Structure-Function Analyses of the ATX1 Metallochaperone*
Matthew E.
Portnoy
§,
Amy C.
Rosenzweig¶
,
Tracey
Rae
**,
David L.
Huffman
**,
Thomas V.
O'Halloran¶
, and
Valeria Cizewski
Culotta

From the
Departments of Environmental Health Sciences
and Biochemistry, Johns Hopkins University School of Public Health,
Baltimore, Maryland 21205 and the Departments of ¶ Biochemistry,
Molecular Biology and Cell Biology and of
Chemistry,
Northwestern University, Evanston, Illinois, 60208
 |
ABSTRACT |
Saccharomyces cerevisiae Atx1p represents
a member of the family of metallochaperone molecules that escort copper
to distinct intracellular targets. Atx1p specifically delivers copper
to the Ccc2p copper transporter in the Golgi. Additionally, when
overproduced, Atx1p substitutes for superoxide dismutase 1 in
preventing oxidative damage; however the mechanistic overlap between
these functions is unresolved. The crystal structure of Atx1p has been
solved recently. By examining a surface electrostatic potential
distribution, multiple conserved lysines are revealed on one face of
Atx1p. An additional conserved lysine (Lys65) lies in close
proximity to the metal binding site. Through site-directed mutagenesis,
residues in the metal binding region including Lys65 were
found to be necessary for both copper delivery to Ccc2p and for Atx1p
antioxidant activity. Copper trafficking to Ccc2p also relied on the
lysine-rich face of Atx1p. Surprisingly however, elimination of these
lysines did not inhibit the antioxidant activity of Atx1p. We provide
evidence that Atx1p does not suppress oxidative damage by a
metallochaperone mechanism but may directly consume superoxide.
Purified Cu-Atx1p reacts noncatalytically with superoxide anion
in vitro. We conclude that the copper-trafficking and
antioxidant functions of Atx1p arise from chemically and structurally
distinct attributes of this metallochaperone.
 |
INTRODUCTION |
Copper is required by all organisms as a cofactor for specific
enzymes that participate in oxygen chemistry. In eukaryotes, copper
metalloenzymes are found in multiple cellular locations including the
cytosol, mitochondria, and cell surface (1). Until recently, it was
unknown how copper could be widely distributed in the cell for the
activation of many copper enzymes. A new class of small proteins termed
copper chaperones, or metallochaperones, deliver copper to specific
intracellular targets (for review, see Ref. 2). All of the copper
chaperones isolated to date were first identified in the bakers' yeast
Saccharomyces cerevisiae, and functional homologues have
been noted in Arabidopsis thaliana, Caenorhabditis elegans,
mice, and humans (3-7). Three copper metallochaperones have been
characterized thus far: (i) yeast Lys7p (human CCS), which delivers
copper to superoxide dismutase 1 (Sod1p) in the cytosol (8); (ii) yeast
and human Cox17p, which direct copper to the mitochondria for
activation of cytochrome oxidase (5, 9); and (iii) yeast Atx1p (human
HAH1 or ATOX1), which specifically carries copper to the secretory
pathway for incorporation into copper enzymes destined for the cell
surface or extracellular milieu (4, 10, 11).
The target of copper delivery by Atx1p is a P-type copper transporting
ATPase confined to a late Golgi compartment (10-12). In humans, this
intracellular copper pump is encoded by the Wilson and Menkes disease
genes (13), and a functional homologue (Ccc2p) has been characterized
in yeast (14, 15). Via the action of Ccc2p, copper is incorporated into
a multi-copper oxidase Fet3p, which translocates to the plasma membrane
and works in conjunction with the iron permease to mediate high
affinity iron uptake (14, 16). Atx1p was not originally identified as a
metallochaperone for Ccc2p, but rather as an antioxidant molecule
(hence the name Atx1) capable of suppressing oxidative damage in yeast
lacking SOD1 (17). Currently, nothing is known about the
mechanism by which ATX1 acts as antioxidant.
Atx1p is a prototype for a structural family of metal binding proteins
and domains. Included in this family are MerP, a bacterial carrier for
mercury ions (18), and the metal binding domains of the Ccc2p and
Wilson and Menkes copper transporters (11). Homology among these
various domains extends throughout the 8-kDa polypeptide segment,
including an invariant metal binding motif, MXCXXC. The NMR structures for MerP (18) and an
Atx1-like domain of the Menkes transporter (19) have been obtained, and
very recently, the crystal structure of Atx1p has been solved (36). A
common structural unit has been revealed, consisting of a
a
a
fold and a metal binding site on the polypeptide surface
(18, 19, 36).
Using the crystal structure as a guide, we have employed a
structure-function approach to probe the biological roles of Atx1p. Our
studies have revealed that the antioxidant and metallochaperone functions of Atx1p require overlapping but distinct regions of the
polypeptide. Furthermore, we provide evidence that the ATX1 antioxidant function does not occur by a metallochaperone mechanism, but Atx1p itself may be acting as a scavenger of free radicals.
 |
EXPERIMENTAL PROCEDURES |
Yeast Strains, Media, and Growth Conditions--
The S. cerevisiae atx1
strain SL215 has been described previously
(10). MP101 was constructed by creating an atx1
mutation in SFY526 (CLONTECH) using the pDA1-His deletion
plasmid (10). SL106 is a sod1
atx1
strain
obtained by introducing an
atx1
::LEU2 mutation in KS107 (20).
The sod1
atx1
strain PS114 was constructed by deleting SOD1 and ATX1 of SY1699 (21) using
the pKS3 and pDA1-His deletion plasmids (10, 20). Stocks of yeast
strains were maintained on a standard yeast extract-peptone-dextrose
medium (22). Measurements of ferrous iron uptake were conducted in quadruplet samples precisely as described (23).
Plasmids--
pMP025 was generated by mobilizing the
ATX1 fragment from pRSA-1 (17) through digestion with
BamHI and XhoI and by inserting this fragment
into the same sites of the URA3 2-µm plasmid, pRS426 (24).
The majority of the mutant ATX1 alleles (K24,28E; K24,28A; K61,62E; K65E; K24,28,61,62E) were created by standard 4-primer polymerase chain reaction mutagenesis (25) using outer primers engineered with BamHI and XhoI restriction sites
to facilitate ligation of the polymerase chain reaction product into
the pRS413 (CEN) and pRS423 (2-µm) vectors (24). The
remaining mutants (K61,62A; K65A; K65F) were generated using the
QuikChangeTM site-directed mutagenesis kit from Stratagene
(per instructions of the manufacturer) and the ATX1
expression vectors p413-A1 (CEN) and pRS-A1 (2-µm) as
templates. All mutants were confirmed by dideoxy automated sequencing
(Johns Hopkins University CORE sequencing facility). The Atx1-Gal4 DNA
binding domain fusions (for 2-hybrid analysis) were created as
described previously (11), using the mutant alleles of ATX1
as template for polymerase chain reaction.
Biochemical Analyses--
Two-hybrid studies were conducted
essentially as described using the MATCHMAKER System
(CLONTECH) (11) and the yeast strain MP101.
For Western blot analysis, yeast cells were grown overnight in
selecting synthetic dextrose medium to confluence, and protein extracts
were prepared as described previously (10). Extracts were subjected to
SDS-polyacrylamide gel electrophoresis using 14% precast Tris-glycine
gels (Novex) followed by immunoblot analysis using a rabbit anti-Atx1p
antibody diluted 1:1,000. The secondary antibody consisted of
anti-rabbit IgG conjugated to horseradish peroxidase (Amersham
Pharmacia Biotech) diluted 1:10,000. Detection employed the ECL kit
(Amersham Pharmacia Biotech), according to the specifications of the manufacturer.
The in vitro SOD activity studies utilized a purified Cu(I)
Atx1p prepared according to previously described protocols (11), with
the exception that Cu(I)(CH3CN)4PF6
was used to load the protein rather than the combination of
CuSO4 and dithiothreitol. Superoxide scavenging activity
was measured by the standard cytochrome c/xanthine oxidase
assay (26) in buffers supplemented with 1.0 mM EDTA.
 |
RESULTS |
Structural Features of Atx1p--
The x-ray structure of Atx1p was
recently solved at 1.02-Å resolution (36). Using the refined
coordinates, a surface electrostatic potential distribution was
generated with the program GRASP (27) (Fig.
1A). Examination of this
distribution reveals a region comprising multiple lysine residues that
generate a positively charged face on the protein surface (Fig.
1A). These basic residues are highly conserved between the
yeast and human metallochaperones (Fig. 1B) and include:
Lys24 and Lys28 located on
-helix 1 and loop
2, Lys61 and Lys62 located on
-helix 2, and
Lys65 which is found in close proximity to the metal ion
coordinated by cysteines at positions 15 and 18 (Fig. 1A).
All three regions were targeted for mutagenesis, and substitutions
engineered at these sites are summarized in Fig. 1B.

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Fig. 1.
X-ray crystal structure and sequence
alignment of Atx1p. A, An electrostatic surface
representation of the Hg(II) form of Atx1p with residues selected for
mutagenesis indicated. The positively, negatively charged and neutral
amino acids are represented in blue, red and white, respectively.
B, amino acid alignment of Atx1p and HAH1. Stars indicate
amino acid identity, dots indicate amino acid similarity and arrows
designate mutational substitutions.
|
|
The Effects of ATX1 Mutations on the Delivery of Copper to
Ccc2p--
atx1
strains are defective for iron uptake
because of lack of copper incorporation into Fet3p (10). Therefore,
iron uptake provides a simple assay for monitoring Atx1p activity and
can be used to obtain a qualitative estimate for how well copper is delivered from Atx1p to Ccc2p. Low copy CEN plasmids
expressing mutant alleles of ATX1 (Fig. 1B) were
used to transform an atx1
strain and were tested for the
ability to support high affinity uptake of 55Fe. The
majority of these Atx1p mutant variants accumulated to near wild-type
levels when expressed in the atx1
strain (Fig. 2A).

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Fig. 2.
The effect of ATX1 mutations
on ferrous iron uptake. The atx1 strain SL215 was
transformed with pRS413 (VECTOR) or with the same vector
expressing either wild type or mutant Atx1 molecules. A,
extracts were prepared and subjected to Western blot analysis using
rabbit anti-Atx1 IgG (10). B, cells were assayed for uptake
of 55Fe as described (35). Ferrous uptake is expressed as a
percentage of that obtained with wild type samples, and the low level
of uptake from the vector control was subtracted from all samples.
Results represent the average of four samples, and error
bars represent standard deviation.
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|
The lysine patches represented by K24,28 and K61,62 of Atx1p were found
to be critical for the delivery of copper ions to Ccc2p. Altering
K24,28 to acidic glutamates (K24,28E) resulted in an Atx1p molecule
that was severely crippled in supporting iron uptake (Fig.
2B). Additionally, changing K61,62 to negatively charged
glutamates and neutral alanines also reduced iron uptake, but to a
lesser extent than was observed with mutations at K24,28 (Fig.
2B). Changing the overall charge of the Atx1p basic region from positive to negative in the K24,28,61,62E allele resulted in an
Atx1p molecule that showed negligible capacity for supporting iron
(Fig. 2B), underscoring the importance of the Atx1p
lysine-rich face in the delivery of copper to Ccc2p and Fet3p. The very
low level of iron uptake seen with K24,28,61,62E Atx1p may in fact represent an overestimation of mutant protein activity because iron
starvation is known to induce Fet3p synthesis (28).
We recently observed that the two cysteines of the Atx1p metal binding
site (MTCXXC) are essential for copper delivery to Ccc2p
(29). We additionally tested the role of Lys65 adjacent to
this metal binding site. As seen in Fig. 2B, the K65E allele
of Atx1p was severely crippled in this assay. Surprisingly, variants
K65A and K65F each exhibited nearly wild type levels of activity (Fig.
2B) even though these alleles were expressed to very low
degrees. Thus, a negative charge adjacent to the metal binding site of
Atx1p prohibits copper delivery to Ccc2p, whereas a basic, neutral, or
hydrophobic residue at this position appears well tolerated.
As a second assay for Atx1p activity, we monitored physical interaction
between Atx1p and Ccc2p by use of the 2-hybrid system. Consistent with
previous results (11), a 2-hybrid signal can be detected in cells
expressing Gal4 fusions to wild type Atx1p and to the metal binding
domains of Ccc2p (Fig. 3, A and
B). This interaction requires a functional metal binding
site of Atx1p because a C15,18S mutation abolished the signal (Fig.
3A). Lys65 adjacent to the metal site, as well
as lysines in the basic face of Atx1p, likewise seemed important for
physical interaction with Ccc2p (Fig. 3B). It seemed
surprising that K61,62E, which exhibited 50% activity in the iron
uptake assay, was unscored for interaction with Ccc2p. As would be
expected for a transient reaction, the 2-hybrid signal obtained with
Atx1p is normally very weak (11), and any reduction in Atx1p
recognition of Ccc2p is likely to lower the signal to levels beyond our
detection.

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Fig. 3.
Two-hybrid analysis of ATX1
mutants. Strain MP101 was transformed with the pGAD424
vector expressing the Gal4AD-Ccc2a fusion (11) and also with pGBT9
alone (CONTROL) or with the same vector expressing Gal4DB
fusions to either wild type Atx1p or the designated mutant alleles.
A and B represent two independent studies.
Results are presented as the average of dual transformants; error
bars represent standard deviation.
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The Effects of ATX1 Mutations on the Antioxidant Role of
Atx1p--
To define the regions of Atx1p needed for antioxidant
protection, the various Atx1p mutants were expressed from a multi-copy 2-µm yeast vector in a strain lacking both Atx1p and Sod1p. As seen
in Fig. 4, all mutants tested accumulated to
the same high level obtained with wild type Atx1p.

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Fig. 4.
The effect ATX1 mutations on
suppression of oxidative damage. The atx1
sod1 strain SL106 was transformed with the multi-copy
plasmid pRS423 (VECTOR) or with the same vector expressing
either wild type or the indicted mutant alleles of Atx1p. 1 × 108 cells were spotted onto medium lacking lysine and were
incubated either aerobically (+O2) or in anaerobic culture
jars ( O2). The bottom panel shows Western blot
analysis expression of the Atx1p variants as described in Fig.
2A.
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The antioxidant activity of each mutant was monitored by assaying for
the ability to overcome the aerobic lysine auxotrophy of
sod1
cells. In the presence of oxygen (but not under
anaerobic conditions), sod1
strains fail to grow on
medium lacking lysine as a result of oxidative damage to component(s)
of the lysine biosynthetic pathway (30, 31). This defect is
suppressed by overexpressing wild type ATX1 (Fig. 4). Full
suppression of oxidative damage was also achieved with Atx1p mutants
targeting lysines 24 and 28. Moreover, the K24,28,61,62E variant of
Atx1p, which was completely inactive for copper activation of Ccc2p and
Fet3p (Fig. 2B), fully suppressed the aerobic lysine
auxotrophy of the sod1
strain (Fig. 4). It was curious
that the K61,62E allele failed to suppress oxidative damage, but the
significance of this result is difficult to reconcile because these
substitutions have no deleterious effect when present in combination
with K24,28E (Fig. 4). In any case, changing the overall charge of the
lysine-rich face from basic to acidic (in the case of K24,28,61,62E)
resulted in an Atx1p molecule that was still wild type for suppressing oxidative damage.
In comparison to results obtained with the lysine patch mutants,
mutations directed at the metal binding region of Atx1p greatly inhibited its antioxidant activity. We previously found that the C15,18S mutant lacking the copper binding cysteine ligands fails to
suppress the sod1
aerobic lysine auxotrophy (29).
Mutations that targeted Lys65 positioned near the metal
site also affected antioxidant function. Alleles K65E and K65F were
completely inactive, and the K65A mutant was only partially functional
in this assay (Fig. 4).
How Does ATX1 Function As an Antioxidant?--
We addressed
whether ATX1 suppresses oxidative damage by acting as a
metallochaperone for a molecule other than Ccc2p. By definition, copper
chaperones are only required under conditions of limiting cellular
copper (9, 10, 32, 37). Therefore, if Atx1p suppresses oxidative damage
by acting as a metallochaperone, it should be possible to mimic this
activity by high copper. However, we noted that treatment with high
copper did not substitute for multi-copy ATX1 in suppressing
sod1
defects (Fig.
5A). By contrast, the same
treatment with copper quite effectively substituted for Atx1p in
delivering copper to Ccc2p, as monitored by high affinity iron uptake
(Fig. 5B). In fact, high copper treatment resulted in
decreased physical interaction between Atx1p and Ccc2p, as revealed by
2-hybrid analysis (Fig. 5C), emphasizing the notion that
copper chaperones are not required when copper is plentiful. Therefore,
while Atx1p delivers copper to Ccc2p and Fet3p only under metal
limiting conditions, the antioxidant behavior of Atx1p cannot be
explained by a similar metallochaperone mechanism.

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Fig. 5.
The effects of increasing copper availability
on Atx1p activity. A, the atx1
sod1 strain PS114 transformed with (+) or without ( )
multi-copy ATX1 on plasmid pMP025 was inoculated at an
A600 of 0.03 into synthetic dextrose medium
lacking lysine and containing the indicated concentrations of copper
sulfate and was incubated at 30 °C for 24 h prior to measuring
total growth at an A600. Results are
representative of two samples. B, measurements of ferrous
iron uptake were conducted in the atx1 strain SL215 as
described in Fig. 2 with cells grown in the presence of the indicated
copper concentrations. C, 2-hybrid analysis conducted as
described in Fig. 3 of cells co-transformed with pGBT9 expressing the
Gal4DB-Atx1 fusion and either pGAD424 alone (VECTOR) or
pGAD424 expressing the Gal4AD-Ccc2a fusion (CCC2a). Cells
were grown in minimal selecting medium at the indicated copper
concentrations.
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We addressed whether Atx1p itself may serve as an antioxidant. By the
standard cytochrome c/xanthine oxidase assay for SOD activity (26), a purified Cu-Atx1p did exhibit the ability to consume
superoxide, as demonstrated by the inhibition of cytochrome c reduction rate at the reaction onset (Fig.
6, slope A). The activity obtained
with 720 nM Cu-Atx1p was equivalent to that seen with
0.84 ± 0.09 nM Sod1 dimer, or 1.7 ± 0.18 nM Sod1 monomer; hence, the reaction of superoxide with
Cu-Atx1 is approximately 430-fold less efficient than with Sod1.
Moreover, with Atx1p, the rate of cytochrome c reduction
toward the later portion of the assay, fit to slope B in Fig. 6,
returned to the rate of the control sample, indicating that purified
Atx1p does not act catalytically as a dismutase in vitro.
Despite this limited activity, our calculations indicate that when
Atx1p is over-produced in vivo, the corresponding superoxide
consumption capacity may be sufficient to substitute for Sod1p (see
"Discussion").

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Fig. 6.
Superoxide scavenging by Cu(I)-Atx1p in
vitro. The nonlinear trace is typical cytochrome
c/xanthine oxidase assay containing 1.5 mM
Cu-Atx1p. A, at the onset of the kinetic assay, this slope
reflects the rate of cytochrome c reduction by superoxide in
competition with Cu-Atx1p scavenging of superoxide. B, after
the sacrificial consumption of Cu-Atx1p in the assay solution, this
slope is equivalent to the rate of cytochrome c reduction in
a control assay solution without Cu-Atx1p.
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 |
DISCUSSION |
The x-ray structure of Atx1p reveals an intriguing patch of lysine
residues along one face of the molecule, as well as an additional
lysine (Lys65) adjacent to the MXCXXC
metal binding site. We find that sequences in the metal coordination
site, including the two copper binding cysteines and the neighboring
Lys65, are necessary for both Atx1p activation of
Ccc2p/Fet3p and for its antioxidant role. In comparison, the basic face
of Atx1p (residues K24,28,61,62) is important only for copper
distribution to Ccc2p but not for the antioxidant function of Atx1p,
supporting the proposal that this region may be uniquely important for
interactions between Atx1p and Ccc2p (36). A structurally characterized
metal binding domain of the Menkes protein (19) contains an acidic face, and a number of these acidic residues are conserved in Ccc2p (36). The mutagenesis data are consistent with the hypothesis that
docking of Atx1p with Ccc2p involves electrostatic interactions between
the basic face of the metallochaperone and the corresponding acidic
face in its target protein (36).
Previously, Gitlin and co-workers reported that lysines 57 and 60 of
the HAH1 human homologue to Atx1p (corresponding to lysines 62 and 65 of Atx1p) are more critical for the antioxidant function of this
metallochaperone than for delivery of copper to Ccc2p (33). Although
these conclusions appear to be at odds with our findings, the results
of the two studies are in fact, compatible. With the crystal structure
of Atx1p now in hand, allele K57,60G of HAH1 is predicted to impinge on
both the metal binding region (Lys60) and on the basic
patch (Lys57) of the metallochaperone and should,
therefore, affect both activities of the metallochaperone. Accordingly,
Gitlin and co-workers observed a partial reduction in copper delivery
to Fet3p and complete inhibition of antioxidant activity with this
mixed allele (33).
What is the significance of the essential basic residue near the metal
binding site of Atx1p? This lysine is conserved among all known members
of the Atx1p metallochaperone family, including those from plants (3),
nematodes (6), and mammals (4, 7). In comparison, a hydrophobic
aromatic residue is present at the corresponding position in 5 of 6 Atx1-like domains in the Wilson and Menkes proteins (19) and in both of
the metal binding domains of Ccc2p (11, 36). In the case of Atx1p,
Lys65 may affect the kinetics or thermodynamics of
copper-protein interaction through hydrogen bonding to a coordinated
cysteine sulfur atom. Lys65 is predicted to partially
neutralize the net negative charge that results from a coordination of
two (or more) cysteinate anions to Cu(I) and would thereby stabilize
the copper-chaperone complex. A working hypothesis is that allosteric
conformation changes at Lys65 may occur in the docking of
the chaperone with its partner Ccc2p (11).
The studies presented here provide several lines of evidence that the
antioxidant activity of Atx1p is distinct from its ability to act as a
metallochaperone. First, the basic surface residues that are critical
for copper delivery to Ccc2p are not needed for the antioxidant
activity of Atx1p. Second, unlike other metallochaperone functions (9,
10, 32, 37), high levels of copper cannot substitute for the
antioxidant activity of Atx1p, indicating that Atx1p does not suppress
oxidative damage by delivering copper to another molecule.
Finally, we provide evidence that Atx1p itself acts as a scavenger of
superoxide anion, albeit a poor one compared with Sod1p. Yet the level
of activity observed with purified Atx1p in vitro may be
sufficient to suppress sod1
deficiency in vivo
based on the following calculations: approximately 430 molecules of
Atx1p are needed to neutralize an equivalent amount of superoxide as 1 molecule of Sod1p. We observed that Atx1p is normally present at
1.5 × 104 molecules per cell (unpublished), but when
over-produced, these levels reach ca. 5 × 105
molecules per cell (10). Because full suppression of aerobic lysine
auxotrophy is achieved with
8 × 102 molecules
of active Sod1p per cell (34),1
it is conceivable that the superoxide consumption by multi-copy Atx1p
is sufficient to substitute for Sod1p in this regard. This antioxidant
activity appears to be physiologic and not simply an artifact of
ATX1 overexpression because complete deletion of Atx1p
causes paraquat sensitivity (17). We conclude that this small copper
binding molecule has evolved with dual and separable functions that aid
in the handling of copper in aerobic environments.
 |
ACKNOWLEDGEMENT |
We thank Daniel Yuan for helpful discussions.
 |
FOOTNOTES |
*
This work was supported by the Johns Hopkins University
NIEHS Center and by National Institutes of Health Grants ES 08996 (to
V. C. C.), GM54111 (to T. V. O.); by a supplement from NIGMS, National Institutes of Health, to this same grant (to A. C. R. and
T. V. O.), and by funds from Northwestern University and the Robert
H. Lurie Cancer Center (to A. C. R).The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
§
Supported by NCI training grant T32CA 09110.
**
Supported by National Institutes of Health Post-doctoral
Fellowships GM16618 and GM19457.

To whom correspondence should be addressed: Johns Hopkins
University, 615 N. Wolfe St., Rm. 7023, Baltimore, MD 21205. Tel.: 410-955-3029; Fax: 410-955-0116; vculotta{at}jhsph.edu.
1
This calculation is based on reports that yeast
cells harbor ~4 × 104 active monomers Sod1p/cell
(37), and that
2% of the total Sod1p of a yeast cell yields wild
type levels of protection against oxidative damage (34). The lower
limit of Sod1p that is required for antioxidant protection has not been
determined but is expected to be far less than the upper limit of
8 × 102 molecules/cell.
 |
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