation of all sugars in lignocellulose for conversion to bioethanol ( dos Reis et al, 2016 ). Prior to this study, there have been no reports on sugar transporter modulation in A. terreus during lignocellulose breakdown.…”

Identification and Characterization of a Cellodextrin Transporter in Aspergillus niger

“…Herein, the sequence of ctA was deposited in GenBank under the accession number of MH648002. The cellodextrin transporter CtA in A. niger shares 36% identity with CDT-1 (Galazka et al, 2010), but only 29% identity with the cellobiose transporter CltA in A. nidulans (dos Reis et al, 2016). Phylogenetic analysis further revealed that CtA was closely related to the well characterized cellodextrin transporter CDT-1 from N. crassa and CdtC from P. oxalicum, but did not group with CltA from A. nidulans (Figure 2).…”

“…In the past years, several cellodextrin transporters have been identified in cellulolytic fungi. Two cellodextrin transporters (CDT-1 and CDT-2) were identified in cellulolytic fungus Neurospora crassa (Galazka et al, 2010), and three genes, CdtC, CdtD, and CdtG, encoding cellodextrin transporters have been reported in cellulolytic fungus Penicillium oxalicum (Li et al, 2013), and the transporter Stp1 and CltA involving cellobiose uptake were identified in Trichoderma reesei and Aspergillus nidulans, respectively (Zhang et al, 2013;dos Reis et al, 2016).…”

CAZyome and Sugar Transportome Unveil Singular Aspects of the Lignocellulolytic Enzyme System of Penicillium echinulatum 2HH

“…In Aspergillus nidulans, CltA is a cellobiose-specific transporter, while CltB/LacpB is able to transport cellobiose and lactose. However, this protein is a cellulose signaling sensor rather than a cellobiose transporter [23]. Still in A. nidulans, deletion of cltB/lacpB resulted in reduced growth and extracellular cellulase activity, indicating that cellulose and lactose catabolic systems operate with common components.…”

“…Among these sugars, filamentous fungi are able to transport disaccharides such as cellobiose into the cell through specific transporters. Cellobiose and other cellodextrins can act as signal transducers in two ways: i) cellodextrins are transported into cells activating intracellular sensors, and ii) extracellular cellodextrins activate plasma membrane sensors, such as transporter-like proteins or protein-coupled G receptors [23].…”