Mutations in the regulatory subunit of yeast acetohydroxyacid synthase affect its activation by MgATP

A pyruvate carbon flux tugging strategy for increasing 2,3-butanediol production and reducing ethanol subgeneration in the yeast Saccharomyces cerevisiae

BACKGROUND

The yeast Saccharomyces cerevisiae is a promising host cell for producing a wide range of chemicals. However, attempts to metabolically engineer Crabtree-positive S. cerevisiae invariably face a common issue: how to reduce dominant ethanol production. Here, we propose a yeast metabolic engineering strategy for decreasing ethanol subgeneration involving tugging the carbon flux at an important hub branching point (e.g., pyruvate). Tugging flux at a central glycolytic overflow metabolism point arising from high glycolytic activity may substantially increase higher alcohol production in S. cerevisiae. We validated this possibility by testing 2,3-butanediol (2,3-BDO) production, which is routed via pyruvate as the important hub compound.

RESULTS

By searching for high-activity acetolactate synthase (ALS) enzymes that catalyze the important first-step reaction in 2,3-BDO biosynthesis, and tuning several fermentation conditions, we demonstrated that a stronger pyruvate pulling effect (tugging of pyruvate carbon flux) is very effective for increasing 2,3-BDO production and reducing ethanol subgeneration by S. cerevisiae. To further confirm the validity of the pyruvate carbon flux tugging strategy, we constructed an evolved pyruvate decarboxylase (PDC)-deficient yeast (PDCΔ) strain that lacked three isozymes of PDC. In parallel with re-sequencing to identify genomic mutations, liquid chromatography-tandem mass spectrometry analysis of intermediate metabolites revealed significant accumulation of pyruvate and NADH in the evolved PDCΔ strain. Harnessing the high-activity ALS and additional downstream enzymes in the evolved PDCΔ strain resulted in a high yield of 2,3-BDO (a maximum of 0.41 g g^-1 glucose consumed) and no ethanol subgeneration, thereby confirming the utility of our strategy. Using this engineered strain, we demonstrated a high 2,3-BDO titer (81.0 g L^-1) in a fed-batch fermentation using a high concentration of glucose as the sole carbon source.

CONCLUSIONS

We demonstrated that the pyruvate carbon flux tugging strategy is very effective for increasing 2,3-BDO production and decreasing ethanol subgeneration in Crabtree-positive S. cerevisiae. High activity of the common first-step enzyme for the conversion of pyruvate, which links to both the TCA cycle and amino acid biosynthesis, is likely important for the production of various chemicals by S. cerevisiae.

SCF(β-TRCP)-mediated degradation of NEDD4 inhibits tumorigenesis through modulating the PTEN/Akt signaling pathway

The HECT domain-containing ubiquitin E3 ligase NEDD4 is widely expressed in mammalian tissues and plays a crucial role in governing a wide spectrum of cellular processes including cell growth, tissue development and homeostasis. Recent reports have indicated that NEDD4 might facilitate tumorigenesis through targeted degradation of multiple tumor suppressor proteins including PTEN. However, the molecular mechanism by which NEDD4 stability is regulated has not been fully elucidated. Here we report that SCF(β-TRCP) governs NEDD4 protein stability by targeting it for ubiquitination and subsequent degradation in a Casein Kinase-I (CKI) phosphorylation-dependent manner. Specifically, depletion of β-TRCP, or inactivation of CKI, stabilized NEDD4, leading to down-regulation of its ubiquitin target PTEN and subsequent activation of the mTOR/Akt oncogenic pathway. Furthermore, we found that CKIδ-mediated phosphorylation of Ser347 and Ser348 on NEDD4 promoted its interaction with SCF(β-TRCP) for subsequent ubiquitination and degradation. As a result, compared to ectopic expression of wild-type NEDD4, introducing a non-degradable NEDD4 (S347A/S348A-NEDD4) promoted cancer cell growth and migration. Hence, our findings revealed the CKI/SCF(β-TRCP) signaling axis as the upstream negative regulator of NEDD4, and further suggested that enhancing NEDD4 degradation, presumably with CKI or SCF(β-TRCP) agonists, could be a promising strategy for treating human cancers.

Acetolactate synthases MoIlv2 and MoIlv6 are required for infection-related morphogenesis in Magnaporthe oryzae

Amino acids are important components in the metabolism of a variety of pathogens, plants and animals. Acetolactate synthase (ALS) catalyses the first common step in leucine, isoleucine and valine biosynthesis, and is the target of several classes of inhibitors. Here, MoIlv2, an orthologue of the Saccharomyces cerevisiae ALS catalytic subunit Ilv2, and MoIlv6, an orthologue of the S. cerevisiae ALS regulatory subunit Ilv6, were identified. To characterize MoILV2 and MoILV6 functions, we generated the deletion mutants ΔMoilv2 and ΔMoilv6. Phenotypic analysis showed that both mutants were auxotrophic for leucine, isoleucine and valine, and were defective in conidial morphogenesis, appressorial penetration and pathogenicity. Further studies suggested that MoIlv2 and MoIlv6 play a critical role in maintaining the balance of intracellular amino acid levels. MoIlv2 and MoIlv6 are both localized to the mitochondria and the signal peptide of MoIlv6 is critical for its localization. In summary, our evidence indicates that MoIlv2 plays a crucial role in isoleucine and valine biosynthesis, whereas MoIlv6 contributes to isoleucine and leucine biosynthesis; both genes are required for fungal pathogenicity. This study indicates the potential of targeting branched-chain amino acid biosynthesis for anti-rice blast management.

Structure and mechanism of inhibition of plant acetohydroxyacid synthase

Plants and microorganisms synthesize valine, leucine and isoleucine via a common pathway in which the first reaction is catalysed by acetohydroxyacid synthase (AHAS, EC 2.2.1.6). This enzyme is of substantial importance because it is the target of several herbicides, including all members of the popular sulfonylurea and imidazolinone families. However, the emergence of resistant weeds due to mutations that interfere with the inhibition of AHAS is now a worldwide problem. Here we summarize recent ideas on the way in which these herbicides inhibit the enzyme, based on the 3D structure of Arabidopsis thaliana AHAS. This structure also reveals important clues for understanding how various mutations can lead to herbicide resistance.

Acetohydroxyacid synthase and its role in the biosynthetic pathway for branched-chain amino acids

The branched-chain amino acids are synthesized by plants, fungi and microorganisms, but not by animals. Therefore, the enzymes of this pathway are potential target sites for the development of antifungal agents, antimicrobials and herbicides. Most research has focused upon the first enzyme in this biosynthetic pathway, acetohydroxyacid synthase (AHAS) largely because it is the target site for many commercial herbicides. In this review we provide a brief overview of the important properties of each enzyme within the pathway and a detailed summary of the most recent AHAS research, against the perspective of work that has been carried out over the past 50 years.