Aspergillus nidulans protein kinase A plays an important role in cellulase production.

Leandro José De Assis,Laure Nicolas Annick Ries,Gustavo Henrique Goldman,Marcela Savoldi,Thaila Fernanda Dos Reis,Neil Andrew Brown

doi:10.1186/s13068-015-0401-1

Abstract

BackgroundThe production of bioethanol from lignocellulosic feedstocks is dependent on lignocellulosic biomass degradation by hydrolytic enzymes. The main component of lignocellulose is cellulose and different types of organisms are able to secrete cellulases. The filamentous fungus Aspergillus nidulans serves as a model organism to study cellulase production and the available tools allow exploring more in depth the mechanisms governing cellulase production and carbon catabolite repression.ResultsIn A. nidulans, microarray data identified the cAMP-dependent protein kinase A (PkaA) as being involved in the transcriptional modulation and the production of lignocellulolytic enzymes in the presence of cellulose. Deletion of pkaA resulted in increased hydrolytic enzyme secretion, but reduced growth in the presence of lignocellulosic components and various other carbon sources. Furthermore, genes involved in fungal development were increased in the ΔpkaA strain, probably leading to the increased hyphal branching as was observed in this strain. This would allow the secretion of higher amounts of proteins. In addition, the expression of SynA, encoding a V-SNARE synaptobrevin protein involved in secretion, was increased in the ΔpkaA mutant. Deletion of pkaA also resulted in the reduced nuclear localization of the carbon catabolite repressor CreA in the presence of glucose and in partial de-repression when grown on cellulose. PkaA is involved in the glucose signaling pathway as the absence of this protein resulted in reduced glucose uptake and lower hexokinase/glucokinase activity, directing the cell to starvation conditions. Genome-wide transcriptomics showed that the expression of genes encoding proteins involved in fatty acid metabolism, mitochondrial function and in the use of cell storages was increased.ConclusionsThis study shows that PkaA is involved in hydrolytic enzyme production in A. nidulans. It appears that this protein kinase blocks the glucose pathway, hence forcing the cell to change to starvation conditions, increasing hydrolytic enzyme secretion and inducing the usage of cellular storages. This work uncovered new regulatory avenues governing the tight interplay between the metabolic states of the cell, which are important for the production of hydrolytic enzymes targeting lignocellulosic biomass. Deletion of pkaA resulted in a strain with increased hydrolytic enzyme secretion and reduced biomass formation.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0401-1) contains supplementary material, which is available to authorized users.

Highlights

The production of bioethanol from lignocellulosic feedstocks is dependent on lignocellulosic biomass degradation by hydrolytic enzymes
In the presence of glucose, the carbon source favored by most organisms, the secretion of these plant cell wall-degrading enzymes and the utilization of alternative carbon sources are repressed by carbon catabolite repression (CCR), which is mediated by the CreA transcriptional repressor [7]
The growth of ΔpkaA mutant was dramatically reduced in liquid glucose-containing minimal media, the growth rate was comparable to the wild-type strain when grown in liquid complete complete media (YG) media (24 h, wild type = 0.116 ± 0.010 g/107 conidia; ΔpkaA = 0.167 ± 0.1018 g/107 conidia)

Summary

Introduction

The production of bioethanol from lignocellulosic feedstocks is dependent on lignocellulosic biomass degradation by hydrolytic enzymes. In the presence of glucose, the carbon source favored by most organisms, the secretion of these plant cell wall-degrading enzymes and the utilization of alternative carbon sources are repressed by carbon catabolite repression (CCR), which is mediated by the CreA transcriptional repressor [7]. In the presence of glucose, CreA has been shown to repress the transcription of genes encoding enzymes important for the utilization of alternative carbon sources [8], such as proline, ethanol, xylan [9], cellulose [10, 11] and arabinan [12, 13]. The reversible phosphorylation of target proteins is performed by the opposing activities of kinases and phosphatases This post-translational mechanism is important for modulating protein structure, function and location, playing a crucial role in many cell signaling mechanisms including the regulation of CCR [14]. Deletion of SNF1 homologues in filamentous fungi, including A. nidulans, has been shown to influence CreA de-repression and reduce hydrolytic enzyme production [8, 17,18,19]

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