Tài liệu hạn chế xem trước, để xem đầy đủ mời bạn chọn Tải xuống
1
/ 13 trang
THÔNG TIN TÀI LIỆU
Thông tin cơ bản
Định dạng
Số trang
13
Dung lượng
2,54 MB
Nội dung
Metalexchangeinmetallothioneins–anovel structurally
significant Cd
5
species inthealphadomainof human
metallothionein 1a
Kelly E. Rigby Duncan, Christopher W. Kirby and Martin J. Stillman
Department of Chemistry, The University of Western Ontario, London, Canada
Cadmium is a known carcinogen that interferes with
cellular signaling and the regulation of gene expression
[1]. Metallothionein, a cysteine-rich metal-binding pro-
tein, has been shown to protect the cell from toxicity
by sequestering the cadmium ions via the cysteinyl
thiolate ligands [2–4]. Cellular response to cadmium is
dependent on the level of exposure, such that high
concentrations induce cytotoxicity whereas low to
moderate concentrations result in gene dysregulation
and uncontrollable growth. The mechanism of cad-
mium-induced toxicity is complex; however, evidence is
mounting that suggests a role for Cd
2+
in the inhibi-
tion of DNA repair processes [5]. Specifically, Cd
2+
is
thought to impair DNA damage recognition by inter-
fering with the interaction of key nucleotide excision
repair component proteins at the damage site by
Keywords
113
Cd NMR spectroscopy; circular dichroism
spectroscopy; ESI mass spectrometry;
metal exchange; metallothionein
Correspondence
M. J. Stillman, Department of Chemistry,
Chemistry Building, The University of
Western Ontario, London, ON,
Canada N6A 5B7
Fax: +1 519 661 3022
Tel: +1 519 661 3821
E-mail: martin.stillman@uwo.ca
Website: http://www.uwo.ca/chem/
(Received 7 January 2008, revised 27
February 2008, accepted 4 March 2008)
doi:10.1111/j.1742-4658.2008.06375.x
Metallothioneins (MTs) are cysteine-rich, metal-binding proteins known to
provide protection against cadmium toxicity in mammals. Metal exchange
of Zn
2+
ions for Cd
2+
ions inmetallothioneins is a critical process for
which no mechanistic or structural information is currently available. The
recombinant humanadomainofmetallothionein isoform 1a, which
encompasses the metal-binding cysteines between Cys33 and Cys60 of the
a domainof native humanmetallothionein 1a, was studied. Characteristi-
cally this fragment coordinates four Cd
2+
ions to the 11 cysteinyl sulfurs,
and is shown to bind an additional Cd
2+
ion to form anovel Cd
5
a-MT
species. This species is proposed here to represent an intermediate in the
metal-exchange mechanism. The ESI mass spectrum shows the appearance
of charge state peaks corresponding to a Cd
5
a species following addition
of 5.0 molar equivalents of Cd
2+
to a solution of Cd
4
a-MT. Significantly,
the structurally sensitive CD spectrum shows a sharp monophasic peak at
254 nm for the Cd
5
a speciesin contrast to the derivative-shaped spectrum
of the Cd
4
a-MT species, with peak maxima at 260 nm (+) and 240 nm
()), indicating Cd-induced disruption ofthe exciton coupling between the
original four Cd
2+
ions inthe Cd
4
a species. The
113
Cd chemical shift of
the fifth Cd
2+
is significantly shielded (approximately 400 p.p.m.) when
compared with the data for the Cd
2+
ions in Cd
4
a-MT by both direct and
indirect
113
Cd NMR spectroscopy. Three ofthe four original NMR peaks
move significantly upon binding the fifth cadmium. Evidence from indirect
1
H-
113
Cd HSQC NMR spectra suggests that the coordination environment
of the additional Cd
2+
is not tetrahedral to four thiolates, as is the case
with the four Cd
2+
ions inthe Cd
4
a-MT, but has two thiolate ligands as
part of its ligand environment, with additional coordination to either water
or anions in solution.
Abbreviations
MT, metallothionein; a-rhMT 1a, recombinantly prepared adomainofhumanmetallothionein isoform 1a.
FEBS Journal 275 (2008) 2227–2239 ª 2008 The Authors Journal compilation ª 2008 FEBS 2227
substitution for a Zn
2+
ion inthe four-cysteine
Zn-finger protein xeroderma pigmentosum group A
protein. This is one of many examples where Cd
2+
has
been shown to replace Zn
2+
in Zn-finger proteins,
resulting in structural alterations and ultimately func-
tional inhibition. Indeed, substitution of Cd
2+
for the
Zn
2+
ions inthe two-finger Tramtrack (TTK) peptide
reduces the affinity of this peptide for its DNA-binding
sequence [6]. However, incubation ofthe Cd-substi-
tuted peptide with Zn-substituted metallothionein
reverses the effect and restores DNA-binding ability.
Similarly, Cd
2+
coordination to the Zn-finger protein
TFIIIA was shown to inhibit DNA association at the
internal control region ofthe 5S ribosomal RNA gene;
however, metalexchange between Cd-TFIIIA and
Zn-MT resulted in reconstitution ofthe functional
Zn-finger protein. It is evident from these examples
that metallothionein is the primary defender against
Cd-induced toxicity, and that its role extends beyond
merely sequestering the ‘free’ ion upon cellular expo-
sure. Extraction ofthe Cd
2+
ion from the affected
protein results in liberation ofthe essential Zn
2+
ion
from themetallothionein pool; thus the metallothion-
ein exhibits a dual function with respect to metal
replacement.
The physiological effects described above indicate
that metalexchange or metal replacement in metallo-
thioneins is a critically important process that requires
mechanistic consideration; however, details of this pro-
cess are completely lacking. Based on examination of
the structures, we and others have previously proposed
that thedomain crevice acts as the initiation site for
the exchange reaction due to exposure of one edge of
the metal-thiolate cluster to the surrounding environ-
ment [7–9]. Upon incorporation ofthe incoming Cd
2+
ion into the metal-thiolate cluster at the crevice site,
rearrangement ofthe cluster is proposed to take place,
resulting in expulsion ofa previously coordinated
Zn
2+
ion from thedomain to reduce the stoichiometry
back to four metal ions. This mechanism would
require an intermediate that includes the metals of the
domain plus the incoming metal. However, to date, no
experimental data have been published to support this
hypothesis.
In this paper, the first structural evidence to support
the formation ofa cluster-expanded adomain is
described. CD, NMR spectroscopic and MS data show
the formation ofanovel and structurally modified
Cd
5
a-MT 1aspecies upon titration of Cd
4
a-MT 1a
with a moderate excess of Cd
2+
. The additional Cd
2+
ion is proposed to coordinate to two cysteinyl sulfurs
positioned near the crevice site ofthe domain, with the
remainder ofthe ligand sphere probably completed
with either water or chloride ions based on indirect
1
H-
113
Cd HSQC NMR data. We propose that this
cluster-expanded Cd
5
a species represents a model for
the intermediate inthe Cd ⁄ Zn metal-exchange reaction
pathway for this particular metallothionein isoform.
Results
The sequence ofthe thrombin-cleaved isolated a do-
main prepared recombinantly in Escherichia coli as an
S-tag fusion protein (herein referred to as a-rhMT 1a),
as used in this study, is shown in Fig. 1A. This
sequence encompasses the metal-binding cysteines
between Cys33 and Cys60 oftheadomainof native
human metallothionein1a but also includes amino
acids not found inthe native protein. The four diva-
lent metal ions are labeled as 1, 5, 6 and 7 in accor-
dance with the original NMR numbering for
two-domain mammalian metallothionein (2, 3 and 4
are assigned to the three divalent metal ions in the
b domain) [10,11]. The 11 cysteinyl sulfurs are labeled
according to the residue number inthe natural
two-domain humanmetallothionein1a sequence [12].
Existence ofthe Cd
4
(S
cys
)
11
species is well docu-
mented for theadomainof mammalian metallothione-
ins as the result of structural characterization using a
variety of techniques including NMR spectroscopy
[11,13] and X-ray crystallography [14]. The isolated
Cd
4
(S
cys
)
11
cluster is shown in Fig. 1B: each cadmium
ion (green sphere) coordinates tetrahedrally to four
cysteinyl sulfurs (yellow spheres) such that five of the
11 cysteinyl sulfurs act as bridging ligands between
two metal centers and the remaining six act as terminal
ligands by coordinating to a single metal center. To
date, this is the maximum structurally characterized
Cd-to-cysteine stoichiometry observed for the single
a domain. These results are based largely on studies
carried out on a variety of mammalian MT species
including rabbit, rat and human [15–17].
The numbering ofthe cadmium ions and the cyste-
inyl sulfurs in Fig. 1B correspond with those in the
sequence shown in Fig. 1A. Figure 1C shows the
space-filling and ribbon model representations of
Cd
4
a-rhMT 1a, emphasizing the wrapping ofthe poly-
peptide backbone ina left-handed coil around the
metal-thiolate cluster, which is shown inthe space-
filling model as located inthe center ofthe domain.
Metal exchangeof Zn
4
a-rhMT 1a with Cd
2+
The exchange reaction ofthe Zn-substituted a domain
with Cd
2+
was investigated by ESI mass spectrometry.
The Zn-substituted metallothionein was prepared by
A novel Cd
5
a metallothioneinspecies K. E. Rigby Duncan et al.
2228 FEBS Journal 275 (2008) 2227–2239 ª 2008 The Authors Journal compilation ª 2008 FEBS
demetallation of recombinantly isolated Cd
4
a-rhMT
1a at low pH followed by removal ofthe Cd
2+
ions
using size-exclusion chromatography. Reconstitution
with Zn
2+
was achieved by raising the pH in the
presence of stoichiometric amounts of Zn
2+
. The top
spectrum of Fig. 2 shows that the Zn
4
a species is the
sole species formed inthe reconstitution process. In all
the spectra, the measured charge states were +4 and
+5, with the +4 state predominant. Addition of 1.5
molar equivalents of Cd
2+
to the Zn
4
a sample results
in the formation of mixed-metal species, with the
Zn
3
Cd
1
a and Zn
2
Cd
2
a species predominating. The rel-
ative abundance shifted to primarily the Zn
1
Cd
3
a and
Cd
4
a species upon titration with 3.4 molar equivalents
of Cd
2+
. When 4.7 molar equivalents of Cd
2+
are
added, the Cd
4
a species is the predominant species,
indicating a near stoichiometric replacement of the
Zn
2+
ions with the incoming Cd
2+
ions ina non-
cooperative manner. This is consistent with other
reports for the Zn ⁄ Cd metal-exchange reaction [18,19].
However, titration with a moderate excess of Cd
2+
results inthe appearance ofa Cd
5
a species, which is
shown in Fig. 2 to be present as a minor contributor
upon addition of 4.7 molar equivalents, and is the
dominating species with the addition of 8.2 molar
equivalents of Cd
2+
. This newly identified Cd
5
a spe-
cies was further characterized by CD and UV absorp-
tion spectroscopy and ESI mass spectrometry by
titration of recombinantly prepared Cd
4
a-rhMT 1a
isolated directly from the E. coli source with excess
Cd
2+
.
Titration of Cd
4
a-rhMT 1a with excess Cd
2+
:CD
and ESI-MS results
The CD spectrum obtained for the Cd-coordinated
a domain as isolated from the recombinant prepara-
tion in E. coli is shown in Fig. 3A. Asignificant fea-
ture ofthe spectrum with no excess of Cd
2+
is the
biphasic, derivative-shaped signal, with positive
extrema at 260 and 220 nm and a negative extremum
at 240 nm. Many previous studies have reported this
CD spectrum as characteristic ofthe mammalian Cd
4
a
species, and it has been described as being due to exci-
ton splitting between the symmetric pairs of
[Cd(S
cys
)
4
]
2
groups inthe Cd
4
(S
cys
)
11
binding site [18–
22]. This result confirms the correct folding and
domain stoichiometry ofthe recombinantly synthesized
a domain as being the Cd
4
a species. However, closer
inspection ofthe CD spectrum reveals a poorly
defined, weak and atypical shoulder at 254 nm, indi-
cating a coexisting secondary speciesof lower abun-
dance that lacks the exciton coupling property, as a
pure Cd
4
a sample results ina point of inflection at this
wavelength. Such aspecies was found during metal
replacement and cadmium-loading experiments for
A
BC
Fig. 1. (A) Recombinant sequence oftheadomainofhuman MT 1a showing the connectivities ofthe four divalent metal cations to the 11
cysteinyl sulfurs. (B) Isolated Cd
4
(S
cys
)
11
cluster present intheadomainofhuman MT 1a. (C) Space-filling and ribbon representations of the
recombinant Cd
4
a-rhMT 1a. Numbering ofmetal ions is based on NMR numbering of mammalian MT [11] and cysteine numbering is based
on the natural human MT 1a sequence [12]. Atom legend: gray = C, blue = N, red = O, yellow = S, green = Cd.
K. E. Rigby Duncan et al. Anovel Cd
5
a metallothionein species
FEBS Journal 275 (2008) 2227–2239 ª 2008 The Authors Journal compilation ª 2008 FEBS 2229
Cd < 4 molar equivalents; however, as we show
below, the present peak is due to aspecies with
Cd > 4 molar equivalents. Based on the ESI mass
spectrometric data for the Zn ⁄ Cd metal exchange, this
species can be identified as a Cd
5
a species, and conse-
quently should increase in abundance upon titration
with excess Cd
2+
. We note similar broadening of the
CD spectrum for the Cd-substituted two-domain
ba-MT 1a as isolated from E. coli [23].
Addition of Cd
2+
to the solution of Cd
4
a, up to 5.0
molar equivalents, resulting ina total of 9.0 molar
equivalents of Cd
2+
in solution, results ina significant
shift inthe CD spectrum, leading stepwise to a mono-
phasic peak at 254 nm and a reduction in peak inten-
sity ofthe band at 223 nm to negative DA values.
Despite thesignificant change observed inthe CD
spectrum, the corresponding UV absorption spectrum
shows very little change upon addition of excess Cd
2+
(Fig. 3B). The loss of exciton coupling inthe CD spec-
trum following titration of excess Cd
2+
into the
protein sample must be due to an alteration of the
metal-thiolate cluster arrangement, which we associate
Fig. 2. ESI mass spectra recorded for the titration of Zn
4
a-rhMT 1a
with Cd
2+
at pH 7.4. Spectral changes were recorded as aliquots of
Cd
2+
(3.3 mM) were titrated into a solution of Zn
4
a-rhMT 1a (15 lM)
at 22 °C. Spectra were recorded at Cd
2+
molar equivalent values of
0.0, 1.5, 3.4, 4.7 and 8.2.
A
B
C
Fig. 3. (A) CD and (B) UV absorption spectral changes observed
upon titrating Cd
4
a-rhMT 1a with an additional 5.0 molar equiva-
lents of Cd
2+
at pH 7.4 and 22 °C. (C) The ratio of CD peak inten-
sity at 254 nm per 264 nm versus molar equivalents of Cd
2+
added
to a sample of Cd
4
a-rhMT 1a as a measure of Cd
5
a-rhMT 1a
species formation.
A novel Cd
5
a metallothioneinspecies K. E. Rigby Duncan et al.
2230 FEBS Journal 275 (2008) 2227–2239 ª 2008 The Authors Journal compilation ª 2008 FEBS
with loss of symmetry ofthe Cd
4
a structure. This spec-
trum is reminiscent ofthe CD spectrum recorded for
the adomain with up to three Cd
2+
ions [18,24].
Retention ofthe overall CD envelope shape indicates
that no significant changes inthe wrapping of the
polypeptide backbone are induced by the additional
Cd
2+
. We propose that titration of excess Cd
2+
alters
only the metal-thiolate cluster stoichiometry to give
the lower-symmetry Cd
5
a species (see below).
Cycling the pH from neutral to acidic and then back
to neutral pH results in demetallation and subsequent
metallation ofthe protein, which can be monitored by
both CD spectroscopy and ESI mass spectrometry.
This reaction sequence, when applied to the Cd
4
a sam-
ple, results in restoration ofthe Cd
4
stoichiometry
upon raising the pH from 2 to 7 (data not shown).
The presence of an additional 5.0 molar equivalents of
Cd
2+
(total ratio of Cd
2+
: MT of 9.0) results in for-
mation ofthe Cd
5
a species, which also reforms repro-
ducibly upon cycling the pH (data not shown).
A plot ofthe ratio DA
254
⁄ DA
264
from the CD spec-
trum approximates the ratio of Cd
5
a ⁄ Cd
4
a. Figure 3C
shows that up to 5.0 additional molar equivalents are
required for nearly all themetallothioneinspecies to
be converted into the Cd
5
a form.
The ESI mass spectrum obtained for the purified
Cd-coordinated adomain as isolated from the recom-
binant preparation in E. coli is shown in Fig. 4A. The
measured charge state distribution ranges from +3 to
+5, with the +4 charge state as the predominant
peak. Reconstruction ofthe mass spectrum results in a
single, principal species with a measured mass of
4526.4 Da, corresponding to the Cd
4
a-rhMT species
(calculated mass 4524.6 Da). Closer inspection of the
original mass spectrum shows the presence ofa minor
peak with a measured m ⁄ z of 927, corresponding to
the +5 charge state ofa Cd
5
a-rhMT species. This
result confirms the existence ofa Cd
5
a species as a
minor contributor to the equilibrium ofthe Cd-coordi-
nated adomainofhuman MT 1a.
Addition of 5.0 molar equivalents of Cd
2+
to the
Cd
4
a-rhMT 1a solution results inthe ESI mass spec-
trum shown in Fig. 4B. Peaks corresponding to the
Cd
4
a-rhMT species are no longer detected, and
instead a new set of peaks are observed that are
consistent with the formation of 100% Cd
5
a-rhMT
species with a reconstructed mass of 4633.2 Da (cal-
culated mass 4635.0 Da). The measured charge state
distribution remains +3 to +5, with the +4 charge
state predominating; however, the relative abundance
of the +5 charge state has increased significantly
compared to the corresponding peak inthe mass
spectrum ofthe Cd
4
a-rhMT species. Previous ESI-MS
studies of globular proteins have shown a correlation
between the observed charge state distribution and
the solution polypeptide conformation [25–27]. In the
case of metallothionein, Palumaa et al. have reported
ESI-MS data showing a higher charge state distribu-
tion for the Cd
4
a domainofhuman MT 3 compared
with the Zn
4
a MT 3 by one unit [28]. Similarly, the
Cd
3
b domainofhuman MT 3 was shown to be sig-
nificantly more open in conformation compared with
the Zn
3
b domainof MT 3, indicating non-isostructur-
al replacement of Cd
2+
for Zn
2+
in this particular
MT isoform. This charge state distribution was inter-
preted by the authors as being due to a slight open-
ing ofthe polypeptide backbone to accommodate the
slightly larger cadmium ions. These data strongly sup-
port our interpretation that the increased relative
abundance ofthe higher +5 charge state species
observed inthe ESI mass spectrum ofthe Cd
5
a pre-
sented here compared with the Cd
4
a species is due to
expansion ofthemetal binding domain to accommo-
date the fifth Cd
2+
. This suggests that the additional
Cd
2+
ion is inserted into the core ofthe domain,
becoming part of an expanded metal-thiolate cluster.
113
Cd NMR spectroscopy was therefore used to probe
the metal-thiolate cluster arrangement inthe newly
identified Cd
5
a species.
A
B
Fig. 4. (A) ESI mass spectrum observed for the Cd-coordinated
a-rhMT 1a following isolation and purification ofthe recombinant
protein from E. coli. Reconstruction ofthe mass spectrum results
in a measured mass of 4526.4 Da corresponding to the Cd
4
a-rhMT
species (calculated mass 4524.6 Da). (B) ESI mass spectral
changes observed upon titrating the Cd
4
a-rhMT 1a sample from (A)
with an additional 5.0 molar equivalents of Cd
2+
at pH 7.4 and
22 °C. The reconstructed mass for Cd
5
a-rhMT 1a was 4633.2 Da
(calculated mass 4635.0 Da).
K. E. Rigby Duncan et al. Anovel Cd
5
a metallothionein species
FEBS Journal 275 (2008) 2227–2239 ª 2008 The Authors Journal compilation ª 2008 FEBS 2231
Titration of Cd
4
a-rhMT 1a with excess Cd
2+
:
113
Cd
NMR results
113
Cd NMR spectroscopy was used in this study to
further investigate the nature ofthe metal-thiolate
binding site intheadomainofhuman MT 1a both
before and after the addition of excess Cd
2+
.
Direct 1D
113
Cd NMR (
1
H-decoupled) spectroscopic
techniques to probe for formation ofa novel
Cd
5
a-rhMT 1a species
The 1D
113
Cd NMR (
1
H-decoupled) spectrum of
Cd
4
a-rhMT 1a as isolated directly from recombinant
overexpression in E. coli and prepared in 10 mm
Tris ⁄ HCl buffer (pH 7.4) is shown in Fig. 5A. The
natural isotopic abundance of
113
Cd was used in this
experiment, despite the low value of 12.26%, in order
to observe the naturally occurring speciation. Six sig-
nals were observed inthe Cd
4
a-rhMT 1a spectrum at
670, 633, 630, 626, 611 and 599 p.p.m. The chemical
shift values ofthe five most deshielded peaks observed
in the NMR spectrum are inthe range of 670 to
611 p.p.m., and are in agreement with those reported
previously for the four cadmium ions inthea domain
of the native two-domain human MT isoform 1 [10].
These five peaks are labeled in Fig. 5A as 1, 5, 5¢,6
and 7, respectively, in accordance with the original
NMR numbering assignments. Splitting ofthe peak
assigned to themetalin site 5 has been noted previ-
ously and is attributed to heterogeneity in that particu-
lar site inthe metal-thiolate cluster [10].
The sixth peak observed at 599 p.p.m. is more
shielded than the other peaks and has not been
reported previously for human MT 1 isoforms. Based
on the ESI-MS and CD spectroscopic results, this peak
is predicted to be due to the Cd
5
a-rhMT species, which
has been shown in this report to be a minor contribu-
tor to the equilibrium together with the Cd
4
a-rhMT
species.
Fig. 5. (A) Direct 1D
113
Cd NMR (
1
H-decoupled) spectrum (133 MHz) for a-rhMT 1a following isolation and purification ofthe recombinant
protein from E. coli with the natural isotopic abundance of
113
Cd, showing primarily the
113
Cd
4
a-rhMT 1a species. (B) Direct 1D
113
Cd
NMR (
1
H-decoupled) spectrum (133 MHz) for isotopically labeled
113
Cd
4
a-rhMT 1a titrated with an additional 10.0 molar equivalents of
113
Cd
2+
to form
113
Cd
5
a-rhMT 1a. The spectrum of
113
Cd
5
a-rhMT 1a (B) is a combination of two separate spectra acquired inthe regions
585–705 p.p.m. and 220–245 p.p.m. Samples were prepared in 10 m
M Tris ⁄ HCl pH 7.4, and buffer-exchanged into 10% D
2
O for the Cd
4
a-
rhMT 1a sample and > 70% D
2
O for the
113
Cd
5
a-rhMT 1a sample. All spectra were acquired at 25 °C.
A novel Cd
5
a metallothioneinspecies K. E. Rigby Duncan et al.
2232 FEBS Journal 275 (2008) 2227–2239 ª 2008 The Authors Journal compilation ª 2008 FEBS
A1D
113
Cd NMR (
1
H-decoupled) spectrum of iso-
topically enriched
113
Cd
4
a-rhMT 1a titrated with an
additional 10.0 molar equivalents of
113
Cd
2+
(total
ratio of Cd
2+
: MT of 14.0) is shown in Fig. 5B. For
easier viewing, the spectrum is divided into two parts,
the region on the left covers the chemical shift range
585–705 p.p.m. and that on the right covers the range
215–245 p.p.m. There are five relatively sharp signals
at 685, 647, 630, 599 and 224 p.p.m. The peak detected
at 599 p.p.m. confirms the presence ofa small popula-
tion ofthe Cd
5
a speciesinthe naturally isolated Cd
4
a
sample, as this peak was observed inthe 1D spectrum
of this sample. The four most deshielded peaks
detected between 600–700 p.p.m. inthe Cd
5
a spectrum
are assigned to the four
113
Cd
2+
ions that are known
to bind to theadomainof mammalian metallothion-
ein ina tetrahedral coordination to four thiolate
ligands. The relative assignment of these peaks to the
four cadmium sites is comparable to that of the
113
Cd
4
a-rhMT speciesin that the order is 1, 5, 6 and 7
for the peaks 685, 647, 630 and 599, respectively. How-
ever, the observed chemical shifts have changed signifi-
cantly with the addition ofthe fifth Cd
2+
ion. Peaks 1,
5 and 6 have shifted upfield by 15, 14 and 3 p.p.m.,
respectively, while peak 7 has shifted downfield by
12 p.p.m. In addition, the peaks labeled 5 and 5¢ in the
spectrum of Cd
4
a-rhMT 1a (Fig. 5A) have collapsed
into a single peak inthe spectrum acquired with excess
113
Cd
2+
(Fig. 5B), indicating a loss of heterogeneity at
that site inthe metal-thiolate cluster. This is expected
if a fifth Cd
2+
ion results in strain inthe binding site,
reducing fluxionality ofthemetal cluster.
The signal detected at 224 p.p.m. is assigned to the
additional fifth Cd
2+
ion, confirming the CD spectro-
scopic and mass spectrometric data regarding the for-
mation ofa Cd
5
a species. This peak is significantly
shielded compared to the other four peaks inthe spec-
trum (approximately 400 p.p.m.), indicating that the
coordination environment around this additional Cd
2+
is not tetrahedral to four thiolate ligands. A previous
study reporting the chemical shifts of inorganic cad-
mium(II)-thiolate complexes correlated signals with
chemical shifts inthe region of 224 p.p.m. with octahe-
dral complexes ofthe form Cd(RS)
2
(OH
2
)
4
[29,30].
Although the current data do not provide enough
information to verify this exact ligand field assignment,
as chloride ions are equally as likely as water molecules
to participate as ligands, it does provide support for
two thiolate groups acting as ligands for the fifth Cd
2+
ion and an increase in coordination number from four
to six. This proposed ligand field assignment suggests
partial insertion ofthe fifth Cd
2+
ion into the metal-
thiolate cluster ina manner that allows solvent access.
Given this restriction, the most likely position for the
fifth Cd
2+
ion is the crevice site ofthedomainin which
a number ofthe cysteinyl sulfurs that make up the
metal-thiolate cluster are solvent-exposed. To further
explore this possibility, indirect 2D NMR methods
were employed as a means of probing the coordination
environment around the additional Cd
2+
ion, with par-
ticular emphasis on identifying the two cysteinyl sulfurs
that are proposed to ligate the fifth Cd
2+
ion.
Indirect 2D
1
H–
113
Cd NMR spectroscopic techniques
for identification ofthe binding site for the fifth Cd
2+
ion inthe Cd
5
a-rhMT 1a cluster
The indirect 2D
1
H–
113
Cd NMR approach exploits the
3
J scalar coupling between the cysteine b protons and
the coordinated cadmium ions as a means of mapping
out the metal-thiolate cluster connectivities and identi-
fying the cysteine residues that are coupled to the fifth
Cd
2+
ion. By focusing on the
1
H chemical shift range
of 2.3–3.6 p.p.m., corresponding to the cysteine H
b
protons only, the tetrathiolate connectivities of the
Cd(S
cys
)
4
units can be identified. Furthermore,
sequence assignment ofthe cysteine residues is possible
through identification of bridging versus terminal
cysteine ligands inthe 2D spectrum.
Two-dimensional
1
H–
113
Cd HSQC NMR spectra
were acquired for the
113
Cd
5
a-rhMT 1a formed by
titration of
113
Cd
4
a-rhMT 1a with an additional 10.0
molar equivalents of
113
Cd
2+
(total ratio of
Cd
2+
: MT of 14.0). Figure 6 shows a combination of
two separate spectra acquired in the
113
Cd chemical
shift ranges of 585–705 p.p.m. and 215–245 p.p.m. to
allow visualization ofthe correlations between all five
Cd
2+
ions inthe Cd
5
a cluster with the 11 cysteinyl sul-
fur residues. The H
b
–
113
Cd
3
J scalar couplings were set
to 66 and 40 Hz for acquisition between 585 and
705 p.p.m. and 215 and 245 p.p.m., respectively. Four
strong peaks were observed in the
113
Cd dimension of
the 2D spectrum of
113
Cd
5
a-rhMT 1ainthe range
585–705 p.p.m. (Fig. 6), at chemical shift values that
are in agreement with those observed inthe 1D
113
Cd
NMR (
1
H-decoupled) spectrum for the Cd
5
a species
(Fig. 5B, sites 1, 5, 6 and 7). The fifth Cd
2+
peak was
observed inthe 2D spectrum acquired in the
113
Cd
chemical shift range of 215–245 p.p.m., with an exact
chemical shift of 224 p.p.m., which is also in agree-
ment with the observed chemical shift inthe 1D
113
Cd
NMR (
1
H-decoupled) spectrum (Fig. 5B, site X).
Identification ofthe specific bridging versus terminal
cysteine residues is possible in this
113
Cd-decoupled
spectrum, and a nearly complete assignment of the
cysteine residues inthe 2D spectrum has been
K. E. Rigby Duncan et al. Anovel Cd
5
a metallothionein species
FEBS Journal 275 (2008) 2227–2239 ª 2008 The Authors Journal compilation ª 2008 FEBS 2233
accomplished. The bridging cysteines are identified by
a solid line in Fig. 6, because a single H
b
chemical shift
correlates to two different
113
Cd atoms. The numbers
written beside each peak in Fig. 6 correspond to the
sequence number ofthe cysteine residues as labeled in
Fig. 1A. This assignment is the most probable solution
that satisfies the known connectivities inthe metal-
thiolate cluster (Fig. 1A,B).
Although the H
b
–
113
Cd correlations are known for
the four tetrahedral thiolate-coordinated
113
Cd
2+
ions,
the unknown correlations of interest are those of the
fifth Cd
2+
. The two peaks correlating to the fifth
Cd
2+
at 224 p.p.m. were observed at
1
H chemical shift
values of 2.97 and 3.59 p.p.m., which are consistent
with the H
b
chemical shifts of cysteine residues 34 and
either 57 or 59, respectively, as shown by the dotted
lines in Fig. 6. This result substantiates our interpreta-
tion of cluster expansion to a Cd
5
(S
cys
)
11
species upon
titration with excess Cd
2+
, as opposed to the fifth
Cd
2+
ion attaching as an adduct to the surface of the
domain. The detection of only two correlations also
confirms that the coordination sphere ofthe fifth
Cd
2+
ion includes two cysteine residues as predicted
by the highly shielded chemical shift [30]. Figure 7
shows a space-filling model of Cd
4
a-rhMT 1a with a
view ofthe crevice site showing the exposed edge of
the metal-thiolate cluster. Cys34 is one ofthe sulfur
atoms exposed in this site (highlighted in purple),
which supports the NMR results indicating that this
sulfur atom is involved in coordination ofthe fifth
Cd
2+
ion. Cys57 and Cys59 are not present inthe cre-
vice site, so it is not immediately obvious how the sec-
ond sulfur is involved inthe coordination. One could
envision a potential cluster rearrangement upon coor-
dination to Cys34 that brings Cys57 or Cys59 into
position for coordination.
Discussion
The in vitro reactivity ofmetallothionein with Cd
2+
has been well documented by reports on the native
1
H (p.p.m.)
113
CD (p.p.m.)
Fig. 6. A combination of two indirect 2D
1
H–
113
Cd HSQC NMR spectra for isotopically enriched
113
Cd
4
a-rhMT 1a titrated with an additional
10.0 molar equivalents of
113
Cd
2+
to form the
113
Cd
5
a-rhMT 1a species. The spectra were recorded in the
1
H chemical shift range 2.3–
3.7 p.p.m. and the
113
Cd ranges 590–690 p.p.m. (
3
J = 66 Hz) and 220–245 p.p.m. (
3
J = 40 Hz). Both spectra were acquired at 25 °C using
an inverse single-axis z-gradient HCX probe with X tuned to
113
Cd.
Fig. 7. Space-filling model of Cd
4
a-rhMT 1ain two orientations
rotated by 90°, emphasizing the crevice site on the domain. One of
the proposed ligands ofthe fifth Cd
2+
ion, Cys34, is highlighted by
the arrow and the sulfur atom of this residue is shown in purple.
Atom legend: gray = C, blue = N, red = O, yellow = S, green = Cd.
A novel Cd
5
a metallothioneinspecies K. E. Rigby Duncan et al.
2234 FEBS Journal 275 (2008) 2227–2239 ª 2008 The Authors Journal compilation ª 2008 FEBS
and ⁄ or recombinant two-domain protein, in addition
to the isolated fragments, from many mammalian spe-
cies including human, rat, rabbit and mouse [18,19,
31–33]. These spectroscopic studies involve reaction of
Zn-containing or metal-free forms of metallothionein
with sub-stoichiometric, stoichiometric or excess
amounts of Cd
2+
. Significantly, titration of Zn
7
-MT
with sub-stoichiometric molar equivalents of Cd
2+
resulted ina gradual red shift ofthe charge-transfer
band inthe CD spectrum, indicating mixed-metal
speciation before saturation with seven Cd
2+
ions.
This result is consistent with the isolation of the
mixed-metal Cd
5
Zn
2
-MT species from in vivo sources
such as rabbit liver upon exposure of these animals to
Cd
2+
, indicating a non-cooperative mechanism of
metal replacement [19]. The available X-ray, NMR
and CD data are consistent with the Cd
4
(S
cys
)
11
cluster
in theadomain being adamantane-like in structure.
Thus, the Cd
4
a domain observed from many mamma-
lian species is characterized by a derivative-shaped CD
signal and a distinct NMR spectrum.
Despite the highly conserved positioning of the
cysteine residues inthe mammalian sequence of metal-
lothionein and the structural consistencies, the behav-
ior of Cd
7
ba-MT towards excess Cd
2+
has been
shown to differ depending not only on thespecies but
also on the isoform or sub-isoform ofthe protein
within a particular species. Mouse MT 1, the sequence
of which is shown in Fig. 8, has been shown by CD
spectroscopy to have the capability of expanding
beyond the typical seven Cd
2+
ions per metallothion-
ein molecule [31,33]. Significant changes inthe CD
spectrum ofthe isolated b domainof mouse MT 1
when binding additional Cd
2+
were interpreted by the
authors as the result ofa Cd-induced, rearranged
peptide conformation. Unfortunately there were no
mass spectral data to confirm the actual number of
Cd
2+
ions bound. Reaction ofthe isolated a domain
of mouse MT 1 with excess Cd
2+
did not induce a
change inthe CD spectroscopic profile, indicating that
this cluster did not expand beyond the Cd
4
(S
cys
)
11
stoi-
chiometry. The Cd-substituted two-domain human
MT 3 has been shown to coordinate additional Cd
2+
ions with stoichiometries of up to Cd
13
ba-MT; how-
ever, species with more than nine equivalents of Cd
2+
are reported as probably being due to adducts on the
surface ofthe protein. The additional two Cd
2+
ions
leading to the Cd
8
ba-MT and Cd
9
ba-MT species of
human MT 3 are proposed to bind into the cluster
regions; however, the ESI-MS results reported did not
provide the structural information necessary for locali-
zation of these metal ions [28,32,34]. ESI mass spectral
data have been reported for the single adomain of
human MT 2, prepared as a synthetic peptide, in the
presence of excess Cd
2+
, in which Cd
5
a was detected
as a minor species [35]. However, structural data on
this species were not provided, leaving open the possi-
bility that the fifth Cd
2+
acts as an adduct, as is com-
monly observed in mass spectroscopy and would lead
to the increased mass found inthe ESI mass spectrum.
Interestingly, thestructurally sensitive CD spectro-
scopic data reported for rabbit liver MT 2a and rat
liver MT 2, isolated from natural sources, showed no
change inthe CD spectrum upon addition of excess
Cd
2+
, indicating that these particular metallothionein
species do not have the capability to expand to larger
metal clusters at reasonably low levels of excess Cd
2+
[18,36]. ESI mass spectral data for the two-domain
rabbit MT 1a also indicated that this protein was satu-
rated at seven Cd
2+
ions, similar to the data for the
rabbit MT 2a isoform [34].
While ESI-MS data have been reported previously
that do show Cd
2+
binding greater than seven for the
two-domain ba full chain ofhuman MT 1a [23], no
structural data have been provided to indicate the sig-
nificance ofthe ‘over-loaded’ species. The experimental
data presented here show unambiguously that a Cd
5
a
species of recombinantly prepared human MT 1a is
Fig. 8. Sequence comparison ofhuman MT 1a with other isoforms ofhumanmetallothionein as well as metallothionein from other mamma-
lian species for which spectroscopic data have been reported.
K. E. Rigby Duncan et al. Anovel Cd
5
a metallothionein species
FEBS Journal 275 (2008) 2227–2239 ª 2008 The Authors Journal compilation ª 2008 FEBS 2235
formed that involves ametal binding site that is differ-
ent from the Cd
4
a site. All three techniques used pro-
vide specific information. First, the ESI-MS data
confirm the increased metal binding stoichiometry and
indicate a slight unwinding ofthe peptide backbone.
Second, the CD spectroscopic data show a disruption
of the exciton coupling with molar equivalents of
Cd
2+
> 4, confirming disruption ofthe Cd
4
binding
site. Finally, the NMR data verify the coordination of
the fifth Cd
2+
ion by thiolate ligands within the clus-
ter, and, more specifically, within the crevice site of the
protein.
The biological significance ofthe Cd
5
a species is an
interesting question. We propose that the Cd
5
a species
described here may be a model ofthe intermediate in
the Cd ⁄ Zn metal-exchange reaction, a critically impor-
tant process for cadmium detoxification. However, the
data presented here only apply to thehuman metallo-
thionein isoform 1a, although a number of reports of
‘over-loaded’ metallothioneins for other mammalian
species are noted above. The ESI mass spectral data
obtained for the replacement reaction ofthe Zn-substi-
tuted adomain with Cd
2+
(Fig. 2) show a range of
mixed-metal species with a stoichiometry of no more
than four metal ions in any given species. The total
loading of four divalent metal ions that is observed
until all ofthe Zn
2+
ions have been replaced can be
explained in terms ofthe relative binding constants of
the two metals. The incoming metal by necessity has a
higher binding constant than the metals already pres-
ent, so the five-metal intermediate, when both Cd
2+
and Zn
2+
are present, is short-lived as Zn is immedi-
ately displaced. Inthe case ofhuman MT 1a, when
four Cd
2+
ions are bound, we propose that the fifth
Cd
2+
ion is simply trapped at theexchange site as no
exchange can take place.
Experimental procedures
Materials
The chemicals used were cadmium sulfate (Fisher Scientific,
Ottawa, Canada), cadmium(113) chloride (Trace Sciences
International Inc., Richmond Hill, Canada), deuterium
oxide (Cambridge Isotopes Laboratories Inc., Andover,
MA, USA), ultrapure Tris buffer, tris(hydroxymethyl)ami-
nomethane (ICN Biomolecules, Irvine, CA, USA), zinc
sulfate (Caledon Laboratory Chemicals, Georgetown,
Canada), ammonium formate buffer (Sigma-Aldrich,
Oakville, Canada), ammonium hydroxide (BDH Chemi-
cals ⁄ VWR, Mississauga, Canada), formic acid (J. T. Baker
Chemical Co., Phillipsburg, NJ, USA) and hydrochloric
acid (Caledon Laboratory Chemicals). All solutions were
produced using > 16 MWÆcm deionized water (Barnstead
Nanopure Infinity, Van Nuys, CA, USA). HiTrapÔ SP HP
ion-exchange columns (Amersham Biosciences ⁄ GE Health-
care, Piscataway, NJ, USA), superfine G-25 Sephadex
(Amersham Biosciences), a stirred ultrafiltration cell (Am-
icon Bioseparations ⁄ Millipore, Bedford, MA, USA) with
YM-3 membrane (3000 molecular weight cut-off) and a Mi-
crocon YM-3 centrifugal filtration device (Amicon Biosepa-
rations ⁄ Millipore) were used inthe protein purification
steps.
Protein preparation
The recombinant adomainofhumanmetallothionein 1a
(sequence shown in Fig. 1A) was produced by overexpres-
sion in E. coli BL21(DE3) cells as an S-tag fusion protein
in the presence of Cd
2+
as described previously [24]. Fol-
lowing isolation and purification, the S-tag was cleaved
from the protein by incubation with thrombin.
Metal exchangeof Zn
4
a-rhMT 1a with Cd
2+
Metal-free apo-a-rhMT was prepared by eluting the throm-
bin-cleaved Cd-bound protein from a G-25 column equili-
brated with deionized water that had been pH-adjusted
with HCOOH to pH 2.8. Elution ofthe protein using an
eluant of low pH effectively removes themetal ions from
the protein, which are then separated from the protein band
as a result ofthe size-exclusion processes. Preparation of
apo-MT by this method simultaneously desalts the solution
by the same size-exclusion process. The metal-free protein
was reconstituted by adding 4.0 molar equivalents of Zn
2+
(3.0 mm stock solution) and increasing the pH to 7.4. The
a-rhMT solution was determined to have a concentration
of 15 lm based on UV absorption at 220 nm
(e
220
= 40 000 LÆmol
)1
Æcm
)1
) and atomic absorption spec-
troscopy following complete metallation with Zn
2+
. Cad-
mium solutions were prepared in 25 mm ammonium
formate pH 7.4 (for MS studies) to a final concentration of
3.3 mm as determined by atomic absorption spectroscopy.
The final samples were thoroughly evacuated and argon-
saturated to remove the bulk ofthe oxygen from the solu-
tions in order to deter oxidation ofthe protein.
Cd
2+
was added incrementally to the solution of Zn
4
a to
8.2 molar equivalents, with thorough mixing after each
titration. Mass spectra were acquired at each addition after
a 2–5 min delay to allow the reaction to reach equilibrium
conditions. Mass spectra were acquired on a Micromass
LCT ESI-TOF mass spectrometer (Waters Micromass,
Mississauga, Canada) at room temperature (22 °C), and
recorded using the mass lynx software package version
4.0. The ESI-TOF spectrometer was calibrated with a solu-
tion of NaI. The scan conditions for the spectrometer were:
capillary, 3000.0 V; sample cone, 39.0 V; RF lens, 450.0 V;
A novel Cd
5
a metallothioneinspecies K. E. Rigby Duncan et al.
2236 FEBS Journal 275 (2008) 2227–2239 ª 2008 The Authors Journal compilation ª 2008 FEBS
[...]... K-S, Onosaka S & Tanaka K (1985) Synthesis ofa nonacosapeptide (betafragment) corresponding to the N-terminal sequence 1–2 9 ofhuman liver metallothionein II and its heavy metal- binding properties FEBS Lett 183, 37 5–3 78 18 Stillman MJ, Cai W & Zelazowski AJ (1987) Cadmium binding to metallothioneinsDomain specificity in reactions ofa and b fragments, apometallothionein, and zinc metallothionein with... properties ofmetallothionein Met Ions Biol Syst 15, 21 3–2 73 23 Chan J, Huang Z, Watt I, Kille P & Stillman MJ (2007) Characterization ofthe conformational changes in recombinant humanmetallothioneins using ESI-MS and molecular modeling Can J Chem 85, 89 8–9 12 24 Rigby DuncanKE & Stillman MJ (2007) Evidence for non-cooperative metal binding to thealphadomainofhumanmetallothionein FEBS J 274, 225 3–2 261... 113Cd 4a- rhMT 1a NMR sample The 113CdCl2 was added to the NMR sample in aliquots of 0.5, 1.0, 5.0 and 10.0 molar equivalents, and the resultant solution monitored by CD spectroscopy Following addition ofthe final aliquot of 113 CdCl2, the NMR sample was argon-saturated and sealed for analysis All ofthe NMR spectra were acquired on a Varian Inova 600 NMR spectrometer using vnmrj 1.1D software (Varian Canada... Binding excess cadmium(II) to Cd7 -metallothionein from recombinant mouse Zn7 -metallothionein 1 UV-VIS absorption and circular dichroism studies and theoretical location approach by surface accessibility analysis J Inorg Biochem 68, 15 7–1 66 Palumaa P, Eriste E, Njunkova O, Pokras L, Jornvall H & Sillard R (2002) Brain-specific metallothionein- 3 has higher metal- binding capacity than ubiquitous metallothioneins. .. metallothioneins and binds metals noncooperatively Biochemistry 41, 615 8–6 163 Capdevila M, Cols N, Romero-Isart N, GonzalezDuarte R, Atrian S & Gonzalez-Duarte P (1997) Recombinant synthesis of mouse Zn3-beta and Zn4 -alpha metallothionein 1 domains and characterization of their cadmium(II) binding capacity Cell Mol Life Sci 53, 68 1–6 88 Palumaa P, Eriste E, Kruusel K, Kangur L, Jornvall H & Sillard R (2003) Metal. .. binding to brain-specific metallothionein- 3 studied by electrospray ionization mass spectrometry Cell Mol Biol 49, 76 3–7 68 Dabrio M, Vyncht GV, Bordin G & Rodriguez AR (2001) Study of complexing properties ofthealpha and beta metallothionein domains with cadmium and ⁄ or zinc using electrospray ionization mass spectrometry Anal Chim Acta 435, 31 9–3 30 Zelazowski AJ, Szymanska JA, Law AYC & Stillman... originÒ version 7.0383 (OrginLab Corp., Northampton, MA, USA) The CD spectra are measured as DA, which required conversion ofthe measured ellipticity in degrees divided by a factor of 33 UV spectra were acquired using a Cary 5G UV-Vis-NIR spectrophotometer (Varian Canada, Mississauga, Canada) ina 1 cm quartz cuvette at room temperature (22 °C) and recorded using the Cary win uv scan software application... themetal clusters in rabbit liver metallothionein Proc Natl Acad Sci USA 77, 709 4–7 098 12 Richards RI, Heguy A & Karin M (1984) Structural and functional analysis ofthehumanmetallothionein1A gene: differential induction by metal ions and glucocorticoids Cell 37, 26 3–2 72 FEBS Journal 275 (2008) 222 7–2 239 ª 2008 The Authors Journal compilation ª 2008 FEBS K E Rigby Duncan et al 13 Otvos JD & Armitage... on a Jasco J810 spectropolarimeter (Jasco, Easton, MD, USA) ina 1 cm quartz cuvette at room temperature (22 °C) and recorded using spectra manager version 1.52.01 software (Jasco) The wavelength range of 20 0–3 00 nm was scanned continuously at a rate of 50 nmÆmin)1 with a band width of 2 nm All spectra were baseline-corrected using 10 mm Tris ⁄ HCl The spectral data were organized and plotted using... was Anovel Cd 5a metallothioneinspecies added as the sulfate salt dissolved in 25 mm ammonium formate buffer (pH 7.4) to a final concentration of 3.3 mm Mass spectra were acquired on a Micromass LCT ESI-TOF mass spectrometer (Waters Micromass) at room temperature (22 °C) and recorded using the mass lynx software package version 4.0 The observed spectra were reconstructed using the max ent 1 program . Metal exchange in metallothioneins – a novel structurally
significant Cd
5
species in the alpha domain of human
metallothionein 1a
Kelly E. Rigby Duncan,. in Fig. 1A. This
sequence encompasses the metal- binding cysteines
between Cys33 and Cys60 of the a domain of native
human metallothionein 1a but also includes