Zinc transporters that regulate vacuolar zinc storage in Saccharomyces cerevisiae
2000; Springer Nature; Volume: 19; Issue: 12 Linguagem: Inglês
10.1093/emboj/19.12.2845
ISSN1460-2075
Autores Tópico(s)Aluminum toxicity and tolerance in plants and animals
ResumoArticle15 June 2000free access Zinc transporters that regulate vacuolar zinc storage in Saccharomyces cerevisiae Colin W. MacDiarmid Colin W. MacDiarmid Department of Nutritional Sciences, 217 Gwynn Hall, University of Missouri-Columbia, Columbia, MO, 65211 USA Search for more papers by this author L.Alex Gaither L.Alex Gaither Department of Nutritional Sciences, 217 Gwynn Hall, University of Missouri-Columbia, Columbia, MO, 65211 USA Search for more papers by this author David Eide Corresponding Author David Eide Department of Nutritional Sciences, 217 Gwynn Hall, University of Missouri-Columbia, Columbia, MO, 65211 USA Search for more papers by this author Colin W. MacDiarmid Colin W. MacDiarmid Department of Nutritional Sciences, 217 Gwynn Hall, University of Missouri-Columbia, Columbia, MO, 65211 USA Search for more papers by this author L.Alex Gaither L.Alex Gaither Department of Nutritional Sciences, 217 Gwynn Hall, University of Missouri-Columbia, Columbia, MO, 65211 USA Search for more papers by this author David Eide Corresponding Author David Eide Department of Nutritional Sciences, 217 Gwynn Hall, University of Missouri-Columbia, Columbia, MO, 65211 USA Search for more papers by this author Author Information Colin W. MacDiarmid1, L.Alex Gaither1 and David Eide 1 1Department of Nutritional Sciences, 217 Gwynn Hall, University of Missouri-Columbia, Columbia, MO, 65211 USA *Corresponding author. E-mail: [email protected] The EMBO Journal (2000)19:2845-2855https://doi.org/10.1093/emboj/19.12.2845 PDFDownload PDF of article text and main figures. ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InMendeleyWechatReddit Figures & Info All cells regulate their intracellular zinc levels. In yeast, zinc uptake is mediated by Zrt1p and Zrt2p, which belong to the ZIP family of metal transporters. Under zinc limitation, ZRT1 and ZRT2 transcription is induced by the Zap1p transcriptional activator. We describe here a new component of zinc homeostasis, vacuolar zinc storage, that is also regulated by Zap1p. Zinc-replete cells accumulate zinc in the vacuole via the Zrc1p and Cot1p transporters. Our results indicate that another zinc transporter, Zrt3p, mobilizes this stored zinc in zinc-limited cells. ZRT3 is a Zap1p-regulated gene whose transcription increases in low zinc. Zrt3p is also a member of the ZIP family and it localizes to the vacuolar membrane. The effects of ZRT3 mutation and overexpression on cell growth, cellular zinc accumulation and intracellular labile zinc pools are all consistent with its proposed role. Furthermore, we demonstrate that zrt3 mutants inefficiently mobilize stored zinc to offset deficiency. Thus, our studies define a system of zinc influx and efflux transporters in the vacuole that play important roles in zinc homeostasis. Introduction Zinc is an essential nutrient for life. As a structural component of the zinc finger motifs found in many transcription factors and as a catalytic co-factor for RNA polymerase, zinc is required for gene transcription (Vallee and Falchuk, 1993). Zinc is also a cofactor of many essential enzymes. Consistent with the importance of zinc to cellular metabolism, zinc deficiency can have severe consequences. In mammals, for example, even moderate zinc deficiency can cause anemia, loss of appetite, immune system defects, developmental problems and teratogenesis. However, excess zinc can also be detrimental to cells. For this reason, the cytoplasmic concentration of loosely bound, labile zinc must be tightly controlled. This homeostatic regulation is mediated through a variety of mechanisms, including the control of zinc uptake (Guerinot and Eide, 1999), intracellular binding of the metal by metallothioneins (Palmiter, 1998) and zinc efflux from the cell (Nies and Silver, 1995; Palmiter and Findley, 1995). In this report, we describe the transporters involved in the regulated storage of zinc in an intracellular organelle. Studies of the yeast Saccharomyces cerevisiae have led to rapid progress in understanding zinc homeostasis at the molecular level (Guerinot and Eide, 1999). Zinc uptake in S.cerevisiae is mediated primarily by two different systems in the plasma membrane: a high affinity system encoded by ZRT1 (Zhao and Eide, 1996a) and a lower affinity system encoded by ZRT2 (Zhao and Eide, 1996b). Zrt1p and Zrt2p are related proteins belonging to the 'ZIP' superfamily, whose members are found in many different eukaryotic organisms (Eng et al., 1998). Other characterized members of this family also play roles in metal ion transport. Zip1p–Zip4p and Irt1p of Arabidopsis transport zinc (and in the case of Irt1p, iron and manganese) across the membranes of plant cells (Grotz et al., 1998; Korshunova et al., 1999). Furthermore, two human ZIPs, hZip1 and hZip2, have recently been implicated in mammalian zinc transport (Costello et al., 1999; Gaither and Eide, 2000). Expression of both ZRT1 and ZRT2 increases in zinc-limited cells (Zhao and Eide, 1996a,b). This induction is mediated by the Zap1p transcriptional activator, which also up-regulates its own expression under zinc limitation (Zhao and Eide, 1997). The activity of Zap1p is repressed in zinc-replete cells. While the mechanism of this regulation is unknown, Zap1p is likely to sense the concentration of a labile pool of cytoplasmic/nuclear zinc. Zap1p binds to a conserved 11 bp sequence, called the zinc-responsive element (ZRE), present in the ZRT1, ZRT2 and ZAP1 promoters (Zhao et al., 1998). A consensus sequence derived from the ZREs in these promoters is 5′-ACCYYNAAGGT-3′. One ZRE, in either orientation relative to the gene, is sufficient to confer Zap1p-regulated expression on a promoter. Another potential participant in yeast zinc homeostasis is the vacuole. The vacuole accumulates several divalent cations, including Ca2+, Co2+, Fe2+, Mn2+ and Ni2+ (Dunn et al., 1994; Bode et al., 1995; Ramsay and Gadd, 1997; Nishimura et al., 1998; Paidhungat and Garrett, 1998). This compartment has also been implicated as a site of zinc detoxification. For example, increased zinc sensitivity has been observed in mutants defective in either vacuole biogenesis or acidification (Eide et al., 1993; Ramsay and Gadd, 1997). The requirement for vacuole acidification suggests that the accumulation of zinc may be coupled to the discharge of the proton gradient across the vacuolar membrane. Two proteins implicated in transporting zinc into the vacuole are Zrc1p and Cot1p. While Cot1p was originally thought to be in mitochondria (Conklin et al., 1992), more recent studies have established that both it and Zrc1p are located predominantly in the vacuolar membrane (Li and Kaplan, 1998). Overexpression of either leads to increased zinc resistance, while mutation of each gene decreases tolerance (Kamizono et al., 1989; Conklin et al., 1992, 1994). Zrc1p and Cot1p are both members of the CDF family of transporters (Paulsen and Saier, 1997), which also includes the mammalian ZnT proteins. ZnT-1 is a probable zinc transporter responsible for efflux across the plasma membrane (Palmiter and Findley, 1995), whereas ZnT-2 transports zinc into late endosomes (Palmiter et al., 1996a; Kobayashi et al., 1999). ZnT-3 localizes to synaptic vesicles and facilitates transport of zinc into this compartment for release with neurotransmitters (Palmiter et al., 1996b). ZnT-4 is expressed in intestinal cells (Murgia et al., 1999) and the mammary gland where it is required for zinc transport into milk (Huang and Gitschier, 1997). Given the similar function of these diverse family members, it is likely that Zrc1p and Cot1p mediate zinc transport into the vacuole. To identify additional Zap1p-regulated genes involved in yeast zinc homeostasis, we scanned the complete genome sequence for genes with promoters that contain potential ZREs. Our characterization of one such gene, YKL175w (which we have designated ZRT3 for zinc regulated transporter 3), indicates that this gene encodes a new member of the ZIP transporter family. Zrt3p's apparent function is to transport stored zinc out of the vacuole during the transition from zinc-replete to zinc-limiting conditions. These studies demonstrate the importance of organellar zinc storage and a new role for ZIP transporters in eukaryotic cells. Furthermore, the identification of Zrt3p as a ZIP transporter allows the extension of this family, formerly limited to eukaryotes, to include previously unknown members from the archaea and eubacteria. Results Identification of the ZRT3 gene Thirty genes in the yeast genome contain one or more consensus ZRE within 1000 bp upstream of their predicted ATG start codons. One of these genes, YKL175w (now called ZRT3), was chosen for further analysis because of its similarity to ZIP transporters (see below). A single potential ZRE (ACCCTTAAGGT) is located in the ZRT3 promoter from −145 to −155 bp upstream of the open reading frame. To test whether ZRT3 is regulated by Zap1p, the ZRT3 promoter was fused to the Escherichia coli lacZ gene. Analysis of β-galactosidase activity in a wild-type strain bearing this reporter showed that the ZRT3 promoter was highly expressed in zinc-limited cells, but not in zinc-replete cells (Figure 1A). Moreover, the ZRT3–lacZ expression profile was almost identical to that observed with a ZRT1–lacZ promoter fusion over a wide range of zinc concentrations. Analysis of ZRT3–lacZ expression in strains bearing mutations in the ZAP1 gene further indicated that ZRT3 induction was dependent on Zap1p; a zap1 mutant did not induce ZRT3–lacZ activity under zinc limitation (Figure 1B). Moreover, a strain carrying the ZAP1-1up mutation, an allele that increases expression of Zap1p-regulated genes in zinc-replete cells (Zhao and Eide, 1997), showed an increased level of ZRT3–lacZ expression (Figure 1C). These results demonstrate that the ZRT3 gene is a target for Zap1p regulation. Figure 1.ZRT3 is a Zap1p target gene. (A) Wild-type (DY1457) cells bearing ZRT1–lacZ (pGI3) or ZRT3–lacZ (pAG3) reporters were grown in LZM supplemented with the indicated concentration of ZnCl2 and assayed for β-galactosidase activity. (B) Wild-type (DY1457) and zap1 mutant (ZHY6) cells bearing pAG3 were grown in high (+; SD supplemented with 2 μM ZnCl2) or low (–; SD supplemented with 1 mM EDTA, 50 μM ZnCl2) zinc medium. (C) Wild type (DY1457) and ZAP1-1up mutants (ZHY7; up) bearing pAG3 were grown in a zinc-replete medium (SD + 2 μM ZnCl2). One representative experiment is shown in each panel; data are the means of three replicates and the error bars represent ± 1 SD. Download figure Download PowerPoint Zrt3p is a member of the ZIP transporter family The ZRT3 gene encodes a protein with a predicted molecular mass of 55 kDa (Figure 2A). While Zrt3p is not closely related to Zrt1p and Zrt2p, a number of observations led us to conclude that Zrt3p is also a member of the ZIP family. First, both Zrt3p and most known ZIP transporters contain eight predicted transmembrane (TM) domains (Eng et al., 1998). Secondly, many of the ZIP proteins have a long loop region, termed the 'variable region', between TM3 and TM4 (Grotz et al., 1998). Zrt3p also shares this feature. Furthermore, in most of the ZIPs, the variable region contains the repeat sequence (HX)n (where n ≥ 3), as does Zrt3p. While the function of this motif is unknown, its presence in Zrt3p provides an additional link between this protein and other ZIP family transporters. Clusters of histidines are also found elsewhere in the variable region and in one other location, between TM7 and TM8, in Zrt3p. Figure 2.ZRT3 encodes a member of the ZIP superfamily. (A) The amino acid sequence of Zrt3p, with predicted transmembrane domains shaded and clusters of histidines underlined. (B) Sequence similarity of Zrt3p to ZIP family members and newly identified proteins. The segments containing predicted transmembrane domains 4–8 of these proteins are shown. Boxed and shaded residues match the consensus identity at each position, the amphipathic regions of TM4 and TM5 are underlined. The DDBJ/EMBL/GenBank protein sequence accession numbers for the new ZIP family members are: C.elegans 1, AAB37816; S.pombe, CAA22478; M.thermoautotrophicum, AAB84979; E.coli, P24198; P.horikoshii, BAA29722; Myxococcus xanthus, Q06916; B.burgdorferi, AAC66605; Thermotoga maritima, AAD36803; Pyrococcus abyssi, CAB50314; Aeropyrum pernix, BAA80692; Aquifex aeolicus, AAC07647; ZRT3, CAA82017. Accession numbers for previously described ZIP family members are: ZRT1, CAA96975.1; ZRT2, 6323159; ZIP1, AAC24197; ZIP2, AAC24198; ZIP3, AAC24199; hZIP1, AAD27717; hZIP2, AAB34328; C.elegans 2, AAB37814. The last 11 residues of the C.elegans 2 sequence are not shown. Download figure Download PowerPoint Database searches indicated that Zrt3p is related to some previously identified ZIP family members (e.g. AAB37814 of Caenorhabditis elegans). The greatest sequence conservation among all of the members of the ZIP family, and their similarity with Zrt3p, extends from TM4 through TM8 (Figure 2B). A specific feature shared among the ZIPs is the amphipathic nature of the TM4 and TM5 helices, and this property is also true of Zrt3p. This amphipathicity suggested that TM4 and TM5 may in part comprise the transmembrane aqueous channel through which metal ions travel (Eng et al., 1998). Within TM4, the ZIPs have an invariant histidine followed by an adjacent polar or charged residue. These residues are conserved in Zrt3p. Similarly, TM5 of Zrt3p and the ZIPs contain a largely conserved histidine and nearby glutamate that are also essential for Irt1p function. Thus, based on shared protein topology and sequence similarity, as well as the conservation of presumably critical residues in TM4 and TM5, we conclude that Zrt3p is a bona fide member of the ZIP family of transporters. Database searches using the Zrt3p sequence also identified a number of closely related gene products of unknown function and several are included in the alignment shown in Figure 2B. These proteins derive from the archaea (e.g. Methanobacterium thermoautotrophicum, Pyrococcus horikoshii) and eubacteria (e.g. Borrelia burgdorferi, E.coli) as well as other eukaryotes (e.g. Schizosaccharomyces pombe). Thus, Zrt3p provides a link between the previously recognized eukaryotic ZIPs and their prokaryotic and more distant eukaryotic relatives. Effect of the zrt3 mutation on cell growth Recognition of Zrt3p as a ZIP family member suggested that this protein might be a zinc transporter. To determine its function, the ZRT3 gene was deleted in a haploid yeast strain. The resulting zrt3 mutant was viable in both rich (YPD) and minimal synthetic defined (SD) media, indicating that Zrt3p is not required for growth under standard laboratory conditions. To test whether Zrt3p is important under zinc limitation, growth of wild-type and zrt3 mutant strains was examined in a low zinc medium (LZM) supplemented with increasing concentrations of zinc (Figure 3A). No difference in growth yield was observed between these two strains, indicating that Zrt3p is not required in low or replete zinc conditions. Figure 3.Effect of the zrt3 mutation on zinc-limited growth. (A) Wild-type (DY1457) and zrt3 mutant (CMK1) strains were grown to stationary phase in SD medium and inoculated into LZM supplemented with the indicated concentration of ZnCl2 at a starting A600 of 0.01. After 20 h incubation at 30°C, the culture A600 values were determined; the values are the means of three independent experiments and the error bars represent ± 1 SD. (B) Growth of wild-type (CM30), zrt1 zrt2 (CM34) and zrt1 zrt2 zrt3 (CM37) strains in LZM-EDTA with the indicated concentrations of zinc added. Otherwise, the experiment was performed as described for (A). The means of two independent experiments are shown and the error bars represent ± 1 SD. Download figure Download PowerPoint Because Zrt3p is related to the Zrt1p and Zrt2p zinc uptake transporters, we suspected that these proteins might play overlapping roles. To test this hypothesis, we compared growth of a zrt1 zrt2 double mutant to that of an isogenic zrt1 zrt2 zrt3 triple mutant over a range of zinc availability. LZM is zinc limiting because it contains EDTA, a high affinity zinc chelator. Because zrt1 zrt2 mutants have impaired zinc uptake, their zinc requirement was assessed in LZM prepared without EDTA (LZM-EDTA). This medium is less zinc limiting than LZM at a given concentration of total zinc because citrate, the predominant chelator in LZM-EDTA, binds the metal with lower affinity than EDTA. While the zrt1 zrt2 mutant reached its maximum growth yield at 50 μM total zinc, the zrt1 zrt2 zrt3 mutant required 10-fold more zinc (500 μM) to attain its maximum yield (Figure 3B). The increased zinc requirement of the triple mutant suggests that these three genes overlap in function, and that Zrt3p also transports Zn2+ into the cytoplasm where it is available for use in zinc-dependent processes. Effects of ZRT3 mutation and overexpression on zinc accumulation If Zrt3p mediates zinc uptake across the plasma membrane, mutation of this gene would impair zinc accumulation, as do the zrt1 and zrt2 mutations (Zhao and Eide, 1996a,b). To test this prediction, we measured the total cell-associated zinc content of wild-type and zrt3 mutant cells after growth in zinc-limited medium supplemented with a range of zinc concentrations (Figure 4A). Surprisingly, rather than a decrease, the zrt3 mutant showed a marked increase in total cellular zinc when grown in LZM containing 10–1000 μM zinc. No difference between the strains was seen at lower zinc concentrations (0.3–3 μM zinc in LZM). While the ZRT3–lacZ fusion results argue that ZRT3 expression is low in zinc-replete cells (Figure 1A), the effects of the zrt3 mutation on zinc accumulation (Figure 4A) show that the gene is expressed at a significant level in these cells. Figure 4.Effects of zrt3 mutation and ZRT3 overexpression on cellular zinc accumulation. (A) Wild-type (DY1457) and zrt3 mutant (CMK1) strains were grown to exponential phase in LZM supplemented with the indicated concentration of ZnCl2. Cells were harvested and assayed for cell-associated zinc levels by atomic absorption spectroscopy (AAS). (B) Wild-type cells (DY1457) transformed with a centromeric plasmid containing the ZRT3 gene (pCM2) or the vector alone (pFL39) were assayed as described for (A). For (A) and (B), values are the means of three independent experiments and the error bars indicate ± 1 SD. Download figure Download PowerPoint We also determined the effect of ZRT3 overexpression on zinc accumulation. High level overexpression of ZRT3 from the galactose-inducible GAL1 promoter or a high copy plasmid vector resulted in poor growth. This effect was not suppressed by either zinc supplements or limitation, suggesting that the phenotype may result from fortuitous toxicity. When a single-copy plasmid clone of the wild-type ZRT3 gene was introduced into a wild-type strain, potentially resulting in overexpression due to a doubling of gene dosage, no growth defect was observed. When these cells were grown in LZM with added zinc, a significant decrease in zinc accumulation was observed relative to the vector-only control (Figure 4B). Again, these effects were only observed at higher zinc concentrations. Given the unexpected effects of zrt3 mutation and overexpression on zinc accumulation, these results demonstrate that Zrt3p clearly plays a different role in zinc metabolism than Zrt1p and Zrt2p. Paradoxically, the results are consistent with a role for Zrt3p in decreasing cellular zinc accumulation. Effect of the ZRT3 gene on the regulatory zinc pool If the excess zinc accumulated in zrt3 mutants was distributed throughout the cell, an increase in the labile pool of cytoplasmic/nuclear zinc sensed by Zap1p might be expected. Although we cannot measure this zinc pool directly, a comparative estimate can be obtained by quantifying Zap1p activity using a ZRE–lacZ reporter (Zhao and Eide, 1996a,b). Wild-type and zrt3 mutant cells bearing a ZRE–lacZ reporter were grown in a zinc-limiting medium (LZM) supplemented over a range of zinc concentrations and then assayed for β-galactosidase activity. Despite the higher accumulation of zinc in the zrt3 mutant (Figure 4A), more extracellular zinc was required to repress the ZRE–lacZ expression in this strain than in wild-type cells (Figure 5A). Interestingly, the increased reporter expression occurred over the same range of zinc concentrations (i.e. >10 μM) that resulted in greater zinc accumulation in the zrt3 mutant. The observed increase in ZRE–lacZ gene expression in the zrt3 mutant was also reflected in increased zinc uptake activity measured in these strains (data not shown), indicating that the effect is not an artifact of the lacZ reporter. These results showed that the excess zinc accumulated by zrt3 mutants is not detectable by the zinc-sensing mechanism of the Zap1p transcription factor. Thus, the zinc may be compartmentalized within an organelle. Figure 5.Effects of the zrt3 mutation on zinc-responsive gene expression. (A) Wild-type (DY1457) and zrt3 (CMK1) strains transformed with the pDg2 ZRE–lacZ reporter fusion were grown in LZM supplemented with the indicated concentration of ZnCl2 and assayed for β-galactosidase activity. (B) The same strains were grown in SD medium supplemented with 0 or 10 μM ZnCl2 and assayed for β-galactosidase activity. For (A) and (B), data points represent the means of three replicates from a representative experiment and the error bars indicate ± 1 SD. Download figure Download PowerPoint An even more striking result was observed when zrt3 mutant cells were grown in a standard minimal medium, SD. In SD, the zrt3 mutant accumulated approximately the same level of zinc as wild type (data not shown), yet ZRE–lacZ activity was >25-fold higher in the zrt3 mutant (Figure 5B). This effect could be suppressed by adding 10 μM Zn2+ to the medium, indicating that in the unsupplemented medium the induction resulted from an apparent cytoplasmic/nuclear zinc deficiency. It is not yet known why the effect of the zrt3 mutation is more pronounced in SD versus LZM medium. Zrt3p is a vacuolar membrane protein One hypothesis that explains these data is that Zrt3p is a zinc transporter responsible for releasing the metal from an intracellular organelle. If zinc transport into this compartment was mediated by other transporters, loss of Zrt3p activity would result in the trapping of zinc within the compartment where it would be unavailable both for controlling Zap1p and for other zinc-dependent processes. To identify the subcellular location of Zrt3p, the protein was epitope tagged with three repeats of the hemagglutinin (HA) epitope. A single copy of the tagged gene was expressed in a zrt3 mutant strain from its own promoter. The zinc accumulation phenotype of the zrt3 mutant was fully complemented by this plasmid (data not shown), indicating that the Zrt3p-3HA fusion was functional. Strains expressing the epitope-tagged and wild-type ZRT3 alleles were grown in zinc-limiting conditions to induce ZRT3 expression, and the distribution of the protein in the cell was determined by indirect immunofluorescence microscopy (Figure 6). While no fluorescence was observed in cells expressing the untagged allele, Zrt3p-3HA fluorescence was seen to outline the periphery of the vacuole, which was clearly visible by Nomarski optics as a single large organelle. These observations demonstrate that Zrt3p is located in the vacuole membrane, and implicate this compartment as the site of zinc accumulation in the zrt3 mutant. Figure 6.Zrt3p is localized to the vacuolar membrane. A zrt3 mutant (CMK1) expressing an epitope-tagged version of Zrt3p (ZRT3-3HA) or the untagged allele (pCM2, ZRT3) were grown to exponential phase in LZM with 1 μM ZnCl2. The cells were fixed, probed for the HA epitope, and viewed by Nomarski or epifluorescence. The indentations visible by Nomarski optics correspond to the vacuoles. Download figure Download PowerPoint An intracellular zinc store buffers against changes in zinc availability Previous studies supported a role for the vacuole in zinc detoxification, but it was not tested whether this sequestered zinc was available for subsequent use in zinc-deficient conditions. Our studies of ZRT3 suggested that the release of vacuolar zinc might help buffer changes in the labile zinc pool. To investigate this question, wild-type cells containing a ZRE–lacZ reporter gene were grown in a zinc-replete medium (SD) supplemented with a range of zinc concentrations. Cell-associated zinc levels were observed to increase in tandem with extracellular zinc concentrations (Figure 7A). Aliquots of these same cells were washed free of extracellular zinc and inoculated into a zinc-deficient medium (LZM with no added zinc). At time 0, all four samples showed similar low levels of ZRE–lacZ expression, indicating that the cells were zinc replete (Figure 7B). After 4 h of growth, a very different result was obtained: the cells grown initially in the unsupplemented medium induced ZRE–lacZ expression to a high level, while cells grown in higher zinc concentrations induced expression to a lesser degree. Figure 7.The mobilization of an intracellular zinc store is dependent on Zrt3p activity. (A) Wild-type (DY1457) cells transformed with a ZRE–lacZ fusion (pDg2) were grown in SD medium supplemented with the indicated amount of added ZnCl2. Cells were harvested, washed with EDTA, and cell-associated zinc levels determined by AAS. Data are the means of three independent experiments (error bars indicate ± 1 SD). (B) The same cells as in (A) were inoculated into a zinc-deficient medium (LZM with no added zinc) and assayed for β-galactosidase activity after 0 and 4 h. (C) Wild-type (DY1457) and zrt3 mutant (CMK1) strains bearing pDg2 were grown to exponential phase in a high zinc medium (SD plus 1 mM ZnCl2), washed free of extracellular zinc, and reinoculated into LZM without added ZnCl2. Cells were harvested at the indicated times and assayed for β-galactosidase activity. The inset graph shows the β-galactosidase activity over the full time course. For (B) and (C), data points represent the means of three replicates from a representative experiment and the error bars indicate ± 1 SD. Download figure Download PowerPoint These results are consistent with an intracellular zinc store being released during the transition from zinc-replete to zinc-deficient conditions. To test whether Zrt3p was required for this mobilization, we compared ZRE–lacZ reporter gene induction in wild-type and zrt3 strains grown initially in high zinc conditions (SD + 1 mM zinc) and then inoculated into zinc-deficient media. Both strains were initially zinc replete and had low levels of ZRE–lacZ expression (Figure 7C). Upon transfer to zinc-deficient conditions, wild-type cells showed little induction of the reporter during the first 6 h of growth and maximum expression was attained after 20 h. In contrast, detectable induction of the ZRE–lacZ occurred in the zrt3 mutant after only 2 h and maximum induction occurred after only 10–12 h in zinc-deficient conditions. These observations indicate that Zrt3p is required to mobilize zinc stores during the transition from zinc-replete to zinc-deficient conditions. Mutations in cot1 and zrc1 are epistatic to zrt3 Previous studies showed that Zrc1p and Cot1p are vacuolar proteins and suggested that they may function to detoxify zinc (Kamizono et al., 1989; Conklin et al., 1992, 1994; Li and Kaplan, 1998). These observations, along with their membership in the CDF transporter family, indicated that Zrc1p and Cot1p are responsible for transporting zinc into the vacuole. If this hypothesis and our proposed role of Zrt3p are correct, then zrc1 and cot1 mutations should reduce zinc transport into the vacuole and hence limit the overaccumulation of zinc caused by the zrt3 mutation. To test this prediction, we generated a set of isogenic strains carrying all possible combinations of the zrt3, zrc1 and cot1 mutations. Cell-associated zinc levels were measured in these strains after growth in LZM supplemented with 1 mM zinc (Figure 8A); this medium was used instead of SD because zrc1 cot1 double mutants are hypersensitive to zinc in the absence of chelators. The zrc1 and cot1 single mutations both reduced zinc accumulation, with the zrc1 mutation having the greater effect. Inactivation of both genes reduced zinc accumulation even further. This observation supports the hypothesis that these two proteins overlap in function. As expected, deletion of ZRT3 increased zinc accumulation in ZRC1 COT1 cells. Zinc accumulation also increased when the zrt3 mutation was combined with either the zrc1 or cot1 mutations. However, the zrt3 mutation had no effect on zinc accumulation in a zrc1 cot1 double mutant. These results are consistent with Zrc1p and Cot1p together transporting zinc into the same compartment, presumably the vacuole, from which Zrt3p releases it. Figure 8.Effects of zrc1 and cot1 mutations on zinc accumulation and the generation of intracellular zinc stores. (A) Strains of the indicated genotypes (CM100–107) were grown to exponential phase in LZM supplemented with 1 mM ZnCl2, harvested, washed, and assayed for cell-associated zinc levels by AAS. Data are the means of three independent experiments and the error bars indicate ± 1 SD. (B) Cells of the indicated genotypes bearing a ZRE–lacZ reporter (pDg2-1-LEU2) were grown in LZM s
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