Casein kinases are required for the stability of the glucose-sensing receptor Rgt2 in yeast

The advances in creating Crabtree-negative Saccharomyces cerevisiae and the application for chemicals biosynthesis

Saccharomyces cerevisiae is a promising microbial cell factory. However, the overflow metabolism, known as the Crabtree effect, directs the majority of the carbon source toward ethanol production, in many cases, resulting in low yields of other target chemicals and byproducts accumulation. To construct Crabtree-negative S. cerevisiae, the deletion of pyruvate decarboxylases and/or ethanol dehydrogenases is required. However, these modifications compromises the growth of the strains on glucose. This review discusses the metabolic engineering approaches used to eliminate ethanol production, the efforts to alleviate growth defect of Crabtree-negative strains, and the underlying mechanisms of the growth rescue. In addition, it summarizes the applications of Crabtree-negative S. cerevisiae in the synthesis of various chemicals such as lactic acid, 2,3-butanediol, malic acid, succinic acid, isobutanol, and others.

The familial amyotrophic lateral sclerosis-associated A4V SOD1 mutant is not able to regulate aerobic glycolysis

Under certain stress conditions, astrocytes operate in aerobic glycolysis, a process controlled by pyruvate dehydrogenase (PDH) inhibition through its E1 α subunit (Pda1) phosphorylation. This supplies lactate to neurons, which save glucose to obtain NADPH to, among other roles, counteract reactive oxygen species. A failure in this metabolic cooperation causes severe damage to neurons. In this work, using humanized Saccharomyces cerevisiae cells in which its endogenous Cu/Zn Superoxide Dismutase (SOD1) was replaced by human ortholog, we investigated the role of human SOD1 (hSOD1) in aerobic glycolysis regulation and its implications to amyotrophic lateral sclerosis (ALS), a neurodegenerative disease. Yeast cells ferment glucose even in the presence of oxygen and switch to respiratory metabolism after glucose exhaustion. However, like cells of SOD1-knockout strain, cells expressing A4V mutant of hSOD1 growing on glucose showed a respiratory phenotype, i.e., low glucose and high oxygen consumptions and low intracellular oxidation levels in response to peroxide stress, contrary to cells expressing wild-type (WT) SOD1 (yeast or human). The A4V mutation in hSOD1 is linked to ALS. In contrast to WT SOD1 strains, PDH activity of both sod1Δ and A4V hSOD1 cells did not change in response to a metabolic shift toward oxidative metabolism, which was associated to lower Pda1 phosphorylation levels under growth on glucose. Taken together, our results suggest that A4V mutant cannot regulate aerobic glycolysis via Pda1 phosphorylation the same way WT hSOD1, which might be linked to problems observed in the motor neurons of ALS patients with the SOD1 A4V mutation.

Multiple roles for the cytoplasmic C-terminal domains of the yeast cell surface receptors Rgt2 and Snf3 in glucose sensing and signaling

The plasma membrane proteins Rgt2 and Snf3 are glucose sensing receptors (GSRs) that generate an intracellular signal for the induction of gene expression in response to high and low extracellular glucose concentrations, respectively. The GSRs consist of a 12-transmembrane glucose recognition domain and a cytoplasmic C-terminal signaling tail. The GSR tails are dissimilar in length and sequence, but their distinct roles in glucose signal transduction are poorly understood. Here, we show that swapping the tails between Rgt2 and Snf3 does not alter the signaling activity of the GSRs, so long as their tails are phosphorylated in a Yck-dependent manner. Attachment of the GSR tails to Hxt1 converts the transporter into a glucose receptor; however, the tails attached to Hxt1 are not phosphorylated by the Ycks, resulting in only partial signaling. Moreover, in response to non-fermentable carbon substrates, Rgt2 and Hxt1-RT (RT, Rgt2-tail) are efficiently endocytosed, whereas Snf3 and Hxt1-ST (ST, Snf3-tail) are endocytosis-impaired. Thus, the tails are important regulatory domains required for the endocytosis of the Rgt2 and Snf3 glucose sensing receptors triggered by different cellular stimuli. Taken together, these results suggest multiple roles for the tail domains in GSR-mediated glucose sensing and signaling.

Casein kinase 1 controls components of a TORC2 signaling network in budding yeast

Tor kinases play diverse and essential roles in control of nutrient signaling and cell growth. Tor kinases are assembled into two large multiprotein complexes referred to as Tor Complex 1 and Tor Complex 2 (TORC1 and TORC2). In budding yeast, TORC2 controls a signaling network that relays signals regarding carbon source that strongly influence growth rate and cell size. However, the mechanisms that control TORC2 signaling are poorly understood. Activation of TORC2 requires Mss4, a phosphoinositol kinase that initiates assembly of a multi-protein complex at the plasma membrane that recruits and activates downstream targets of TORC2. Localization of Mss4 to the plasma membrane is controlled by phosphorylation and previous work suggested that yeast homologs of casein kinase 1γ, referred to as Yck1 and Yck2, control phosphorylation of Mss4. Here, we generated a new analog-sensitive allele of YCK2 and used it to test whether Yck1/2 influence signaling in the TORC2 network. We found that multiple components of the TORC2 network are strongly influenced by Yck1/2 signaling.

The role of salt bridge networks in the stability of the yeast hexose transporter 1

BACKGROUND

The yeast S. cerevisiae preferably metabolizes glucose through aerobic glycolysis. Glucose transport is facilitated by multiple hexose transporters (Hxts), and their expression and activity are tightly regulated by multiple mechanisms. However, detailed structural and functional analyses of Hxts remain limited, largely due to the lack of crystal structure.

METHODS

Homology modeling was used to build a 3D structural model for the yeast glucose transporter Hxt1 and investigate the effects of site directed mutations on Hxt1 stability and glucose transport activity.

RESULTS

The conserved salt bridge-forming residues observed in the human Glut4 and the yeast glucose receptor Rgt2 were identified within and between the two 6-transmembrane spanning segments of Hxt1. Most of the RGT2 mutations that disrupt the salt bridge networks were known to cause constitutive signal generation, whereas the corresponding substitutions in HXT1 were shown to decrease Hxt1 stability. While substitutions of the two residues in the salt bridge 2 in Glut4-E329Q and E393D-were reported to abolish glucose transport, the equivalent substitutions in Hxt1 (D382Q and E454D) did not affect Hxt1 glucose transport activity.

CONCLUSIONS

Substitutions of equivalent salt bridge-forming residues in Hxt1, Rgt2, and Glut4 are predicted to lock them in an inward-facing conformation but lead to different functional consequences.

GENERAL SIGNIFICANCE

The salt bridge networks in yeast and human glucose transporters and yeast glucose receptors may play different roles in maintaining their structural and functional integrity.