Expression and secretion of defined cutinase variants by Aspergillus awamori

Biodegradation of plastic polymers by fungi: a brief review

Plastic polymers are non-degradable solid wastes that have become a great threat to the whole world and degradation of these plastics would take a few decades. Compared with other degradation processes, the biodegradation process is the most effective and best way for plastic degradation due to its non-polluting mechanism, eco-friendly nature, and cost-effectiveness. Biodegradation of synthetic plastics is a very slow process that also involves environmental factors and the action of wild microbial species. In this plastic biodegradation, fungi play a pivotal role, it acts on plastics by secreting some degrading enzymes, i.e., cutinase`, lipase, and proteases, lignocellulolytic enzymes, and also the presence of some pro-oxidant ions can cause effective degradation. The oxidation or hydrolysis by the enzyme creates functional groups that improve the hydrophilicity of polymers, and consequently degrade the high molecular weight polymer into low molecular weight. This leads to the degradation of plastics within a few days. Some well-known species which show effective degradation on plastics are Aspergillus nidulans, Aspergillus flavus, Aspergillus glaucus, Aspergillus oryzae, Aspergillus nomius, Penicillium griseofulvum, Bjerkandera adusta, Phanerochaete chrysosporium, Cladosporium cladosporioides, etc., and some other saprotrophic fungi, such as Pleurotus abalones, Pleurotus ostreatus, Agaricus bisporus and Pleurotus eryngii which also helps in degradation of plastics by growing on them. Some studies say that the degradation of plastics was more effective when photodegradation and thermo-oxidative mechanisms involved with the biodegradation simultaneously can make the degradation faster and easier. This present review gives current knowledge regarding different species of fungi that are involved in the degradation of plastics by their different enzymatic mechanisms to degrade different forms of plastic polymers.

Conditional Expression of the Small GTPase ArfA Impacts Secretion, Morphology, Growth, and Actin Ring Position in Aspergillus niger

In filamentous fungi, growth and protein secretion occurs predominantly at the tip of long, thread like cells termed hyphae. This requires coordinated regulation of multiple processes, including vesicle trafficking, exocytosis, and endocytosis, which are facilitated by a complex cytoskeletal apparatus. In this study, functional analyses of the small GTPase ArfA from Aspergillus niger demonstrate that this protein functionally complements the Saccharomyces cerevisiae ARF1/2, and that this protein is essential for A. niger. Loss-of-function and gain-of-function analyses demonstrate that titration of arfA expression impacts hyphal growth rate, hyphal tip morphology, and protein secretion. Moreover, localization of the endocytic machinery, visualized via fluorescent tagging of the actin ring, was found to be abnormal in ArfA under- and overexpressed conditions. Finally, we provide evidence that the major secreted protein GlaA localizes at septal junctions, indicating that secretion in A. niger may occur at these loci, and that this process is likely impacted by arfA expression levels. Taken together, our results demonstrate that ArfA fulfills multiple functions in the secretory pathway of A. niger.

Unfolded protein response in filamentous fungi-implications in biotechnology

The unfolded protein response (UPR) represents a mechanism to preserve endoplasmic reticulum (ER) homeostasis that is conserved in eukaryotes. ER stress caused by the accumulation of potentially toxic un- or misfolded proteins in the ER triggers UPR activation and the induction of genes important for protein folding in the ER, ER expansion, and transport from and to the ER. Along with this adaptation, the overall capacity for protein secretion is markedly increased by the UPR. In filamentous fungi, various approaches to employ the UPR for improved production of homologous and heterologous proteins have been investigated. As the effects on protein production were strongly dependent on the expressed protein, generally applicable strategies have to be developed. A combination of transcriptomic approaches monitoring secretion stress and basic research on the UPR mechanism provided novel and important insight into the complex regulatory cross-connections between UPR signalling, cellular physiology, and developmental processes. It will be discussed how this increasing knowledge on the UPR might stimulate the development of novel strategies for using the UPR as a tool in biotechnology.

Engineering of protein folding and secretion-strategies to overcome bottlenecks for efficient production of recombinant proteins

SIGNIFICANCE

Recombinant protein production has developed into a huge market with enormous positive implications for human health and for the future direction of a biobased economy. Limitations in the economic and technical feasibility of production processes are often related to bottlenecks of in vivo protein folding.

RECENT ADVANCES

Based on cell biological knowledge, some major bottlenecks have been overcome by the overexpression of molecular chaperones and other folding related proteins, or by the deletion of deleterious pathways that may lead to misfolding, mistargeting, or degradation.

CRITICAL ISSUES

While important success could be achieved by this strategy, the list of reported unsuccessful cases is disappointingly long and obviously dependent on the recombinant protein to be produced. Singular engineering of protein folding steps may not lead to desired results if the pathway suffers from several limitations. In particular, the connection between folding quality control and proteolytic degradation needs further attention.

FUTURE DIRECTIONS

Based on recent understanding that multiple steps in the folding and secretion pathways limit productivity, synergistic combinations of the cell engineering approaches mentioned earlier need to be explored. In addition, systems biology-based whole cell analysis that also takes energy and redox metabolism into consideration will broaden the knowledge base for future rational engineering strategies.

Protein folding and conformational stress in microbial cells producing recombinant proteins: a host comparative overview

Different species of microorganisms including yeasts, filamentous fungi and bacteria have been used in the past 25 years for the controlled production of foreign proteins of scientific, pharmacological or industrial interest. A major obstacle for protein production processes and a limit to overall success has been the abundance of misfolded polypeptides, which fail to reach their native conformation. The presence of misfolded or folding-reluctant protein species causes considerable stress in host cells. The characterization of such adverse conditions and the elicited cell responses have permitted to better understand the physiology and molecular biology of conformational stress. Therefore, microbial cell factories for recombinant protein production are depicted here as a source of knowledge that has considerably helped to picture the extremely rich landscape of in vivo protein folding, and the main cellular players of this complex process are described for the most important cell factories used for biotechnological purposes.

Cutinase-like enzyme from the yeast Cryptococcus sp. strain S-2 hydrolyzes polylactic acid and other biodegradable plastics

A purified lipase from the yeast Cryptococcus sp. strain S-2 exhibited remote homology to proteins belonging to the cutinase family rather than to lipases. This enzyme could effectively degrade the high-molecular-weight compound polylactic acid, as well as other biodegradable plastics, including polybutylene succinate, poly (epsilon-caprolactone), and poly(3-hydroxybutyrate).

Molecular characteristics of strains of the cameroon family, the major group of Mycobacterium tuberculosis in a country with a high prevalence of tuberculosis

A preliminary investigation of the genetic biodiversity of Mycobacterium tuberculosis complex strains in Cameroon, a country with a high prevalence of tuberculosis, described a group of closely related M. tuberculosis strains (the Cameroon family) currently responsible for more than 40% of smear-positive pulmonary tuberculosis cases. Here, we used various molecular methods to study the genetic characteristics of this family of strains. Cameroon family M. tuberculosis strains (i) are part of the major genetic group 2 and lack the TbD1 region like other families of epidemic strains, (ii) lack spacers 23, 24, and 25 in their direct repeat (DR) region, (iii) have an identical number of repeats in 8 of 12 variable-number tandem repeats of mycobacterial interspersed repetitive unit (MIRU-VNTR) loci, (iv) have similar IS6110-restriction fragment length polymorphism (RFLP) multiband patterns (10 to 15 copies) with seven common IS6110 bands, (v) do not have an IS6110 element in their DR locus, and (vi) have four IS6110 elements in open reading frames (adenylate cyclase, phospholipase C, moeY, and ATP binding genes). Analysis by spoligotyping, MIRU-VNTR, and IS6110-RFLP typing methods revealed differences not observed in previous studies; polymorphism as assessed by MIRU-VNTR typing was lower than suggested by spoligotyping, and in rare cases, strains with identical IS6110-RFLP patterns had spoligotypes differing by as much as 15 spacers. Our findings confirm the recent expansion of this family in Cameroon and indicate that the interpretation of molecular typing results has to be adapted to the characteristics of the strain population within each setting. The knowledge of this particular genotype, with its large involvement in tuberculosis in Cameroon, allows greater refinement of tuberculosis transmission studies by interpreting data in the context of this geographic area.

Modulation of Aspergillus awamori thaumatin secretion by modification of bipA gene expression

Two different strains, Aspergillus awamori TGDTh-4 and A. awamori TGP-3 overexpressing a synthetic gene encoding the plant sweet protein thaumatin, showed an unfolded protein response. To facilitate protein secretion, the chaperone BiPA gene was expressed in A. awamori under control of the strong constitutive promoter of the gpdA gene. A good correlation was observed between the level of the bipA transcript in different strains and the amount of thaumatin secreted. Thaumatin secretion was increased 2- to 2.5-fold in transformants overexpressing the bipA gene compared with the parental strain. Secretion of the homologous proteins alpha-amylase and glucoamylase was not affected by the bipA gene overexpression. The requirement for BiPA for secretion of thaumatin was confirmed by attenuation of the endogenous bipA gene expression with an antisense RNA cassette. The decrease in bipA expression reduced the amount of secreted thaumatin up to 80% without affecting the secretion of the homologous alpha-amylase and glucoamylase proteins. The BiPA protein is, therefore, very important for secretion of some heterologous proteins, such as thaumatin in A. awamori.

Improvement of foreign-protein production in Aspergillus niger var. awamori by constitutive induction of the unfolded-protein response

Unfolded-protein response (UPR) denotes the upregulation of endoplasmic reticulum (ER)-resident chaperone and foldase genes and numerous other genes involved in secretory functions during the accumulation of unfolded proteins into the ER. Overexpression of individual foldases and chaperones has been used in attempts to improve protein production in different production systems. We describe here a novel strategy to improve foreign-protein production. We show that the constitutive induction of the UPR pathway in Aspergillus niger var. awamori can be achieved by expressing the activated form of the transcription factor hacA. This induction enhances the production of Trametes versicolor laccase by up to sevenfold and of bovine preprochymosin by up to 2.8-fold in this biotechnically important fungus. The regulatory range of UPR was studied by analyzing the mRNA levels of novel A. niger var. awamori genes involved in different secretory functions. This revealed both similarities and differences to corresponding studies in Saccharomyces cerevisiae.

Recent studies of protein secretion by filamentous fungi

Filamentous fungi have been widely exploited for the homologous and heterologous protein production, because of the high capacity of their protein secretion machinery. However, the production of heterologous proteins is often limited while the production of homologous proteins can be very high. Various researches have reported the methods for overcoming this problem and some techniques, such as the fusion gene system, improve the production of heterologous proteins. Recently, the molecular biological study of solid-state culture attracts the attention, because the long history of biological studies has shown that the productivity of protein in the solid-state culture frequently exceeds the productivity of protein in the submerged culture. The recent researches of solid-state culture have revealed the new aspects of protein production in filamentous fungi. Solid-state specific gene expression was observed in the glaB and pepA genes of Aspergillus oryzae. A GC-box and HSE element of the glaB promoter region affected solid-state specific gene expression of this gene. Solid-state culture-specific release of enzymes from the cell wall was also observed in the production of beta-glucosidases in Aspergillus kawachii. Extracellular soluble polysaccharide (ESP) from A. kawachii was concerned with the location of beta-glucosidases. Moreover, ESP and the cell wall fraction of A. kawachii were shown to be involved in the stability of beta-glucosidases. The knowledge of the molecular biology of solid-state culture should provide new approaches for the production of both homologous and heterologous proteins in both submerged culture and solid-state culture of filamentous fungi.

Calnexin overexpression increases manganese peroxidase production in Aspergillus niger

Heme-containing peroxidases from white rot basidiomycetes, in contrast to most proteins of fungal origin, are poorly produced in industrial filamentous fungal strains. Factors limiting peroxidase production are believed to operate at the posttranslational level. In particular, insufficient availability of the prosthetic group which is required for peroxidase biosynthesis has been proposed to be an important bottleneck. In this work, we analyzed the role of two components of the secretion pathway, the chaperones calnexin and binding protein (BiP), in the production of a fungal peroxidase. Expression of the Phanerochaete chrysosporium manganese peroxidase (MnP) in Aspergillus niger resulted in an increase in the expression level of the clxA and bipA genes. In a heme-supplemented medium, where MnP was shown to be overproduced to higher levels, induction of clxA and bipA was also higher. Overexpression of these two chaperones in an MnP-producing strain was analyzed for its effect on MnP production. Whereas bipA overexpression seriously reduced MnP production, overexpression of calnexin resulted in a four- to fivefold increase in the extracellular MnP levels. However, when additional heme was provided in the culture medium, calnexin overexpression had no synergistic effect on MnP production. The possible function of these two chaperones in MnP maturation and production is discussed.

The secretion pathway in filamentous fungi: a biotechnological view

The high capacity of the secretion machinery of filamentous fungi has been widely exploited for the production of homologous and heterologous proteins; however, our knowledge of the fungal secretion pathway is still at an early stage. Most of the knowledge comes from models developed in yeast and higher eukaryotes, which have served as reference for the studies on fungal species. In this review we compile the data accumulated in recent years on the molecular basis of fungal secretion, emphasizing the relevance of these data for the biotechnological use of the fungal cell and indicating how this information has been applied in attempts to create improved production strains. We also present recent emerging approaches that promise to provide answers to fundamental questions on the molecular genetics of the fungal secretory pathway.

From gene to product in yeast: production of fungal cutinase

In the mid-1970s, information technology and recombinant DNA technology were considered as the breakthrough technologies of the final quarter of the 20th century. Now, about 25 years later, information technology has penetrated deeply into our society and nearly everyone uses this technology. Compared to the formidable success of information technology, the progress in the commercialization of recombinant DNA technology is moderate, even when taking into account that all that is related to the technological application of biological sciences needs extensive safety testing. However, there are signs that the speed of this commercialization will increase in the first decade of the 21st century. Moreover, new breakthroughs in our understanding of the complete genetic make up of eukaryotes will contribute to this increase in speed. An important aspect of the commercialization of this technology is the development of cells as factories for the production of valuable and/or useful molecules. Lower eukaryotes, such as yeasts and molds, are the most promising candidates to become the factories of the future, but at present these factories still contains a lot of process lines that may be superfluous under the well controlled conditions in fermentors. On the other hand, the speed and yield of these cellular production lines can be increased by eliminating the rate-determining steps of these process lines. In this contribution to the European Union symposium from Cell to Factory, some steps in the improvement of S. cerevisiae as cell factories for (heterologous) hydrophobic molecules are presented.