Anti-Pma1p Antibody [40B7] (A85311)

Yeast mutants lacking the intracellular V-ATPase proton pump (vma mutants) have reduced levels of the Pma1p proton pump at the plasma membrane and increased levels in organelles including the vacuolar lumen. We examined the mechanism and physiological consequences of Pma1p mislocalization. Pma1p is ubiquitinated in vma mutants, and ubiquitination depends on the ubiquitin ligase Rsp5p and the arrestin-related adaptor protein Rim8p. vma mutant strains containing rsp5 or rim8 mutations maintain Pma1p at the plasma membrane, suggesting that ubiquitination is required for Pma1p internalization. Acute inhibition of V-ATPase activity with concanamycin A triggers Pma1p ubiquitination and internalization. In an endocytosis-deficient mutant (end4Δ) Pma1p is ubiquitinated but retained at the plasma membrane during concanamycin A treatment. Consistent with specificity in signaling loss of V-ATPase activity to Pma1p, another plasma membrane transporter, Mup1p, is not internalized in a vma mutant, and loss of the Mup1p adaptor Art1p does not prevent Pma1p internalization in a vma mutant. Very poor growth of vma2 rsp5-1 and vma2 rim8Δ double mutants suggests that Pma1p internalization benefits the vma mutants. We hypothesize that loss of V-ATPase-mediated organelle acidification signals ubiquitination, internalization, and degradation of a portion of Pma1p as a means of balancing overall pH homeostasis.

Long-lived proteins have been implicated in age-associated decline in metazoa, but they have only been identified in extracellular matrices or postmitotic cells. However, the aging process also occurs in dividing cells undergoing repeated asymmetric divisions. It was not clear whether long-lived proteins exist in asymmetrically dividing cells or whether they are involved in aging. Here we identify long-lived proteins in dividing cells during aging using the budding yeast, Saccharomyces cerevisiae. Yeast mother cells undergo a limited number of asymmetric divisions that define replicative lifespan. We used stable-isotope pulse-chase and total proteome mass-spectrometry to identify proteins that were both long-lived and retained in aging mother cells after ∼ 18 cells divisions. We identified ∼ 135 proteins that we designate as long-lived asymmetrically retained proteins (LARPS). Surprisingly, the majority of LARPs appeared to be stable fragments of their original full-length protein. However, 15% of LARPs were full-length proteins and we confirmed several candidates to be long-lived and retained in mother cells by time-lapse microscopy. Some LARPs localized to the plasma membrane and remained robustly in the mother cell upon cell division. Other full-length LARPs were assembled into large cytoplasmic structures that had a strong bias to remain in mother cells. We identified age-associated changes to LARPs that include an increase in their levels during aging because of their continued synthesis, which is not balanced by turnover. Additionally, several LARPs were posttranslationally modified during aging. We suggest that LARPs contribute to age-associated phenotypes and likely exist in other organisms.

Replicative aging in yeast is asymmetric-mother cells age but their daughter cells are rejuvenated. Here we identify an asymmetry in pH between mother and daughter cells that underlies aging and rejuvenation. Cytosolic pH increases in aging mother cells, but is more acidic in daughter cells. This is due to the asymmetric distribution of the major regulator of cytosolic pH, the plasma membrane proton ATPase (Pma1). Pma1 accumulates in aging mother cells, but is largely absent from nascent daughter cells. We previously found that acidity of the vacuole declines in aging mother cells and limits lifespan, but that daughter cell vacuoles re-acidify. We find that Pma1 activity antagonizes mother cell vacuole acidity by reducing cytosolic protons. However, the inherent asymmetry of Pma1 increases cytosolic proton availability in daughter cells and facilitates vacuole re-acidification and rejuvenation.

The activation and localization of the Rho-family GTPase Cdc42p at one pole of a cell is necessary for maintaining an axis of polarized growth in many animal and fungal cells. How the asymmetric distribution of this key regulator of polarized morphogenesis is maintained is not fully understood, though divergent models have emerged from a congruence of multiple studies, including one that posits a role for polarized secretion. Here we show with S. cerevisiae that Cdc42p associates with secretory vesicles in vivo.

The regulator of ATPase of vacuoles and endosomes (RAVE) complex is implicated in vacuolar H(+)-translocating ATPase (V-ATPase) assembly and activity. In yeast, rav1 mutants exhibit a Vma(-) growth phenotype characteristic of loss of V-ATPase activity only at high temperature. Synthetic genetic analysis identified mutations that exhibit a full, temperature-independent Vma(-) growth defect when combined with the rav1 mutation. These include class E vps mutations, which compromise endosomal sorting. The synthetic Vma(-) growth defect could not be attributed to loss of vacuolar acidification in the double mutants, as there was no vacuolar acidification in the rav1 mutant. The yeast V-ATPase a subunit is present as two isoforms, Stv1p in Golgi and endosomes and Vph1p in vacuoles. Rav1p interacts directly with the N-terminal domain of Vph1p. STV1 overexpression suppressed the growth defects of both rav1 and rav1vph1, and allowed RAVE-independent assembly of active Stv1p-containing V-ATPases in vacuoles. Mutations causing synthetic genetic defects in combination with rav1 perturbed the normal localization of Stv1-green fluorescent protein. We propose that RAVE is necessary for assembly of Vph1-containing V-ATPase complexes but not Stv1-containing complexes. Synthetic Vma(-) phenotypes arise from defects in Vph1p-containing complexes caused by rav1, combined with defects in Stv1p-containing V-ATPases caused by the second mutation. Thus RAVE is the first isoform-specific V-ATPase assembly factor.

Our studies revealed that lithocholic acid (LCA), a bile acid, is a potent anti-aging natural compound that in yeast cultured under longevity-extending caloric restriction (CR) conditions acts in synergy with CR to enable a significant further increase in chronological lifespan. Here, we investigate a mechanism underlying this robust longevity-extending effect of LCA under CR. We found that exogenously added LCA enters yeast cells, is sorted to mitochondria, resides mainly in the inner mitochondrial membrane, and also associates with the outer mitochondrial membrane. LCA elicits an age-related remodeling of glycerophospholipid synthesis and movement within both mitochondrial membranes, thereby causing substantial changes in mitochondrial membrane lipidome and triggering major changes in mitochondrial size, number and morphology. In synergy, these changes in the membrane lipidome and morphology of mitochondria alter the age-related chronology of mitochondrial respiration, membrane potential, ATP synthesis and reactive oxygen species homeostasis. The LCA-driven alterations in the age-related dynamics of these vital mitochondrial processes extend yeast longevity. In sum, our findings suggest a mechanism underlying the ability of LCA to delay chronological aging in yeast by accumulating in both mitochondrial membranes and altering their glycerophospholipid compositions. We concluded that mitochondrial membrane lipidome plays an essential role in defining yeast longevity.

G-protein-coupled receptor homo-oligomerization has been increasingly reported. However, little is known regarding the relationship between activation of the receptor and its association/conformational states. The mammalian olfactory receptors (ORs) belong to the G protein-coupled receptor superfamily. In this study, the homo-oligomerization status of the human OR1740 receptor and its involvement in receptor activation upon odorant ligand binding were addressed by co-immunoprecipitation and bioluminescence resonance energy transfer approaches using crude membranes or membranes from different cellular compartments. For the first time, our data clearly show that mammalian ORs constitutively self-associate into homodimers at the plasma membrane level. This study also demonstrates that ligand binding mediates a conformational change and promotes an inactive state of the OR dimers at high ligand concentrations. These findings support and validate our previously proposed model of OR activation/inactivation based on the tripartite odorant-binding protein-odorant-OR partnership.

Vacuolar proton-translocating ATPases (V-ATPases) play a central role in organelle acidification in all eukaryotic cells. To address the role of the yeast V-ATPase in vacuolar and cytosolic pH homeostasis, ratiometric pH-sensitive fluorophores specific for the vacuole or cytosol were introduced into wild-type cells and vma mutants, which lack V-ATPase subunits. Transiently glucose-deprived wild-type cells respond to glucose addition with vacuolar acidification and cytosolic alkalinization, and subsequent addition of K(+) ion increases the pH of both the vacuole and cytosol. In contrast, glucose addition results in an increase in vacuolar pH in both vma mutants and wild-type cells treated with the V-ATPase inhibitor concanamycin A. Cytosolic pH homeostasis is also significantly perturbed in the vma mutants. Even at extracellular pH 5, conditions optimal for their growth, cytosolic pH was much lower, and response to glucose was smaller in the mutants. In plasma membrane fractions from the vma mutants, activity of the plasma membrane proton pump, Pma1p, was 65-75% lower than in fractions from wild-type cells. Immunofluorescence microscopy confirmed decreased levels of plasma membrane Pma1p and increased Pma1p at the vacuole and other compartments in the mutants. Pma1p was not mislocalized in concanamycin-treated cells, but a significant reduction in cytosolic pH under all conditions was still observed. We propose that short-term, V-ATPase activity is essential for both vacuolar acidification in response to glucose metabolism and for efficient cytosolic pH homeostasis, and long-term, V-ATPases are important for stable localization of Pma1p at the plasma membrane.

In metazoans, dynein-dependent vesicle transport is mediated by dynactin, containing an actin-related protein, Arp1p, together with a cargo-selection complex containing a second actin-related protein, Arp11. Paradoxically, in budding yeast, models of dynactin function imply an interaction with membranes, whereas the lack of microtubule-based vesicle transport implies the absence of a cargo-selection complex. Using both genetic and biochemical approaches, we demonstrate that Arp10p is the functional yeast homologue of Arp11, suggesting the possible existence of a pointed-end complex in yeast. Specifically, Arp10p interacts with Arp1p and other dynactin subunits and is dependent on Arp1p for stability. Conversely, Arp10p stabilizes the dynactin complex by association with the Arp1p filament pointed end. Using a novel hRAS-Arp1p one-hybrid assay, we show that Arp1p associates with the plasma membrane dependent on dynactin subunits, but independent of dynein, and sensitive to cell wall damage. We directly show the association of Arp1p with not only the plasma membrane but also with a less dense membrane fraction. Based on the hRAS-Arp1p assay, loss of Arp10p enhances the apparent association of dynactin with the plasma membrane and suppresses the loss of signaling conferred by cell wall damage.

BACKGROUND:

Cancer cells, and a variety of normal cells, exhibit aerobic glycolysis, high rates of glucose fermentation in the presence of normal oxygen concentrations, also known as the Warburg effect. This metabolism is considered abnormal because it violates the standard model of cellular energy production that assumes glucose metabolism is predominantly governed by oxygen concentrations and, therefore, fermentative glycolysis is an emergency back-up for periods of hypoxia. Though several hypotheses have been proposed for the origin of aerobic glycolysis, its biological basis in cancer and normal cells is still not well understood.

RESULTS:

We examined changes in glucose metabolism following perturbations in membrane activity in different normal and tumor cell lines and found that inhibition or activation of pumps on the cell membrane led to reduction or increase in glycolysis, respectively, while oxidative phosphorylation remained unchanged. Computational simulations demonstrated that these findings are consistent with a new model of normal physiological cellular metabolism in which efficient mitochondrial oxidative phosphorylation supplies chronic energy demand primarily for macromolecule synthesis and glycolysis is necessary to supply rapid energy demands primarily to support membrane pumps. A specific model prediction was that the spatial distribution of ATP-producing enzymes in the glycolytic pathway must be primarily localized adjacent to the cell membrane, while mitochondria should be predominantly peri-nuclear. The predictions were confirmed experimentally.

CONCLUSIONS:

Our results show that glycolytic metabolism serves a critical physiological function under normoxic conditions by responding to rapid energetic demand, mainly from membrane transport activities, even in the presence of oxygen. This supports a new model for glucose metabolism in which glycolysis and oxidative phosphorylation supply different types of energy demand. Cells use efficient but slow-responding aerobic metabolism to meet baseline, steady energy demand and glycolytic metabolism, which is inefficient but can rapidly increase adenosine triphosphate (ATP) production, to meet short-timescale energy demands, mainly from membrane transport activities. In this model, the origin of the Warburg effect in cancer cells and aerobic glycolysis in general represents a normal physiological function due to enhanced energy demand for membrane transporters activity required for cell division, growth, and migration.

Fermenting cells growing exponentially on rich (YPAD) medium underwent a transition to a slow-growing state as glucose levels declined and their metabolism shifted to respiration. During exponential growth, Fe import and cell-growth rates were matched, affording an approximately invariant cellular Fe concentration. During the transition period, the high-affinity Fe import rate declined slower than the cell-growth rate declined, causing Fe to accumulate, initially as Fe(III) oxyhydroxide nanoparticles but eventually as mitochondrial and vacuolar Fe. Once the cells had reached slow-growth mode, Fe import and cell-growth rates were again matched, and the cellular Fe concentration was again approximately invariant. Fermenting cells grown on minimal medium (MM) grew more slowly during the exponential phase and underwent a transition to a true stationary state as glucose levels declined. The Fe concentration of MM cells that just entered the stationary state was similar to that of YPAD cells, but MM cells continued to accumulate Fe in the stationary state. Fe initially accumulated as nanoparticles and high-spin Fe(II) species, but vacuolar Fe(III) also eventually accumulated. Surprisingly, Fe-packed 5-day-old MM cells suffered no more reactive oxygen species (ROS) damage than younger cells, suggesting that the Fe concentration alone does not accurately predict the extent of ROS damage. The mode and rate of growth at the time of harvesting dramatically affected cellular Fe content. A mathematical model of Fe metabolism in a growing cell was developed. The model included the import of Fe via a regulated high-affinity pathway and an unregulated low-affinity pathway. The import of Fe from the cytosol to vacuoles and mitochondria and nanoparticle formation were also included. The model captured essential trafficking behavior, demonstrating that cells regulate Fe import in accordance with their overall growth rate and that they misregulate Fe import when nanoparticles accumulate. The lack of regulation of Fe in yeast is perhaps unique compared to the tight regulation of other cellular metabolites. This phenomenon likely derives from the unique chemistry associated with Fe nanoparticle formation.

Many antidepressants are cationic amphipaths, which spontaneously accumulate in natural or reconstituted membranes in the absence of their specific protein targets. However, the clinical relevance of cellular membrane accumulation by antidepressants in the human brain is unknown and hotly debated. Here we take a novel, evolutionarily informed approach to studying the effects of the selective-serotonin reuptake inhibitor sertraline/Zoloft® on cell physiology in the model eukaryote Saccharomyces cerevisiae (budding yeast), which lacks a serotonin transporter entirely. We biochemically and pharmacologically characterized cellular uptake and subcellular distribution of radiolabeled sertraline, and in parallel performed a quantitative ultrastructural analysis of organellar membrane homeostasis in untreated vs. sertraline-treated cells. These experiments have revealed that sertraline enters yeast cells and then reshapes vesiculogenic membranes by a complex process. Internalization of the neutral species proceeds by simple diffusion, is accelerated by proton motive forces generated by the vacuolar H(+)-ATPase, but is counteracted by energy-dependent xenobiotic efflux pumps. At equilibrium, a small fraction (10-15%) of reprotonated sertraline is soluble while the bulk (90-85%) partitions into organellar membranes by adsorption to interfacial anionic sites or by intercalation into the hydrophobic phase of the bilayer. Asymmetric accumulation of sertraline in vesiculogenic membranes leads to local membrane curvature stresses that trigger an adaptive autophagic response. In mutants with altered clathrin function, this adaptive response is associated with increased lipid droplet formation. Our data not only support the notion of a serotonin transporter-independent component of antidepressant function, but also enable a conceptual framework for characterizing the physiological states associated with chronic but not acute antidepressant administration in a model eukaryote.

Control of lipid composition of membranes is crucial to ensure normal cellular functions. Saccharomyces cerevisiae has two different phosphatidylserine decarboxylase enzymes (Psd1 and Psd2) that catalyze formation of phosphatidylethanolamine. The mitochondrial Psd1 provides roughly 70% of the phosphatidylethanolamine (PE) biosynthesis in the cell with Psd2 carrying out the remainder. Here, we demonstrate that loss of Psd2 causes cells to acquire sensitivity to cadmium even though Psd1 remains intact. This cadmium sensitivity results from loss of normal activity of a vacuolar ATP-binding cassette transporter protein called Ycf1. Measurement of phospholipid levels indicates that loss of Psd2 causes a specific reduction in vacuolar membrane PE levels, whereas total PE levels are not significantly affected. The presence of a phosphatidylinositol transfer protein called Pdr17 is required for Psd2 function and normal cadmium tolerance. We demonstrate that Pdr17 and Psd2 form a complex in vivo that seems essential for maintenance of vacuolar PE levels. Finally, we refine the localization of Psd2 to the endosome arguing that this enzyme controls vacuolar membrane phospholipid content by regulating phospholipids in compartments that will eventually give rise to the vacuole. Disturbance of this regulation of intracellular phospholipid balance leads to selective loss of membrane protein function in the vacuole.

Aggregated alpha-synuclein (alpha-syn) fibrils form Lewy bodies (LBs), the signature lesions of Parkinson's disease (PD) and related synucleinopathies, but the pathogenesis and neurodegenerative effects of LBs remain enigmatic. Recent studies have shown that when overexpressed in Saccharomyces cerevisiae, alpha-syn localizes to plasma membranes and forms cytoplasmic accumulations similar to human alpha-syn inclusions. However, the exact nature, composition, temporal evolution, and underlying mechanisms of yeast alpha-syn accumulations and their relevance to human synucleinopathies are unknown. Here we provide ultrastructural evidence that alpha-syn accumulations are not comprised of LB-like fibrils, but are associated with clusters of vesicles. Live-cell imaging showed alpha-syn initially localized to the plasma membrane and subsequently formed accumulations in association with vesicles. Imaging of truncated and mutant forms of alpha-syn revealed the molecular determinants and vesicular trafficking pathways underlying this pathological process. Because vesicular clustering is also found in LB-containing neurons of PD brains, alpha-syn-mediated vesicular accumulation in yeast represents a model system to study specific aspects of neurodegeneration in PD and related synucleinopathies.

The uptake of Urd into the yeast Saccharomyces cerevisiae is mediated by Fui1p, a Urd-specific nucleoside transporter encoded by the FUI1 gene and a member of the yeast Fur permease family, which also includes the uracil, allantoin, and thiamine permeases. When Fui1p was produced in a double-permease knock-out strain (fur4Deltafui1Delta) of yeast, Urd uptake was stimulated at acidic pH and sensitive to the protonophore carbonyl cyanide m-chlorophenylhydrazone. Electrophysiological analysis of recombinant Fui1p produced in Xenopus oocytes demonstrated that Fui1p-mediated Urd uptake was dependent on proton cotransport with a 1:1 stoichiometry. Mutagenesis analysis of three charged amino acids (Glu(259), Lys(288), and Asp(474) in putative transmembrane segments 3, 4, and 7, respectively) revealed that only Lys(288) was required for maintaining high Urd transport efficiency. Analysis of binding energies between Fui1p and different Urd analogs indicated that Fuip1 interacted with C(3')-OH, C(2')-OH, C(5)-H, and N(3)-H of Urd. Fui1p-mediated transport of Urd was inhibited by analogs with modifications at C-5', but was not inhibited significantly by analogs with modifications at C-3', C-5, and N-3 or inversions of configuration at C-2' and C-3'. This characterization of Fui1p contributes to the emerging knowledge of the structure and function of the Fur family of permeases, including the Fui1p orthologs of pathogenic fungi.

Sulfate transporters in plants represent a family of proteins containing transmembrane domains that constitute the catalytic part of the protein and a short linking region that joins this catalytic moiety with a C-terminal STAS domain. The STAS domain resembles an anti-sigma factor antagonist of Bacillus subtilis, which is one distinguishing feature of the SLC26 transporter family; this family includes transporters for sulfate and other anions such as iodide and carbonate. Recent work has demonstrated that this domain is critical for the activity of Arabidopsis thaliana sulfate transporters, and specific lesions in this domain, or the exchange of STAS domains between different sulfate transporters, can severely impair transport activity. In this work we generated a Saccharomyces cerevisiae expression library of the A. thaliana Sultr1;2 gene with random mutations in the linking region-STAS domain and identified STAS domain lesions that altered Sultr1;2 biogenesis and/or function. A number of mutations in the beta-sheet that forms the core of the STAS domain prevented intracellular accumulation of Sultr1;2. In contrast, the linking region and one surface of the STAS domain containing N termini of the first and second alpha-helices have a number of amino acids critical for the function of the protein; mutations in these regions still allow protein accumulation in the plasma membrane, but the protein is no longer capable of efficiently transporting sulfate into cells. These results suggest that the STAS domain is critical for both the activity and biosynthesis/stability of the transporter, and that STAS sub-domains correlate with these specific functions.