Sphingolipid depletion suppresses UPR activation and promotes galactose hypersensitivity in yeast models of classic galactosemia

Classic galactosemia is an inborn error of metabolism caused by deleterious mutations on the GALT gene, which encodes the Leloir pathway enzyme galactose-1-phosphate uridyltransferase. Previous studies have shown that the endoplasmic reticulum unfolded protein response (UPR) is relevant to galactosemia, but the molecular mechanism behind the endoplasmic reticulum stress that triggers this response remains elusive. In the present work, we show that the activation of the UPR in yeast models of galactosemia does not depend on the binding of unfolded proteins to the ER stress sensor protein Ire1p since the protein domain responsible for unfolded protein binding to Ire1p is not necessary for UPR activation. Interestingly, myriocin - an inhibitor of the de novo sphingolipid synthesis pathway - inhibits UPR activation and causes galactose hypersensitivity in these models, indicating that myriocin-mediated sphingolipid depletion impairs yeast adaptation to galactose toxicity. Supporting the interpretation that the effects observed after myriocin treatment were due to a reduction in sphingolipid levels, the addition of phytosphingosine to the culture medium reverses all myriocin effects tested. Surprisingly, constitutively active UPR signaling did not prevent myriocin-induced galactose hypersensitivity suggesting multiple roles for sphingolipids in the adaptation of yeast cells to galactose toxicity. Therefore, we conclude that sphingolipid homeostasis has an important role in UPR activation and cellular adaptation in yeast models of galactosemia, highlighting the possible role of lipid metabolism in the pathophysiology of this disease.

Regulation of sphingolipid synthesis by the G1/S transcription factor Swi4

SBF (Swi4/Swi6 Binding Factor) complex is a crucial regulator of G1/S transition in Saccharomyces cerevisiae. Here, we show that SBF complex is required for myriocin resistance, an inhibitor of sphingolipid synthesis. This phenotype was not shared with MBF complex mutants nor with deletion of the Swi4p downstream targets, CLN1/CLN2. Based on data mining results, we selected putative Swi4p targets related to sphingolipid metabolism and studied their gene transcription as well as metabolite levels during progression of the cell cycle. Genes which encode key enzymes for the synthesis of long chain bases (LCBs) and ceramides were periodically transcribed during the mitotic cell cycle, having a peak at G1/S, and required SWI4 for full transcription at this stage. In addition, HPLC-MS/MS data indicated that swi4Δ cells have decreased levels of sphingolipids during progression of the cell cycle, particularly, dihydrosphingosine (DHS), C24-phytoceramides and C24-inositolphosphoryl ceramide (IPC) while it had increased levels of mannosylinositol phosphorylceramide (MIPC). Furthermore, we demonstrated that both inhibition of de novo sphingolipid synthesis by myriocin or SWI4 deletion caused partial arrest at the G2/M phase. Importantly, our lipidomic data demonstrated that the sphingolipid profile of WT cells treated with myriocin resembled that of swi4Δ cells, with lower levels of DHS, IPC and higher levels of MIPC. Taken together, these results show that SBF complex plays an essential role in the regulation of sphingolipid homeostasis, which reflects in the correct progression through the G2/M phase of the cell cycle.

Rqc1 and other yeast proteins containing highly positively charged sequences are not targets of the RQC complex

Previous work has suggested that highly positively charged protein segments coded by rare codons or poly (A) stretches induce ribosome stalling and translational arrest through electrostatic interactions with the negatively charged ribosome exit tunnel, leading to inefficient elongation. This arrest leads to the activation of the Ribosome Quality Control (RQC) pathway and results in low expression of these reporter proteins. However, the only endogenous yeast proteins known to activate the RQC are Rqc1, a protein essential for RQC function, and Sdd1, a protein with unknown function, both of which contain polybasic sequences. To explore the generality of this phenomenon, we investigated whether the RQC complex controls the expression of other proteins with polybasic sequences. We showed by ribosome profiling data analysis and western blot that proteins containing polybasic sequences similar to, or even more positively charged than those of Rqc1 and Sdd1, were not targeted by the RQC complex. We also observed that the previously reported Ltn1-dependent regulation of Rqc1 is posttranslational, independent of the RQC activity. Taken together, our results suggest that RQC should not be regarded as a general regulatory pathway for the expression of highly positively charged proteins in yeast.

The yeast protein Ubx4p contributes to mitochondrial respiration and lithium-galactose-mediated activation of the unfolded protein response

In the presence of galactose, lithium ions activate the unfolded protein response (UPR) by inhibiting phosphoglucomutase activity and causing the accumulation of galactose-related metabolites, including galactose-1-phosphate. These metabolites also accumulate in humans who have the disease classic galactosemia. Here, we demonstrate that Saccharomyces cerevisiae yeast strains harboring a deletion of UBX4, a gene encoding a partner of Cdc48p in the endoplasmic reticulum-associated degradation (ERAD) pathway, exhibit delayed UPR activation after lithium and galactose exposure because the deletion decreases galactose-1-phosphate levels. The delay in UPR activation did not occur in yeast strains in which key ERAD or proteasomal pathway genes had been disrupted, indicating that the ubx4Δ phenotype is ERAD-independent. We also observed that the ubx4Δ strain displays decreased oxygen consumption. The inhibition of mitochondrial respiration was sufficient to diminish galactose-1-phosphate levels and, consequently, affects UPR activation. Finally, we show that the deletion of the AMP-activated protein kinase ortholog-encoding gene SNF1 can restore the oxygen consumption rate in ubx4Δ strain, thereby reestablishing galactose metabolism, UPR activation, and cellular adaption to lithium-galactose challenge. Our results indicate a role for Ubx4p in yeast mitochondrial function and highlight that mitochondrial and endoplasmic reticulum functions are intertwined through galactose metabolism. These findings also shed new light on the mechanisms of lithium action and on the pathophysiology of galactosemia.

Methyl and Ethylmercury elicit oxidative stress and unbalance the antioxidant system in Saccharomyces cerevisiae

Methylmercury (MeHg) and Ethylmercury (EtHg) are toxic to the central nervous system. Human exposure to MeHg and EtHg results mainly from the consumption of contaminated fish and thimerosal-containing vaccines, respectively. The mechanisms underlying the toxicity of MeHg and EtHg are still elusive. Here, we compared the toxic effects of MeHg and EtHg in Saccharomyces cerevisiae (S. cerevisiae) emphasizing the involvement of oxidative stress and the identification of molecular targets from antioxidant pathways. Wild type and mutant strains with deleted genes for antioxidant defenses, namely: γ-glutamylcysteine synthetase, glutathione peroxidase, catalase, superoxide dismutase, mitochondrial peroxiredoxin, cytoplasmic thioredoxin, and redox transcription factor Yap1 were used to identify potential pathways and proteins from cell redox system targeted by MeHg and EtHg. MeHg and EtHg inhibited cell growth, decreased membrane integrity, and increased the granularity and production of reactive species (RS) in wild type yeast. The mutants were predominantly less tolerant of mercurial than wild type yeast. But, as the wild strain, mutants exhibited higher tolerance to MeHg than EtHg. Our results indicate the involvement of oxidative stress in the cytotoxicity of MeHg and EtHg and reinforce S. cerevisiae as a suitable model to explore the mechanisms of action of electrophilic toxicants.

Induction of triacylglycerol synthesis in yeast by cell cycle arrest

In this study, we found that cell cycle arrest induced by alpha-factor mating pheromone (G1), hydroxyurea (S) or nocodazole (G2/M) was associated to increased lipid droplet (LD) content. To identify novel cell cycle genes involved in LD homeostasis, we screened a deletion library for strains with altered LD levels. Among the mutants related to mitotic cell cycle, we found 24 hits that displayed a significantly higher LD content. Ontology mapping showed that neither a biological process nor a specific cell cycle phase was enriched among the hits. We decided to further study the role of SWI4 on LD homeostasis as it is involved in G1/S transition, a stage where lipolysis is active. The high LD content of swi4Δ mutant was not due to inhibition of lipolysis, but due to an increase in triacylglycerol (TAG) synthesis. In addition, deletion of the AMP kinase gene SNF1 or inhibition of TORC1 activity, both known regulators of LD homeostasis, further increased the LD content of a swi4Δ mutant. These findings highlight a role of the cell cycle regulator SWI4 in the coordination of lipid metabolism which is independent of the TORC1 and SNF1/AMPK pathways.

The galactose-induced decrease in phosphate levels leads to toxicity in yeast models of galactosemia

Classic galactosemia is an inborn error of metabolism caused by deleterious mutations in the GALT gene. A number of evidences indicate that the galactose-1-phosphate accumulation observed in patient cells is a cause of toxicity in this disease. Nevertheless, the consequent molecular events caused by the galactose-1-phosphate accumulation remain elusive. Here we show that intracellular inorganic phosphate levels decreased when yeast models of classic galactosemia were exposed to galactose. The decrease in phosphate levels is probably due to the trapping of phosphate in the accumulated galactose-1-phosphate since the deletion of the galactokinase encoding gene GAL1 suppressed this phenotype. Galactose-induced phosphate depletion caused an increase in glycogen content, an expected result since glycogen breakdown by the enzyme glycogen phosphorylase is dependent on inorganic phosphate. Accordingly, an increase in intracellular phosphate levels suppressed the galactose effect on glycogen content and conferred galactose tolerance to yeast models of galactosemia. These results support the hypothesis that the galactose-induced decrease in phosphate levels leads to toxicity in galactosemia and opens new possibilities for the development of better treatments for this disease.

A Chemogenomic Screen Reveals Novel Snf1p/AMPK Independent Regulators of Acetyl-CoA Carboxylase

Acetyl-CoA carboxylase (Acc1p) is a key enzyme in fatty acid biosynthesis and is essential for cell viability. To discover new regulators of its activity, we screened a Saccharomyces cerevisiae deletion library for increased sensitivity to soraphen A, a potent Acc1p inhibitor. The hits identified in the screen (118 hits) were filtered using a chemical-phenotype map to exclude those associated with pleiotropic drug resistance. This enabled the identification of 82 ORFs that are genetic interactors of Acc1p. The main functional clusters represented by these hits were “transcriptional regulation”, “protein post-translational modifications” and “lipid metabolism”. Further investigation of the “transcriptional regulation” cluster revealed that soraphen A sensitivity is poorly correlated with ACC1 transcript levels. We also studied the three top unknown ORFs that affected soraphen A sensitivity: SOR1 (YDL129W), SOR2 (YIL092W) and SOR3 (YJR039W). Since the C18/C16 ratio of lipid acyl lengths reflects Acc1p activity levels, we evaluated this ratio in the three mutants. Deletion of SOR2 and SOR3 led to reduced acyl lengths, suggesting that Acc1p is indeed down-regulated in these strains. Also, these mutants showed no differences in Snf1p/AMPK activation status and deletion of SNF1 in these backgrounds did not revert soraphen A sensitivity completely. Furthermore, plasmid maintenance was reduced in sor2Δ strain and this trait was shared with 18 other soraphen A sensitive hits. In summary, our screen uncovered novel Acc1p Snf1p/AMPK-independent regulators.

The ACBP gene family in Rhodnius prolixus: Expression, characterization and function of RpACBP-1

The acyl-CoA-binding proteins (ACBP) constitute a family of conserved proteins that bind acyl-CoA with high affinity and protect it from hydrolysis. Thus, ACBPs may have essential roles in basal cellular lipid metabolism. The genome of the insect Rhodnius prolixus encodes five ACBP genes similar to those described for other insect species. The qPCR analysis revealed that these genes have characteristic expression profiles in insect organs, suggesting that they have specific roles in insect physiology. Recombinant RpACBP-1 was able to bind acyl-CoA in an in vitro gel-shift assay. Moreover, heterologous RpACBP-1 expression in acb1Δ mutant yeast rescued the multi-lobed vacuole phenotype, indicating that RpACBP-1 acts as a bona fide acyl-CoA-binding protein. RpACBP-1 knockdown using RNAi caused triacylglycerol accumulation in the insect posterior midgut and a reduction in the number of deposited eggs. The amount of stored triacylglycerol was reduced in flight muscle, and the incorporation of fatty acids in cholesteryl esters was increased in the fat body. These results showed that RpACBP-1 participates in several lipid metabolism steps in R. prolixus.

The unfolded protein response has a protective role in yeast models of classic galactosemia

Classic galactosemia is a human autosomal recessive disorder caused by mutations in the GALT gene (GAL7 in yeast), which encodes the enzyme galactose-1-phosphate uridyltransferase. Here we show that the unfolded protein response pathway is triggered by galactose in two yeast models of galactosemia: lithium-treated cells and the gal7Δ mutant. The synthesis of galactose-1-phosphate is essential to trigger the unfolded protein response under these conditions because the deletion of the galactokinase-encoding gene GAL1 completely abolishes unfolded protein response activation and galactose toxicity. Impairment of the unfolded protein response in both yeast models makes cells even more sensitive to galactose, unmasking its cytotoxic effect. These results indicate that endoplasmic reticulum stress is induced under galactosemic conditions and underscores the importance of the unfolded protein response pathway to cellular adaptation in these models of classic galactosemia.

Expression of the glucose transporter HXT1 involves the Ser-Thr protein phosphatase Sit4 in Saccharomyces cerevisiae

We studied the effect of the loss of the Ser-Thr protein phosphatase Sit4, an important post-translational regulator, on the steady-state levels of the low-affinity glucose transporter Hxt1p and observed a delay in its appearance after high glucose induction, slow growth, and diminished glucose consumption. By analyzing the known essential pathway necessary to induce Hxt1p, we observed a partial inhibition of casein kinase I activity. In both WT and sit4Δ strains, the transcript was induced with no significant difference at 15 min of glucose induction; however, after 45 min, a clear difference in the level of expression was observed being 45% higher in WT than in sit4Δ strain. As at early time of induction, the HXT1 transcript was present but not the protein in the sit4Δ strain we analyzed association of HXT1 with ribosomes, which revealed a significant difference in the association profile; in the mutant strain, the HXT1 transcript associated with a larger set of ribosomal fractions than it did in the WT strain, suggesting also a partial defect in protein synthesis. Overexpression of the translation initiation factor TIF2/eIF4A led to an increase in Hxt1p abundance in the WT strain only. It was concluded that Sit4p ensures that HXT1 transcript is efficiently transcribed and translated thus increasing protein levels of Hxt1p when high glucose levels are present.

High hydrostatic pressure activates gene expression that leads to ethanol production enhancement in a Saccharomyces cerevisiae distillery strain

High hydrostatic pressure (HHP) is a stress that exerts broad effects on microorganisms with characteristics similar to those of common environmental stresses. In this study, we aimed to identify genetic mechanisms that can enhance alcoholic fermentation of wild Saccharomyces cerevisiae isolated from Brazilian spirit fermentation vats. Accordingly, we performed a time course microarray analysis on a S. cerevisiae strain submitted to mild sublethal pressure treatment of 50 MPa for 30 min at room temperature, followed by incubation for 5, 10 and 15 min without pressure treatment. The obtained transcriptional profiles demonstrate the importance of post-pressurisation period on the activation of several genes related to cell recovery and stress tolerance. Based on these results, we over-expressed genes strongly induced by HHP in the same wild yeast strain and identified genes, particularly SYM1, whose over-expression results in enhanced ethanol production and stress tolerance upon fermentation. The present study validates the use of HHP as a biotechnological tool for the fermentative industries.

Heme-induced ROS in Trypanosoma cruzi activates CaMKII-like that triggers epimastigote proliferation. One helpful effect of ROS

Heme is a ubiquitous molecule that has a number of physiological roles. The toxic effects of this molecule have been demonstrated in various models, based on both its pro-oxidant nature and through a detergent mechanism. It is estimated that about 10 mM of heme is released during blood digestion in the blood-sucking bug's midgut. The parasite Trypanosoma cruzi, the agent of Chagas' disease, proliferates in the midgut of the insect vector; however, heme metabolism in trypanosomatids remains to be elucidated. Here we provide a mechanistic explanation for the proliferative effects of heme on trypanosomatids. Heme, but not other porphyrins, induced T. cruzi proliferation, and this phenomenon was accompanied by a marked increase in reactive oxygen species (ROS) formation in epimastigotes when monitored by ROS-sensitive fluorescent probes. Heme-induced ROS production was time-and concentration-dependent. In addition, lipid peroxidation and the formation of 4-hydroxy-2-nonenal (4-HNE) adducts with parasite proteins were increased in epimastigotes in the presence of heme. Conversely, the antioxidants urate and GSH reversed the heme-induced ROS. Urate also decreased parasite proliferation. Among several protein kinase inhibitors tested only specific inhibitors of CaMKII, KN93 and Myr-AIP, were able to abolish heme-induced ROS formation in epimastigotes leading to parasite growth impairment. Taken together, these data provide new insight into T. cruzi- insect vector interactions: heme, a molecule from the blood digestion, triggers epimastigote proliferation through a redox-sensitive signalling mechanism.

A new fluorescence-based method identifies protein phosphatases regulating lipid droplet metabolism

In virtually every cell, neutral lipids are stored in cytoplasmic structures called lipid droplets (LDs) and also referred to as lipid bodies or lipid particles. We developed a rapid high-throughput assay based on the recovery of quenched BODIPY-fluorescence that allows to quantify lipid droplets. The method was validated by monitoring lipid droplet turnover during growth of a yeast culture and by screening a group of strains deleted in genes known to be involved in lipid metabolism. In both tests, the fluorimetric assay showed high sensitivity and good agreement with previously reported data using microscopy. We used this method for high-throughput identification of protein phosphatases involved in lipid droplet metabolism. From 65 yeast knockout strains encoding protein phosphatases and its regulatory subunits, 13 strains revealed to have abnormal levels of lipid droplets, 10 of them having high lipid droplet content. Strains deleted for type I protein phosphatases and related regulators (ppz2, gac1, bni4), type 2A phosphatase and its related regulator (pph21 and sap185), type 2C protein phosphatases (ptc1, ptc4, ptc7) and dual phosphatases (pps1, msg5) were catalogued as high-lipid droplet content strains. Only reg1, a targeting subunit of the type 1 phosphatase Glc7p, and members of the nutrient-sensitive TOR pathway (sit4 and the regulatory subunit sap190) were catalogued as low-lipid droplet content strains, which were studied further. We show that Snf1, the homologue of the mammalian AMP-activated kinase, is constitutively phosphorylated (hyperactive) in sit4 and sap190 strains leading to a reduction of acetyl-CoA carboxylase activity. In conclusion, our fast and highly sensitive method permitted us to catalogue protein phosphatases involved in the regulation of LD metabolism and present evidence indicating that the TOR pathway and the SNF1/AMPK pathway are connected through the Sit4p-Sap190p pair in the control of lipid droplet biogenesis.

The serine/threonine protein phosphatase Sit4p activates multidrug resistance in Saccharomyces cerevisiae

Multidrug resistance in Saccharomyces cerevisiae is frequently associated with gain-of-function mutations in zinc finger-containing transcription factors Pdr1p and Pdr3p. These regulatory proteins activate the expression of several ATP-binding cassette transporter genes, leading to elevated drug resistance. Here, we report that loss of the type 2A-related serine/threonine protein phosphatase Sit4p renders yeast cells sensitive to cycloheximide, azoles, daunorubicin and rhodamine 6G. This effect is a consequence of the decreased transcriptional levels of mainly PDR3 and its target genes, PDR5, SNQ2 and YOR1, which encode multidrug efflux pumps. The multidrug sensitivity of sit4 mutant cells is suppressed by the PDR1-3 mutant allele, which encodes a hyperactive form of Pdr1p. Sit4p is known to associate with regulatory proteins Sap155p, Sap4p, Sap185p and Sap190p. We found that the sap155 mutant strain is sensitive to azoles, but not to cycloheximide, while the sap155sap4 and sap185sap190 mutant strains are sensitive to both drugs. This finding indicates that the Sit4p-Sap protein complex subtly modulates the expression of drug efflux pumps. Drug resistance conferred by the expression of the Candida albicans CDR1 gene, an ortholog of PDR5 in S. cerevisiae, is also positively modulated by Sit4p. These data uncover a new regulatory pathway that connects multidrug resistance to Sit4p function.

A fluorescent mutant of the NM domain of the yeast prion Sup35 provides insight into fibril formation and stability

The Sup35 protein of Saccharomyces cerevisiae forms a prion that generates the [PSI(+)] phenotype. Its NM region governs prion status, forming self-seeding amyloid fibers in vivo and in vitro. A tryptophan mutant of Sup35 (NM(F117W)) was used to probe its aggregation. Four indicators of aggregation, Trp 117 maximum emission, Trp polarization, thio-T binding, and light scattering increase, revealed faster aggregation at 4 degrees C than at 25 degrees C, and all indicators changed in a concerted fashion at the former temperature. Curiously, at 25 degrees C the changes were not synchronized; the first two indicators, which reflect nucleation, changed more quickly than the last two, which reflect fibril formation. These results suggest that nucleation is insensitive to temperature, whereas fibril extension is temperature dependent. As expected, aggregation is accelerated when a small fraction (5%) of the nuclei produced at 4 or 25 degrees C are added to a suspension containing the soluble NM domain, although these nuclei do not seem to propagate any structural information to the growing fibrils. Fibrils grown at 4 degrees C were less stable in GdmCl than those grown at higher temperature. However, they were both resistant to high pressure; in fact, both sets of fibrils responded to high pressure by adopting an altered conformation with a higher capacity for thio-T binding. From these data, we calculated the change in volume and free energy associated with this conformational change. AFM revealed that the fibrils grown at 4 degrees C were statistically smaller than those grown at 25 degrees C. In conclusion, the introduction of Trp 117 allowed us to more carefully dissect the effects of temperature on the aggregation of the Sup35 NM domain.

Overexpression of the aldose reductase GRE3 suppresses lithium-induced galactose toxicity in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, lithium induces a 'galactosemia-like' phenotype as a consequence of inhibition of phosphoglucomutase, a key enzyme in galactose metabolism. Induced galactose toxicity is prevented by deletion of GAL4, which inhibits the transcriptional activation of genes involved in galactose metabolism and by deletion of the galactokinase (GAL1), indicating that galactose-1-phosphate, a phosphorylated intermediate of the Leloir pathway, is the toxic compound. As an alternative to inhibiting entry and metabolism of galactose, we investigated whether deviation of galactose metabolism from the Leloir pathway would also overcome the galactosemic effect of lithium. We show that cells overexpressing the aldose reductase GRE3, which converts galactose to galactitol, are more tolerant to lithium than wild-type cells when grown in galactose medium and they accumulate more galactitol and less galactose-1-phosphate. Overexpression of GRE3 also suppressed the galactose growth defect of the 'galactosemic'gal7- and gal10-deleted strains, which lack galactose-1-P-uridyltransferase or UDP-galactose-4-epimerase activities, respectively. Furthermore, the effect of GRE3 was independent of the inositol monophosphatases INM1 and INM2. We propose that lithium induces a galactosemic state in yeast and that inhibition of the Leloir pathway before the phosphorylation step or stimulation of galactitol production suppresses lithium-induced galactose toxicity.

Ubiquitin Ligase Activity of TFIIH and the Transcriptional Response to DNA Damage

Core transcription factor (TF) IIH purified from yeast possesses an E3 ubiquitin (Ub) ligase activity, which resides, at least in part, in a RING finger (RNF) domain of the Ssl1 subunit. Yeast strains mutated in the Ssl1 RNF domain are sensitive to ultraviolet (UV) light and to methyl methanesulfonate (MMS). This increased sensitivity to DNA-damaging agents does not reflect a deficiency in nucleotide excision repair. Rather, it correlates with reduced transcriptional induction of genes involved in DNA repair, suggesting that the E3 Ub ligase activity of TFIIH mediates the transcriptional response to DNA damage.

The initiation factor eIF4A is involved in the response to lithium stress in Saccharomyces cerevisiae

A gene, TIF2, was identified as corresponding to the translation initiation factor eIF4A and when overexpressed it confers lithium tolerance in galactose medium to Saccharomyces cerevisiae. Incubation of yeast with 6 mm LiCl in galactose medium leads to inhibition of [(35)S]methionine incorporation. By polysome analysis we show that translation is inhibited by lithium at the initiation step, accumulating 80 S monosomes. We further show by immunoblot analysis that when cells are incubated with lithium eIF4A does not sediment with ribosomal subunits. Overexpression of TIF2 overcomes inhibition of protein synthesis and restores its sedimentation with the initiation complex. In vivo, eIF4A is induced by lithium stress. We have shown previously that lithium is highly toxic to yeast when grown in galactose medium mainly due to inhibition of phosphoglucomutase, an enzyme responsible for the entry of galactose into glycolysis. We show that conditions that revert inhibition of phosphoglucomutase also revert inhibition of protein synthesis. Interestingly, glucose starvation leads to loss of polysomes but not to dissociation of eIF4A from the preinitiation complexes. Overexpression of SIT4, a protein phosphatase related to the TOR kinase pathway, reverts inhibition of protein synthesis by lithium and association of eIF4A with the initiation complex.

Phosphoglucomutase is an in vivo lithium target in yeast

Lithium is a drug frequently used in the treatment of manic depressive disorder. We have observed that the yeast Saccharomyces cerevisiae is very sensitive to lithium when growing in galactose medium. In this work we show that lithium inhibits with high affinity yeast (IC50 approximately 0.2 mm) and human (IC50 approximately 1.5 mm) phosphoglucomutase, the enzyme that catalyzes the reversible conversion of glucose 1-phosphate to glucose 6-phosphate. Lithium inhibits the rate of fermentation when yeast are grown in galactose and induces accumulation of glucose 1-phosphate and galactose 1-phosphate. Accumulation of these metabolites was also observed when a strain deleted of the two isoforms of phosphoglucomutase was incubated in galactose medium. In glucose-grown cells lithium reduces the steady state levels of UDP-glucose, resulting in a defect on trehalose and glycogen biosynthesis. Lithium acts as a competitive inhibitor of yeast phosphoglucomutase activity by competing with magnesium, a cofactor of the enzyme. High magnesium concentrations revert lithium inhibition of growth and phosphoglucomutase activity. Lithium stress causes an increase of the phosphoglucomutase activity due to an induction of transcription of the PGM2 gene, and its overexpression confers lithium tolerance in galactose medium. These results show that phosphoglucomutase is an important in vivo lithium target.

Regulation of monovalent ion homeostasis and pH by the Ser-Thr protein phosphatase SIT4 in Saccharomyces cerevisiae

A gene, SIT4, was identified as corresponding to a serine/threonine protein phosphatase and when overexpressed confers lithium tolerance in galactose medium to the budding yeast Saccharomyces cerevisiae. This gene has been previously identified as a regulator of the cell cycle and involved in nitrogen sensing. It is shown that the transcription levels of SIT4 are induced by low concentrations of Li(+) in a time-dependent manner. Na(+) and K(+) at high concentrations, but not sorbitol, also induce transcription. As a response to Na(+) or Li(+) stress, yeast cells lower the intracellular K(+) content. This effect is enhanced in cells overexpressing SIT4, which also increase (86)Rb efflux after the addition of Na(+) or Li(+) to the extracellular medium. Another feature of SIT4-overexpressing cells is that they maintain a more alkaline pH of 6.64 compared with 6.17 in the wild type cells. It has been proposed that the main pathway of salt tolerance in yeast is mediated by a P-type ATPase, encoded by PMR2A/ENA1. However, our results show that in a sit4 strain, expression of ENA1 is still induced by monovalent cations, and overexpression of SIT4 does not alter the amount of ENA1 transcript. These results show that SIT4 acts in a parallel pathway not involving induction of transcription of ENA1 and suggest a novel function for SIT4 in response to salt stress.

An NH2-terminal deleted plasma membrane H+-ATPase is a dominant negative mutant and is sequestered in endoplasmic reticulum derived structures

The NH2-terminus of the plasma membrane H+-ATPase is one of the least conserved segments of this protein among fungi. We constructed and expressed a mutant H+-ATPase from Saccharomyces cerevisiae deleted at an internal peptide within the cytoplasmic NH2-terminus (D44-F116). When the enzyme was subjected to limited trypsinolysis it was digested more rapidly than wild type H+-ATPase. Membrane fractionation experiments and immunofluorescence microscopy, using antibodies against H+-ATPase showed that the mutant ATPase is retained in the endoplasmic reticulum. The pattern observed in the immunofluorescence microscopy resembled structures similar to Russell bodies (modifications of the endoplasmic reticulum membranes) recently described in yeast. When the wild type H+-ATPase was co-expressed with the mutant, wild type H+-ATPase was also retained in the endoplasmic reticulum. Co-expression of both ATPases in a wild type yeast strain was lethal, demonstrating that this is a dominant negative mutant.