(Received for publication, April 22, 1997)
From the Cell Biology and Metabolism Branch, NICHHD,
National Institutes of Health, Bethesda, Maryland 20892, the
¶ Department of Biochemistry, School of Medicine and Biomedical
Sciences, State University of New York, Buffalo, New York 14214, and
the ** Laboratory of Biochemistry, NCI, National Institutes of Health,
Bethesda, Maryland 20892
Copper deprivation of Saccharomyces
cerevisiae induces transcription of the FRE1 and
CTR1 genes. FRE1 encodes a surface reductase capable of reducing and mobilizing copper chelates outside the cell,
and CTR1 encodes a protein mediating copper uptake at the plasma membrane. In this paper, the protein encoded by MAC1
is identified as the factor mediating this homeostatic control. A novel
dominant allele of MAC1, MAC1up2, is
mutated in a Cys-rich domain that may function in copper sensing (a G
to A change of nucleotide 812 resulting in a Cys-271 to Tyr
substitution). This mutant is functionally similar to the MAC1up1 allele in which His-279 in the same domain
has been replaced by Gln. Both mutations confer constitutive
copper-independent expression of FRE1 and CTR1.
A sequence including the palindrome TTTGCTCA ... TGAGCAAA,
appearing within the 5-flanking region of the CTR1
promoter, is necessary and sufficient for the copper- and
MAC1-dependent CTR1 transcriptional
regulation. An identical sequence appears as a direct repeat in the
FRE1 promoter. The data indicate that the signal resulting
from copper deprivation is transduced via the Cys-rich motif of MAC1
encompassing residues 264-279. MAC1 then binds directly and
specifically to the CTR1 and FRE1 promoter
elements, inducing transcription of those target genes. This model
defines the homeostatic mechanism by which yeast regulates the cell
acquisition of copper in response to copper scarcity or excess.
Copper is essential but toxic to cells. Copper is an essential prosthetic group of proteins such as cytosolic superoxide dismutase that is required for detoxification of oxygen free radicals and thus for the fitness of aerobic organisms (1). The copper sites in cytochrome oxidase are essential for the activity of this enzyme and thus for sustained cellular respiration (2). More recently, a role for copper enzymes in iron metabolism has been defined (3). Multi-copper oxidases, ceruloplasmin (mammals) (4, 5) and FET3 (yeast) (6, 7), have been implicated in iron transport. On the other hand, excess copper is toxic. The toxicity of copper may derive from the reaction of Cu(I) with hydrogen peroxide yielding the highly reactive hydroxyl radical that in turn may damage lipids, proteins, or DNA in cells (8). One way that cells solve the problem of acquisition of sufficient copper while avoiding toxic excess is by homeostatic control of copper uptake (2, 9).
In aerated media, copper, like iron, is complexed to media components in its higher valence state. Since data indicate that the metals enter the cell uncomplexed to any extracellular ligand, an initial ligand displacement step must precede the movement of the metal ion across the plasma membrane (9). Ligand coordination to lower valence ions is typically more labile than to the corresponding high valence species (9), and therefore reduction of copper in the environment facilitates cellular uptake. The major surface reductase of Saccharomyces cerevisiae is the FRE1 gene product (10); FRE1 is capable of reducing extracellular copper or iron complexes (11, 12). Consequently, this reductase plays a significant role in cellular copper acquisition (12).
In most laboratory strains of S. cerevisiae, cellular copper acquisition requires CTR1 (13, 14). CTR1 protein exists in the cell as a multimer in the plasma membrane (15), although in some settings it may be internalized into the endocytotic pathway (16). CTR1 contains an unusual amino-terminal domain oriented toward the cell exterior (13). This domain, rich in methionine and serine, is similar to copper-binding domains present in bacterial proteins mediating copper resistance (17, 18). The homology of the methionine-rich domain with copper binding proteins suggests that it will form a pocket with affinity for Cu(I). The subsequent events involved in internalization and translocation of the metal across the cell membrane and into the cytosol have not been characterized.
Expression of FRE1 (11, 12) and CTR1 (15) are homeostatically regulated by copper availability, consistent with the roles of the two gene products in copper acquisition. The regulation of FRE1 (12), like the regulation of CTR1 (15), is mediated at the level of copper-dependent transcription. Copper deprivation induces and copper loading represses transcription of both of these genes. Thus, cellular component(s) must exist through which the signal for available copper levels is transduced into regulated transcription of these genes.
ACE1 must be considered as a candidate for this transducing function (19). ACE1 encodes a yeast protein that binds copper (20) and activates the transcription of target genes in a copper-dependent fashion (21). However, strains deleted for the ACE1 gene maintain homeostatic regulation of the surface reductase and copper uptake (15). Furthermore, ACE1 protein mediates the induction of genes involved in copper detoxification, including CUP1, the yeast metallothionein (19), and SOD1, the copper-zinc superoxide dismutase (22). Because these genes function in cellular protection, the threshold copper concentrations at which these genes are expressed are relatively high (µM concentrations in defined media). In contrast, the copper exposure at which regulation of the copper uptake system occurs is much lower (less than 20 nM in defined media) (7, 15). Also, the copper acquisition system is repressed by copper availability (15), and thus is regulated in a direction opposite from the detoxification system controlled by ACE1 (19). These several factors make ACE1 an unlikely candidate for the copper sensor controlling copper uptake.
A protein with homology to ACE1, called MAC1, must also be considered. The sequence of the MAC1 protein is notable for an amino-terminal domain (residues 1-42) with significant homology to ACE1. The activator for the metallothionein genes in Candida glabrata, AMT1, is also homologous in this region (23). In addition, MAC1 contains a motif conforming to the pattern CysXCysX4CysXCysX2CysX2His. This motif occurs twice in the carboxyl-terminal portion of the protein. It is within the first of these repeats, residues 264-279, that the mutation in the dominant MAC1up1 allele occurs, resulting in a His to Gln substitution at residue 279 (23). FRE1 expression and copper uptake are not repressed by copper in strains containing this dominant MAC1 allele (12, 23). The MAC1 protein, fused to the lacZ gene product for purposes of tracking, has been localized to the nucleus consistent with a role for MAC1 in transcriptional regulation (23).
Here we report evidence that strongly indicates that the MAC1 protein functions as the copper sensor-regulator that controls the expression of surface reductase and copper uptake activity in yeast and thus provides homeostatic control of copper acquisition. We characterize the direct interaction of MAC1 protein with specific sequence elements in the promoters of the CTR1 and FRE1 genes and show that these elements alone support copper- and MAC1-dependent transcriptional activation. The data suggest that MAC1 binding to these elements plays a role in the transduction of environmental copper levels into regulated gene transcription within yeast cells.
The MAC1up1 mutant
isolate, called UPC31 (MATa trp1-1 ade8 his3 gal1
CUP1R MAC1up1), and its parent, strain
BR10 (MATa trp1-1 ade8 his3 gal1 CUP1R MAC1), have been previously described
(23). Strain MA20 (MATa ino1-13 leu2-3,112 gcn4-101
his3-609 FRE1-HIS3::URA3 MAC1up2) was derived
from strain 81 (MAC1) by selecting for histidine prototrophic mutants on plates with iron and copper added to defined medium as described (13). CM3262 (MATa ino1-13
leu2-3,112 gcn4-101 his3-609 ura3-52) (13) was a parent of strain
81 and was used as a control in some experiments. MA20p was derived
from MA20 by incubation in the presence of 5-fluoroorotic acid to eject the integrated FRE1-HIS3::URA3 and thereby restore
uracil auxotrophy which was needed for transformation (24). For
deletion of MAC1, the plasmid pACU1:LEU2 was used. This
plasmid was constructed by cloning a genomic fragment carrying the
MAC1 gene between SacI and XbaI sites
into the compatible SacI and SalI sites of
Bluescript SK+. The genomic sequences were modified by opening at the
unique internal NcoI and EcoRI sites, and a
LEU2 cassette with HindIII linkers was inserted
into this site. The plasmid could then be digested with ApaI
and SacI to release a linear fragment for gene replacement
of MAC1 with LEU2. In this manner, the
MAC1 gene was deleted in strains CM3262 and H1085 creating
3262mac (MATa ino1-13 leu2-3,112 gcn4-101 his3-609
ura3-52
mac1::LEU2) and WY10 (MATa
ura3-52 leu2-3,112
mac1::LEU2), respectively. The presence of the expected gene replacement in these strains was
verified by PCR1 or by Southern blot,
respectively. FTRUNB1 (MATa ino1-13 leu2-3,112
gcn4-101 his3-609 ura3-52
ctr1::URA3) was constructed in the CM3262 background by replacing the CTR1
locus with URA3 as described (13).
Copper concentrations were manipulated in YPD medium (2% yeast extract, 1% peptone, 2% glucose) by the addition of the copper chelator BCS or by the addition of copper sulfate. In some experiments, the strains were grown in defined media containing yeast nitrogen base without amino acids modified to omit iron and copper. This ingredient (6.7 g/liter) was added to glucose (2%), MES buffer (25 mM, pH 6.1), and BCS for copper starvation or copper sulfate for copper loading. Iron was added back as ferric ammonium sulfate in a concentration of 10 µM. A 1-mm loopful of yeast grown on a fresh YPD plate was inoculated into 10 ml of liquid medium and grown overnight with aeration and agitation at 30 °C to late log phase (A600 approximately 2.0). The following day, the culture was diluted into medium of the same composition at an A600 of 0.2 and grown for an additional 5 h prior to harvesting. Methods for crossing, sporulation, spore dissection, and transformation of yeast were as described (25).
Demonstration That MA20 Carried an Allele of MAC1MA20
(MAC1up2 leu2-3,112) was crossed with WY10
(mac1::LEU2 leu2-3,112) and the diploid was
sporulated. Spore clones derived from 12 tetrads were evaluated for
leucine prototrophy and non-repressing reductase activity. This
phenotype has been associated with the MAC1up1
mutation (23). Clones with non-repressing reductase were in all cases
found to be incapable of growing in the absence of leucine, indicating
lack of recombination between the marked MAC1 allele and the
non-repressing reductase phenotype of the MA20 mutant.
The MAC1up2 allele was also rescued by gap repair
(24). A 1.6-kilobase pair NarI fragment containing the
entire MAC1 open reading frame was removed from the
3.0-kilobase pair HindIII MAC1 clone in pSEY8
(23). The resulting gapped pSEY8 that now contained regions 5 and 3
to the MAC1 open reading frame as sticky ends was
transformed into MA20 (MAC1up2), and recircularized
plasmid was recovered. This plasmid mixture was amplified in
Escherichia coli and the recovered
MAC1up2 allele in pSEY8 was sequenced from
162 to
+1585 (numbering relative to MAC1 translation start, with
termination at +1251) by the standard chain termination
method (Sequenase 2.0, U. S. Biochemical Corp.).
CTR1 (and FRE1) mRNA levels in wild type, MAC1up1, and MAC1up2 containing strains were determined by Northern blot analysis following standard protocols (26). For determining CTR1 protein levels a myc-tagged protein fusion was constructed. To do this, the HindIII fragment was excised from plasmid 352-myc (13) and inserted into the HindIII sites of pRS416, creating plasmid 416-CTR1myc. This construct contains a myc epitope tag inserted at the unique EcoRI site of the genomic CTR1 clone and is carried on a centromere-linked vector.
CTR1 protein visualization was as follows. Cultures of transformants with the 352-myc plasmid (myc epitope labeled CTR1 genomic clone) were grown to log phase. Approximately 2 × 107 cells were harvested and lysed by pelleting and resuspending in 150 µl of 1.85 M NaOH, 2% 2-mercaptoethanol. Proteins were precipitated by the addition of 150 µl of 50% trichloroacetic acid and incubation on ice for 30 min. The precipitate was then recovered by centrifuging at 16,000 × g for 10 min prior to resuspending in 45 µl of 2 × Laemmli buffer and 5 µl of 1 M Tris base. Ten microliters or 4 × 106 cell equivalents were loaded per lane of an SDS-polyacrylamide gel. The proteins were blotted to nitrocellulose, and the myc-tagged CTR1 protein was visualized using 9E10 ascites anti-myc monoclonal antibody and peroxidase-conjugated rabbit anti-mouse secondary antibody. The enhanced chemiluminescence (Amersham Corp.) was used to develop the signal from the epitope labeled protein.
Deletion Analysis of the CTR1 PromoterPlasmids containing
portions of the CTR1 promoter fused to the lacZ
gene were constructed as follows. Sequences from the CTR1 promoter, translation start, and codons 1-3 were amplified by PCR
between unique SalI and BamHI sites and fused to
the -galactosidase coding region in plasmid pYEGal4. 5
deletions of
the CTR1 sequences (numbered with respect to the translation
start) were made by PCR, creating plasmids pCTRlac1-413, -334, -333, -311, and -226. For pGC-CTR(
337/
301), the oligonucleotides depicted
in Fig. 6A and flanked by SalI half-sites were
inserted into the XhoI site of pLG699-Z (27). Correctness of
the inserted sequence and orientation of the insert were verified by
direct sequencing of the plasmid. Sequences encoding the FLAG epitope
were introduced 3
of the MAC1 coding sequence and subcloned
into pBluescript IISK(
), creating plasmid pT7MAC1.
DNA Binding Assays
Electrophoretic mobility shift assays (EMSA) were performed with MAC1 protein that was prepared by in vitro transcription and translation. This assay was carried out essentially as described for the AFT1 EMSA (28). MAC1 mRNA was synthesized from the pT7MAC1 plasmid with T7 polymerase and translated in a wheat germ lysate (Promega). The binding reaction was carried out in 20 µl of binding buffer (10 mM HEPES-KOH, pH 7.5, 150 mM KCl, 1 mM dithiothreitol, 0.5 mM EDTA, 0.05% Nonidet P-40, 7.5% glycerol) containing 2 ng of 32P-end-labeled probe, salmon sperm DNA, and in vitro translated MAC1 protein. The binding reaction mixtures were incubated for 10 min at room temperature and then electrophoresed in a 4% native polyacrylamide gel in 6.6 mM Tris-HCl, pH 7.9, 3.3 mM sodium acetate, and 1 mM EDTA at 4 °C.
Enzyme and Copper Uptake AssaysTo evaluate reductase
activity in yeast on solid media, spore clones were streaked onto YPD
plates containing 50 µM copper sulfate and incubated
overnight at 30 °C. The reductase activity was then evaluated by
filter lift assay as described (29). The MAC1up1 or
MAC1up2 clones gave a strong signal in this assay
while the wild-type MAC1 clones were negative. Quantitative
Fe(III) and Cu(II) reductase activity was determined in cell
suspensions as described using bathophenanthroline disulfonate and
bathocuproine disulfonate as indicators of Fe(II) and Cu(I) production,
respectively (12). -Galactosidase activity in yeast transformants
was determined by a standard technique (27). For 67Cu
uptake measurement, strains were grown in YPD to log phase, washed in
assay buffer consisting of 50 mM sodium citrate, pH 6.5, 5% glucose, and incubated at a density of approximately 2 × 108 cells/ml in the presence of 1 µM
67CuSO4 of specific activity 820 dpm/pmol
copper. After 1 h incubation at 30 or 4 °C, washed cells were
collected on glass fiber filters and counted in a scintillation
counter. Copper uptake, reported as pmol/million cells/h, was
calculated after subtracting cell-associated counts obtained at 4 °C
from the cell-associated counts obtained at 30 °C.
The promoter of the FRE1 gene mediates a wide range of
transcriptional changes in response to copper or iron (10, 12). Thus,
mutants selected for FRE1 dysregulation included strains with abnormalities of copper or iron metabolism (13). The selection scheme, which has been described previously (13), involved the fusion
of the FRE1 promoter element to a HIS3 selectable
marker and integration into a his3 deleted strain. The
mutants were selected for the inability to repress FRE1
transcription under conditions of abundant copper and iron. A mutant
strain identified by this method, MA20, exhibited surface Fe(III) and
Cu(II) reductase activity that was poorly repressed by copper (Fig.
1). This phenotype resembled that of the previously
described MAC1up1 mutant (12, 23). The MA20 mutant
was crossed with the parental strain 61, and the diploid, like the
haploid, was found to possess reductase activity that was not
repressible by copper indicating that the mutation behaved as a
dominant (not shown). This dominance also resembled the previously
described MAC1up1 mutant (23). The MA20 strain was
then mated with WY10, a strain carrying a LEU2-marked
MAC1 allele, and the diploid was sporulated. Spore clones
analyzed from 12 independent meioses revealed no recombination between
the LEU2 marker and the non-repressing reductase phenotype,
suggesting that the mutation in MA20 was allelic to MAC1.
Direct rescue of the MAC1 mutant allele was then
accomplished by means of gap repair (24), and the entire
MAC1 coding region in MA20 was sequenced. A single mutation,
a G to A missense mutation at nucleotide 812 (SCMAC1 accession no.
X74551), was identified. This mutation was predicted to alter the
residue at position 271 in the primary sequence from a cysteine to a
tyrosine. Thus, this mutation causes an amino acid substitution within
the same Cys-rich domain, residues 264-279, as the His279
to Gln substitution due to the MAC1up1 allele (23)
(Fig. 2).
To determine if the MAC1up2 allele conferred the
same elevated copper uptake observed in MAC1up1
containing strains (12, 23), high affinity copper uptake was evaluated
in a congenic panel of strains containing the
MAC1up2 allele (MA20), a MAC1 wild-type
allele (CM3262), or a mac1 allele (3262
mac). The
results revealed that the MAC1up2 allele conferred
increased copper uptake (Fig. 3). Evaluation of a
ctr1
-containing strain revealed negligible high affinity copper uptake similar to the mac1
phenotype (13) (Fig.
3). The resemblance between ctr1
and mac1
strains suggested a model in which the nuclear MAC1 protein mediates
the transcriptional activation of CTR1 in response to copper
deprivation. According to this model, CTR1 protein, functioning
downstream of MAC1, acts as the structural mediator of copper uptake at
the plasma membrane.
A prediction of this model is that different MAC1 alleles should
support different patterns of copper-dependent
CTR1 expression. This was tested first by evaluating the
steady-state FRE1 and CTR1 transcript levels in
strains BR10 (MAC1), BR10mac1,
UPC31(MAC1up1), CM3262(MAC1), and MA20
(MAC1up2). These levels were determined by Northern
blot analysis and are shown in Fig. 4A. In
both MAC1 wild-type backgrounds, copper deprivation (BCS
strongly inhibits copper uptake) led to strong induction of
FRE1 and CTR1 mRNA species, and copper
addition strongly repressed these levels. In the
MAC1up1 and MAC1up2 mutants, the
copper-dependent repression was abrogated. Finally, in the
mac1
strain, the CTR1 transcript was
undetectable, and the FRE1 transcript was present in very
low abundance (Fig. 4A), indicating a requirement for MAC1
in the expression of these two genes.
The level of CTR1 protein was also determined in a similar panel of
allelic MAC1 strains grown in different concentrations of
copper. CTR1 protein was assessed by transforming the strains with a
myc-tagged version of the CTR1 gene, visualizing the
myc-CTR1 fusion by Western blot analysis. The protein results precisely followed the Northern data. Like CTR1 mRNA, CTR1 protein
expression was regulated by copper in the wild-type strain, with
maximal expression in the copper-deprived cultures (Fig.
4B). The lower molecular weight band visualized with the
anti-myc epitope antibody may represent a more rapidly migrating form
of CTR1 protein. This CTR1 species may have resulted from improper
glycosylation, since CTR1 protein has been observed to be heavily
glycosylated via O-linkages (12). In contrast, CTR1 protein
expression was constitutive in the MAC1up2 strain
and virtually undetectable in the mac1 strain, regardless of manipulation of the copper in the growth medium.
These effects of MAC1 alleles on CTR1 expression suggested
that MAC1 was likely to be a regulatory protein. To evaluate whether this regulation was occurring at the level of gene transcription or
some other step in gene regulation, a promoter construct that included
431 base pairs of contiguous CTR1 upstream DNA fused to the
lacZ gene was placed into MAC1,
MAC1up2, and mac1-containing strains.
This region attracted attention because it included a perfect
palindrome of the sequence TTTGCTCA, a likely candidate for a
regulatory sequence (cf. Fig. 6A). Furthermore, this same sequence was found in the FRE1 promoter as a
direct repeat in a region shown to confer metal-dependent
expression of that locus (10). These three transformants were grown in the presence of BCS (
Cu) or copper sulfate (+Cu), and
-galactosidase activity was determined as a measure of
CTR1 promoter activity. As the data in Fig. 5
show, in MAC1 wild-type strain CM3262 this CTR1
promoter fragment did confer copper- and MAC1-dependent
expression of the reporter gene (pCTR lacZ-413, first
entry). In contrast, in the MAC1up2 strain,
MA20, this expression was copper-independent, whereas it was
essentially absent in the mac1
strain.
To more closely define this copper-responsive element, progressive 5
deletions of this fragment were constructed and analyzed. Upon deletion
to
334 (leaving the 5
TTTGCTCA intact, pCTR lacZ-334, second
entry) the expression of the reporter gene was qualitatively unaltered in that both copper and MAC1 dependence were proportionately unaffected. However, deletion of even the 5
-terminal T of this sequence (pCTR1 lacZ-333, third entry) significantly reduced
the copper dependence of this expression, as did deletion of the entire 5
-half of the palindrome (pCTR lacZ-311, fourth entry). In
these two constructs, expression was less MAC1-dependent
also as indicated by the increased
-galactosidase activity in the
mac1
-containing strain. Removal of the palindrome
entirely (pCTR lacZ-226) abolished lacZ expression,
suggesting that this region contained elements needed for even basal
transcription. To further test the apparent enhancer activity of the
CTR1 sequences
337 to
301, including the palindrome,
they were cloned in front of a minimal promoter from the
CYC1 gene including the transcription and translation start
sites but lacking any upstream activating sequences (pGC-CTR). As
the data show, these CTR1 sequences alone conferred MAC1-
and copper-dependent expression of lacZ in this
construct (Fig. 5, last entry).
A cis-acting palindromic element in the CTR1 promoter was thus identified as both necessary and sufficient for MAC1- and copper-dependent transcriptional activity. MAC1 action could be mediated through direct interaction with the DNA sequence or indirectly via intermediate proteins. To distinguish between these possibilities we examined the ability of an oligonucleotide containing the palindromic CTR1 target sequence (Fig. 6A) to specifically interact with MAC1 protein in an electrophoretic mobility shift assay (EMSA). In the first set of experiments, shown in Fig. 6B, the MAC1 protein was modified by insertion of a carboxyl-terminal FLAG epitope tag. This fusion protein was synthesized by means of an in vitro transcription-translation system. The MAC1-FLAG protein specifically retarded migration of the labeled oligonucleotide (Fig. 6B, lane 2). Two specific bands appeared with the addition of MAC1 protein to the probe. Various controls demonstrated the specificity of these signals. In the absence of added MAC1 neither band was observed (Fig. 6B, lane 1). Addition of 100-fold molar excess of cold competitor DNA with the same sequence as the labeled oligonucleotide eliminated the shifted signals (Fig. 6B, lane 5). In contrast, MAC1 did not bind to the cis element from the FET3 locus in S. cerevisiae recognized by the iron-regulated transcription factor, AFT1 (28), since an oligonucleotide representing this sequence did not compete with the CTR1 probe oligonucleotide for MAC1 (Fig. 6B, lane 4). Finally, antibody to the FLAG epitope appended to the MAC1 protein resulted in a decrease in the more slowly migrating signal and the appearance of a supershifted band. This was presumed due to delayed migration of the FLAG antibody complex in association with the DNA-MAC1 complex (Fig. 6B, lane 3, indicated by *). The effect of the antibody addition on the intensity of the lower band was minimal. In sum, a specific signal due to MAC1-DNA interaction were observed.
We then used the EMSA to evaluate more precisely the nucleotide
sequence constraints on the protein-DNA interaction. This was done by
adding as cold competitors at 100-fold excess mutated forms of this
CTR1-derived oligonucleotide probe. Mutation of the first
portion of the palindrome (M1, GCT to CGA in the 5-half of the
palindrome, Fig. 6A) partially inhibited the interaction (lane 6, compare with competition by the wild-type
CTR1 promoter fragment, lane 5), whereas
mutations in both the first and second portions of the palindrome (M2,
in which an additional GCT to CGA substitution has been made in the
3
-inverted repeat, Fig. 6A) abolished the interaction
(lane 7, no competition). These results indicate first that
MAC1 does bind to this sequence in vitro and that both the
GCT of the 5
and 3
portions of the palindrome are important in this
regard.
The role of the direct repeat sequence in the 5-flanking region of
FRE1 (Fig. 6A) was also evaluated (Fig.
6B, lanes 8-11). An oligonucleotide including
this direct repeat from FRE1 (Fig. 6A) was able
to compete (again at 100-fold excess) with the CTR1 palindrome for binding to MAC1 protein (lane 8).
Subsequently, a directly labeled oligonucleotide with the same sequence
was tested, and a specific MAC1-dependent shift was
observed in the EMSA (lane 10) which was reduced upon
addition of the FLAG antibody (lane 11).
In this paper, the MAC1 protein was identified as a regulatory
DNA-binding protein through which cellular copper levels are transduced
into the regulated transcription of genes involved in copper
acquisition. Specifically, copper-regulated expression of
FRE1 and CTR1 was found to be altered in
MAC1 mutant strains, with copper-independent expression in
the MAC1up1 and MAC1up2 strains
and negligible expression in the mac1 strains. A
homeostatic feedback loop for the control of cellular copper levels can
thus be defined. Copper uptake into the cell, requiring reduction of copper chelates (FRE1-mediated) and translocation across the plasma membrane (CTR1-mediated), subsequently inhibits transcription of
FRE1 and CTR1 (MAC1-mediated).
To define further this feedback loop, we identified a regulatory sequence from within the CTR1 promoter that was sufficient for conferring copper-dependent and MAC1-dependent expression to a reporter gene. MAC1 was shown in vitro to bind specifically to a motif within this sequence. This sequence element (A/T)TTTGCTCA appears as a palindrome in the native CTR1 promoter region and is capable of direct interaction with MAC1 protein. A direct repeat of an identical sequence appears in the FRE1 promoter region and is also capable of interacting with MAC1 protein. As indicated by the promoter deletion analysis, direct and specific binding of MAC1 to these sequence elements appears required for transcriptional activation of the target genes involved in copper acquisition.
Examination of the MAC1 primary sequence provides some hints as to how this protein-DNA interaction might occur. The amino-terminal domain of MAC1 contains a subdomain (residues 1-42) with strong similarity to the amino-terminal domains of ACE1 and AMT1, proteins implicated in DNA binding and copper-dependent transcriptional activation in S. cerevisiae and C. glabrata, respectively. The GRP motif (residues 37-39 in MAC1), conserved among MAC1, AMT1, and ACE1, is also shared with a larger family of transcription factors (PAXI, HMGI, and Drosophila prd and Hin recombinase) (30). This motif, and in particular the R residue, has been implicated in the AMT1 interaction with the AT-rich minor groove of its binding site (30). The TyrXCysX2CysX3HisX4Cys motif (residues 9-23 in MAC1) is likewise shared with AMT1 and ACE1. In AMT1, this motif appears to be a zinc-binding element (31, 32), and there is evidence for its involvement in the DNA-binding activity of both ACE1 and AMT1. Thus, a potential DNA-binding domain can be identified in MAC1 that resembles the apparent DNA-binding domains in ACE1 and AMT1.
The DNA sequences recognized by ACE1/AMT1 or MAC1 proteins are also
strikingly similar to each other (Fig. 7). The AMT1
metal response element possesses a critical T thought to lie in the minor groove of the DNA helix and to interact with the GRP motif in the
AMT1 protein. This T residue of the metal response element site in AMT1
(nucleotide 195) is conserved in three of four MAC1 interaction sites
(Fig. 7). Only the 5
CTR1 site has an A in this position
instead. The other residues of the core region identified by
methylation interference as contacts for AMT1 are all conserved with
the MAC1 binding site with the exception of the G at position
189
(Fig. 7). The MRE element from the CUP1 (copper thionein) promoter that interacts with ACE1 regulatory protein includes the
sequence TTTTCCG*CTG*A (the asterisk-marked bases indicate methylation protection in the ACE1-bound state) (33). The TTTcCgCT constituents were also shown to be critical by base substitution analysis (34). This pattern of base selectivity is similar to that
shown for the MAC1 binding site in the CTR1 promoter, in which the GCT of TTTTGCTCA was identified as critical for DNA binding.
Thus, ACE1 and MAC1 share primary sequence homology within an amino-terminal domain thought to be important for DNA interaction, and they share features between their DNA recognition sequences. Together these observations could suggest that the protein-DNA complexes may be similar. However, ACE1 and AMT1 activate copper detoxification activity (thionein genes) and are active as trans-factors in a copper-bound state. In contrast, MAC1 activates copper acquisition and the data indicate is most likely active in DNA binding and trans-activation when in a copper-depleted or copper-free form. With this difference in mind, the evolutionary relationship between these copper regulatory proteins is interesting to consider. Coordinated but opposite regulation of iron uptake (transferrin receptor) and iron detoxification (ferritin) by a common regulatory molecule (IRP1) has been described in mammals (35). Perhaps the regulatory molecules MAC1 and ACE1 in S. cerevisiae, controlling copper uptake and copper detoxification, respectively, diverged from a common ancestral regulator.
The copper-regulated activity of MAC1 could occur at least in part as a consequence of copper-regulated DNA binding. However, we were unable to detect a copper dependence of this binding in the EMSA. This result was similar to what has been observed with in vitro AFT1-DNA binding in which this interaction was independent of iron, although in vivo AFT1 trans activity is iron-dependent (28). Despite this negative result for MAC1, a suggestion about how copper might regulate MAC1 is provided by the amino acid sequence. That is, cysteine and histidine residues are known to coordinate copper in proteins, and the Cys-His element in the carboxyl-terminal domain of MAC1 (Fig. 2) might be able to reversibly interact with copper, generating a biological signal. In this model, a copper-depleted MAC1 protein form would bind to DNA followed by transcriptional activation. The gain-of-function alleles of MAC1 fit this model in that they contain mutations in potential copper-coordinating residues in this domain (H279Q, C271Y). Thus, by interfering with the copper binding ability of the Cys-His repeat domain of MAC1, these mutations result in a constitutively active protein.
How MAC1 may function to activate transcription is not known. However, the distribution of charged amino acid residues in MAC1 is asymmetric, with the amino-terminal domain (residues 1-201) strongly basic in nature (31 Arg and Lys, 21 Asp and Glu) and the carboxyl-terminal domain (residues 202-417) predominantly acidic (27 Asp and Glu, 16 Arg and Lys). Also, the putative nuclear targeting domain is found in the amino-terminal region (24). These features are consistent with a model in which the amino-terminal domain is associated with nuclear targeting and DNA binding, whereas the carboxyl-terminal domain is associated with copper sensing and trans-activation. This model remains to be tested. For example, we have not rigorously excluded the possibility that the copper dependence of MAC1 function requires another protein(s), with this other protein being the copper sensor. While this model is not excluded by our data, we believe the model that most simply explains the phenotype of the UP alleles is based on their having a diminished affinity for copper.
The interrelationships between iron and copper homeostasis in yeast are defined by a complex network of genes. Cellular iron uptake requires the activity of the plasma membrane iron transport complex composed of the FTR1 and FET3 gene products (36). FTR1 most likely encodes an iron permease function (36), whereas the FET3 gene encodes a multi-copper oxidase that is required for iron uptake (6). Thus, the cellular copper uptake and utilization pathway mediating delivery of copper to FET3 must be intact for high affinity iron uptake to occur. This copper delivery pathway includes CTR1, required for cellular copper uptake (13). We may now add MAC1 to this scheme, because MAC1 is required for CTR1 expression and consequently is required for iron uptake.
At the level of gene regulation, however, the pathways for responding to changes in iron or copper levels are distinct. Iron deprivation induces increased activity of the iron uptake system through a homeostatic feedback loop involving the AFT1 regulatory protein (28). In iron-deprived cells, AFT1 binds to a specific recognition sequence in the enhancer regions of genes involved in iron uptake, including FET3 and FTR1 (28). AFT1 induces transcription of the target genes under conditions of iron deprivation. A separate but analogous pathway mediates the cellular response to copper deprivation; MAC1 is induced to bind to the specific recognition sequences identified in this paper, leading to expression of the genes involved in copper acquisition (FRE1, CTR1). The two systems are distinct in that the iron regulatory protein, AFT1, is distinct from the copper regulatory protein, MAC1, and each recognizes different DNA sequence elements. Only the FRE1 gene possesses binding sites for both AFT1 and MAC1 (10) and thus constitutes a special case. The dual role of surface reductases in facilitating iron and copper acquisition by reduction of extracellular metal chelates may explain this dual regulation of the FRE1 gene although the MAC1- and copper-dependent regulation of FRE1 seems to predominate as demonstrated by the Northern analysis shown in Fig. 4A.
Humans, much like yeast, have Cu/Zn superoxide dismutase and
copper-based cytochrome oxidase (2), and defects in these proteins have
been implicated in human disease (37, 38). Cytoplasmic thioneins exist
in humans as in yeast and are involved in protection against copper
toxicity (39). Recently, a human homologue of CTR1 has been
identified that is capable of complementing the phenotype of the yeast
ctr1 mutant (40). This protein may be involved in copper
uptake in human cells. Human FRE1 homologues are also likely
to exist (41). Since the problems of acquiring copper and avoiding its
toxicity must be confronted by virtually all organisms,
copper-dependent, homeostatic gene regulation is likely to
occur in humans, too. Whether a MAC1 homologue functions to mediate
copper homeostasis in humans must await further study.