The Nuclear Hexokinase 2 Acts as a Glucose Sensor in Saccharomyces cerevisiae

Proteomic consequences of TDA1 deficiency in Saccharomyces cerevisiae: Protein kinase Tda1 is essential for Hxk1 and Hxk2 serine 15 phosphorylation

Hexokinase 2 (Hxk2) of Saccharomyces cerevisiae is a dual function hexokinase, acting as a glycolytic enzyme and being involved in the transcriptional regulation of glucose-repressible genes. Relief from glucose repression is accompanied by phosphorylation of Hxk2 at serine 15, which has been attributed to the protein kinase Tda1. To explore the role of Tda1 beyond Hxk2 phosphorylation, the proteomic consequences of TDA1 deficiency were investigated by difference gel electrophoresis (2D-DIGE) comparing a wild type and a Δtda1 deletion mutant. To additionally address possible consequences of glucose repression/derepression, both were grown at 2% and 0.1% (w/v) glucose. A total of eight protein spots exhibiting a minimum twofold enhanced or reduced fluorescence upon TDA1 deficiency was detected and identified by mass spectrometry. Among the spot identities are-besides the expected Hxk2-two proteoforms of hexokinase 1 (Hxk1). Targeted proteomics analyses in conjunction with 2D-DIGE demonstrated that TDA1 is indispensable for Hxk2 and Hxk1 phosphorylation at serine 15. Thirty-six glucose-concentration-dependent protein spots were identified. A simple method to improve spot quantification, approximating spots as rotationally symmetric solids, is presented along with new data on the quantities of Hxk1 and Hxk2 and their serine 15 phosphorylated forms at high and low glucose growth conditions. The Δtda1 deletion mutant exhibited no altered growth under high or low glucose conditions or on alternative carbon sources. Also, invertase activity, serving as a reporter for glucose derepression, was not significantly altered. Instead, an involvement of Tda1 in oxidative stress response is suggested.

d-Xylose consumption by nonrecombinant Saccharomyces cerevisiae: A review

Xylose is the second most abundant sugar in nature. Its efficient fermentation has been considered as a critical factor for a feasible conversion of renewable biomass resources into biofuels and other chemicals. The yeast Saccharomyces cerevisiae is of exceptional industrial importance due to its excellent capability to ferment sugars. However, although S. cerevisiae is able to ferment xylulose, it is considered unable to metabolize xylose, and thus, a lot of research has been directed to engineer this yeast with heterologous genes to allow xylose consumption and fermentation. The analysis of the natural genetic diversity of this yeast has also revealed some nonrecombinant S. cerevisiae strains that consume or even grow (modestly) on xylose. The genome of this yeast has all the genes required for xylose transport and metabolism through the xylose reductase, xylitol dehydrogenase, and xylulokinase pathway, but there seems to be problems in their kinetic properties and/or required expression. Self-cloning industrial S. cerevisiae strains overexpressing some of the endogenous genes have shown interesting results, and new strategies and approaches designed to improve these S. cerevisiae strains for ethanol production from xylose will also be presented in this review.

LncRNA loc285194 inhibits tumor growth of laryngeal squamous cell carcinoma cells by downregulating hexokinase 2

LncRNA loc285194 is a well-characterized tumor suppressor in several types of cancer. To the best of our knowledge, the present study is the first to investigate the involvement of lncRNA loc285194 in laryngeal squamous cell carcinoma (LSCC). The expression of loc285194 and hexokinase 2 (HK-2) mRNA in laryngeal biopsies of patients with LSCC or healthy controls was detected by reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The diagnostic value of loc285194 and HK-2 mRNA for LSCC was evaluated using receiver operating characteristic curve analysis. The correlation between expression of loc285194 and HK-2 mRNA was analyzed using the Pearson correlation coefficient. The association between loc285194 and clinicopathological data of patients with LSCC was analyzed using the Chi-square test. LncRNA loc285194 and HK-2 expression vectors were transfected into human LSCC cell lines and the effects on HK-2 expression, lncRNA loc285194 expression and cell proliferation was detected by western blot, RT-qPCR and CCK-8 assays, respectively. It was observed that loc285194 was upregulated, while HK-2 mRNA was downregulated in patients with LSCC compared with healthy controls. The results demonstrated that the downregulation of loc285194 may be a sensitive diagnostic marker for LSCC. The expression levels of loc285194 and HK-2 mRNA were inversely correlated in patients with LSCC, but not in healthy controls. loc285194 expression was significantly associated with tumor size, but not distant tumor metastasis. Loc285194 overexpression significantly inhibited HK-2 expression and LSCC cell proliferation. HK-2 overexpression did not significantly affect Loc285194 expression, but promoted LSCC proliferation and attenuated the inhibitory effects of loc285194 overexpression on LSCC cell proliferation. lncRNA loc285194 may inhibit tumor growth of LSCC cells by downregulating HK-2.

Pentose Metabolism in Saccharomyces cerevisiae: The Need to Engineer Global Regulatory Systems

Extending the host substrate range of industrially relevant microbes, such as Saccharomyces cerevisiae, has been a highly-active area of research since the conception of metabolic engineering. Yet, rational strategies that enable non-native substrate utilization in this yeast without the need for combinatorial and/or evolutionary techniques are underdeveloped. Herein, this review focuses on pentose metabolism in S. cerevisiae as a case study to highlight the challenges in this field. In the last three decades, work has focused on expressing exogenous pentose metabolizing enzymes as well as endogenous enzymes for effective pentose assimilation, growth, and biofuel production. The engineering strategies that are employed for pentose assimilation in this yeast are reviewed, and compared with metabolism and regulation of native sugar, galactose. In the case of galactose metabolism, multiple signals regulate and aid growth in the presence of the sugar. However, for pentoses that are non-native, it is unclear if similar growth and regulatory signals are activated. Such a comparative analysis aids in identifying missing links in xylose and arabinose utilization. While research on pentose metabolism have mostly concentrated on pathway level optimization, recent transcriptomics analyses highlight the need to consider more global regulatory, structural, and signaling components.

Single-cell study links metabolism with nutrient signaling and reveals sources of variability

BACKGROUND

The yeast AMPK/SNF1 pathway is best known for its role in glucose de/repression. When glucose becomes limited, the Snf1 kinase is activated and phosphorylates the transcriptional repressor Mig1, which is then exported from the nucleus. The exact mechanism how the Snf1-Mig1 pathway is regulated is not entirely elucidated.

RESULTS

Glucose uptake through the low affinity transporter Hxt1 results in nuclear accumulation of Mig1 in response to all glucose concentrations upshift, however with increasing glucose concentration the nuclear localization of Mig1 is more intense. Strains expressing Hxt7 display a constant response to all glucose concentration upshifts. We show that differences in amount of hexose transporter molecules in the cell could cause cell-to-cell variability in the Mig1-Snf1 system. We further apply mathematical modelling to our data, both general deterministic and a nonlinear mixed effect model. Our model suggests a presently unrecognized regulatory step of the Snf1-Mig1 pathway at the level of Mig1 dephosphorylation. Model predictions point to parameters involved in the transport of Mig1 in and out of the nucleus as a majorsource of cell to cell variability.

CONCLUSIONS

With this modelling approach we have been able to suggest steps that contribute to the cell-to-cell variability. Our data indicate a close link between the glucose uptake rate, which determines the glycolytic rate, and the activity of the Snf1/Mig1 system. This study hence establishes a close relation between metabolism and signalling.