Department of Biology, University of Texas at
Arlington, Arlington, Texas 76019,1 and
Department of Pharmacology, School of Medicine, Case Western
Reserve University, Cleveland, Ohio 44106-49652
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TEXT |
The CorA Mg2+
transporter was originally identified phenotypically in
Escherichia coli (10, 12) and has been cloned,
sequenced, and characterized from Salmonella typhimurium
(4, 5, 13). In these organisms, a single corA
locus constitutively expresses a polypeptide of 316 amino acids with a
large N-terminal periplasmic domain of about 240 amino acids followed
by three transmembrane domains at the C terminus. CorA mediates the
influx of Mg2+ with a Ka of about 20 µM extracellular Mg2+. Influx of Ni2+ and
Co2+ is also supported by CorA albeit at extracellular
concentrations that are immediately toxic to the cell. At very high
extracellular Mg2+ concentrations (>1 mM), CorA also
mediates Mg2+ efflux. No other genetic locus has been
identified as being necessary for Mg2+ influx via CorA; in
contrast, the efflux seen at high extracellular Mg2+
concentrations is altered by mutations in the corB,
corC, and corD loci and is abolished in a strain
carrying mutations at all three loci (2). Two additional
Mg2+ influx systems in S. typhimurium, MgtA and
MgtB, are sibling P-type ATPases with no sequence similarities to CorA.
Both are highly regulated by Mg2+ concentration in the
growth medium and under normal laboratory growth conditions are almost
completely repressed by the Mg2+ content of the medium
(4, 16, 17).
Work by Silver and colleagues has shown that CorA-like Mg2+
uptake systems are present in E. coli, Bacillus
subtilis, and Rhodobacter capsulatus (6, 11,
12). Using both genomic Southern blot analysis and PCR followed
by Southern blot analysis, we showed that corA or a similar
gene was present in a wide variety of gram-negative and gram-positive
bacteria (14); these results suggest that CorA forms the
dominant Mg2+ transport system of the domain
Bacteria. Recently, several complete microbial genomic
sequences have become available including examples within both the
Bacteria and the domain Archaea. A CorA homolog has been shown to be present in all of these sequences to date except
for those for Mycoplasma genitalium, Mycoplasma
pneumoniae, and Borrelia burgdorferi, confirming the
ubiquity of this class of Mg2+ transporter. However,
comparison of these CorA-like proteins suggests that the major sequence
conservation is within the second and third transmembrane domains
(8, 13). The question therefore arises whether this sequence
conservation reflects a functional similarity. We tested this
possibility by comparing the Mg2+ transport properties of
the putative CorA homolog of the archaeon Methanococcus
jannaschii (MJ-CorA) and the S. typhimurium CorA (ST-CorA).
The M. jannaschii corA gene complements a growth defect
in S. typhimurium.
The genomic sequence of M. jannaschii (1) contains an open reading frame
(accession no. L77117) encoding a polypeptide with sequence similarity
to the ST-CorA (13). The amino acid sequences share a low
identity of about 22% and a similarity of 23% (Fig.
1). Both polypeptides are highly charged,
with predicted pIs of 4.59 and 5.02 for ST-CorA and MJ-CorA,
respectively. In addition, the alignment shown in Fig. 1 reveals 15 sites within the polypeptides with charge reversals. The large majority
of these are changes to positive charges in MJ-CorA, accounting for the
difference in pI. Optimal growth conditions for the marine archaeon
M. jannaschii are reported to be 85°C at >200 atm of pressure in seawater (7). Of relevance is that the
concentration of Mg2+ in seawater is generally about 55 mM.
Moreover, archaeal membrane lipid composition is markedly different
than that in bacteria, with phospholipids containing ether rather than
fatty acyl linkages. Thus it was possible that the relatively low level
of sequence similarity of these two proteins did not reflect a
functional similarity.
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FIG. 1.
Sequence alignment of MJ-CorA and ST-CorA. The two
polypeptides were aligned by eye by using the following sets of amino
acid similarities (with C and P each having no similar amino acids): G,
A, S, and T; I, L, V, and M; H, R, and K; D, E, N, and Q; and F, Y, and
W. The three C-terminal transmembrane domains of CorA (13)
are underlined. Colons indicate identical amino acids, and periods
indicate similar amino acids. S. typh., ST-CorA; M. jann., MJ-CorA.
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An M. jannaschii genomic DNA clone containing the putative
corA gene was obtained from the American Type Culture
Collection as construct AMJFH86 carried on pUC18 in E. coli.
The plasmid was transferred into S. typhimurium JR501 by
transformation for restriction modification and then into strain MM281
(4), yielding strain MM1556. MM281 carries insertions in all
three S. typhimurium Mg2+ transport systems, has
no detectable Mg2+ transport, and requires 10 to 100 mM
Mg2+ for growth. A strain carrying any one of the three
S. typhimurium Mg2+ transporters requires no
supplemental Mg2+ in the growth medium. Thus, MM281 is a
good screen for expression of a protein capable of mediating
Mg2+ uptake. The MJ-CorA in MM1556 was compared to the
ST-CorA encoded by a plasmid-borne S. typhimurium corA
allele (pRS170) in the same MM281 background (MM1278).
Upon transformation into MM281, the M. jannaschii corA gene
complemented the Mg2+ growth defect of this strain and
allowed relatively normal growth on N-minimal medium supplemented with
0.4% glucose and 0.1% Casamino Acids (4, 10) or
Luria-Bertani nutrient agar plates or broth. In N-minimal medium
without added Mg2+, the strain carrying MJ-CorA exhibited a
significant lag period before beginning growth but had the same rate of
exponential growth as a plasmid-borne ST-CorA (Fig.
2). The strain carrying MJ-CorA also
exhibited a slight rightward shift in Mg2+ dependence for
growth compared to the plasmid-borne ST-CorA (Fig. 2, insert). Thus at
the level of growth, the archaeal MJ-CorA is functional as an
Mg2+ transporter in S. typhimurium.
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FIG. 2.
Time and Mg2+ dependence of growth.
Overnight cultures of MM1278 (containing ST-CorA) and MM1556
(containing MJ-CorA) were grown in supplemented N-minimal medium
containing 100 mM Mg2+. Cells were washed twice in the same
medium without Mg2+ before being resuspended in the same
medium. Cells were then aliquoted to tubes containing the same medium
plus the indicated Mg2+ concentration (inset) at an initial
optical density at 600 nm (OD600) of 0.05. Cell growth was
measured as OD600. The inset shows the Mg2+
dependence of growth measured after 17 h, and the main panel shows
the time course of growth at 0.1 mM Mg2+. The experiment
was repeated, and similar results were obtained.
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The M. jannaschii and S. typhimurium CorA
transporters have functionally identical properties.
Cation uptake
was measured by previously published procedures by using
63Ni2+ as an alternative substrate since
28Mg2+ is unavailable (3, 15).
ST-CorA has an apparent affinity for Ni2+ of about 250 µM
compared to an affinity for Mg2+ of 20 to 30 µM
(15). The final Ni2+ concentration in the assay
buffer for these experiments was 100 µM; because this is well below
the apparent affinity for Ni2+, measurement of the cation
concentration required for half-maximal inhibition of uptake is within
a factor of 2 of the actual Ki for that cation.
The apparent affinities of Mg2+ for the two systems as
estimated by inhibition of the uptake of 63Ni2+
(Fig. 3) were virtually identical.
Ca2+ did not significantly inhibit either CorA system.
Co2+ is also transported by ST-CorA (5, 12). In
contrast to the affinities of Ni2+ and Mg2+ for
the two CorA transporters, Co2+ affinity appeared slightly
less for MJ-CorA compared to that for ST-CorA (Fig.
4). Mn2+ inhibition was
difficult to assess accurately because higher concentrations of
Mn2+ were acutely toxic to whole cells and caused extensive
clumping. Inhibition of 63Ni2+ uptake by
Mn2+ in the MJ-CorA transporter system appeared to be
similar to that in the ST-CorA system. Overall, these data indicate
that the inhibition profile of MJ-CorA is remarkably similar to that of
ST-CorA.
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FIG. 3.
Mg2+ and Ca2+ inhibition of
transport. Mg2+ and Ca2+ inhibitions of
63Ni2+ uptake were measured as previously
described (3, 15). The data shown are normalized to the
maximal uptake of each CorA. The data for Mg2+ are the
averages of three independent experiments while those for
Ca2+ are from a single experiment. The Ni2+
concentration used was 100 µM. Uptake was measured for 5 min with
triplicates at each concentration. Uptake by the ST-CorA in the
Mg2+ experiments was 1.0 nmol of
63Ni2+ min 1 unit of optical
density at 600 nm (OD600)1; uptake by the
MJ-CorA was 0.14 nmol of 63Ni2+
min1 unit of OD6001. Uptake in
the Ca2+ experiment was comparable to that in the
Mg2+ experiments.
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FIG. 4.
Co2+ and Mn2+ inhibition of
transport. Co2+ and Mn2+ inhibitions of
63Ni2+ uptake were measured as previously
described (3, 15). The data shown are normalized to the
maximal uptake of each CorA. A single experiment with each cation (with
triplicates at each concentration) was performed and was repeated once
with similar results. The Ni2+ concentration used was 100 µM. Uptake by the ST-CorA was 0.85 nmol of
63Ni2+ min1 unit of optical
density at 600 nm (OD600)1; uptake by the
MJ-CorA was 0.09 nmol of 63Ni2+
min1 unit of OD6001.
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Total uptake by MJ-CorA in MM281 was less than that in MM281 carrying
ST-CorA on a plasmid. In various experiments, the total uptake via
MJ-CorA was 5 to 15% of that via ST-CorA. Because the Ni2+
and Mg2+ affinities of the two systems were comparable,
this implies that the Vmaxs for the two systems
were in approximately the same ratio as total uptakes. There are
several potential reasons for the apparently lower uptake capacity of
the MJ-CorA. First, plasmid copy number for ST-CorA was greater than
that for MJ-CorA. Further, although the apparent ribosomal binding site
sequence for the archaeal gene is comparable to that of a favorable
bacterial sequence, there is no recognizable S. typhimurium
promoter adjacent to the M. jannaschii corA allele;
transcription was presumably via read-through from a plasmid-borne
promoter, likely from the antibiotic promoter. Most importantly, codon
usage in the archaeal MJ-CorA is not optimal for S. typhimurium; this might greatly diminish translation compared to
that of ST-CorA. For these reasons, we did not attempt to quantitate a
maximal transport rate, although taken together, our results suggest
that the Vmax of MJ-CorA would not be markedly
different than that of ST-CorA if expressed on a per polypeptide basis. Thus, the longer lag phase before growth of MM281 carrying MJ-CorA starts is unlikely to be due to an intrinsically poor transport capacity.
Finally, since M. jannaschii is a thermophile with optimal
growth at 85°C, the temperature dependence of uptake in S. typhimurium carrying either MJ-CorA or ST-CorA was examined.
Although S. typhimurium viability decreases greatly upon
exposure to higher temperatures, the cells remain intact and functional
for at least 15 min after heating so transport can be measured. Since
transport was being measured in isogenic cells differing only in the
CorA protein each was expressing, any differences in transport must be
due to the stability of the transporter itself in the context of the S. typhimurium membrane. Cells were heated to the indicated
temperature for 5 min before addition of 63Ni2+
for assessment of transport over the following 5 min. The ST-CorA rapidly lost activity above 44°C while the MJ-CorA was completely stable at 65°C, the highest temperature tested (Fig.
5). This result is reflective of the
normal growth temperatures of the two organisms.
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FIG. 5.
Temperature stability of ST-CorA and MJ-CorA. The
temperature stability of uptake was measured by diluting an aliquot of
cells 1:10 into medium prewarmed to the indicated temperatures. After 5 min, Ni2+ containing 63Ni2+ was
added to a final concentration 100 µM. Uptake was measured for 5 min
as previously described (3, 15). Uptake at 44°C by the
ST-CorA was 1.5 nmol of 63Ni2+
min1 unit of optical density at 600 nm
(OD600)1; uptake by the MJ-CorA at 44°C was
0.22 nmol of 63Ni2+ min1 unit of
OD6001.
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Conclusions.
Three conclusions arise from these results.
First, despite the low sequence similarity of the MJ-CorA and ST-CorA,
functionally the proteins appear to be essentially identical. Both
could mediate sufficient uptake of Mg2+ to allow growth of
the Mg2+-transport-deficient MM281 strain, even when, as is
the case with the MJ-CorA carried in S. typhimurium,
expression of the protein was not optimal. In addition, their cation
inhibition profiles are very similar. This is surprising because as a
marine organism M. jannaschii lives in a medium that
contains not only high NaCl but >50 mM Mg2+. By the
results shown in Fig. 3 and 4, uptake of Mg2+ by the
MJ-CorA should be saturated in its normal environment. In contrast,
S. typhimurium grows in a variety of environments, most of
which are not rich in Mg2+, and thus a high
Mg2+ affinity might be required. Thus, there is no obvious
physiological reason why the MJ-CorA should have such a high affinity,
not just for Mg2+, but also for other divalent cations.
The ST-CorA can also mediate efflux of Mg2+, but only at
high extracellular Mg2+ concentrations which are saturating
for influx (2). Since the MJ-CorA has evolved in an
environment with a high concentration of Mg2+, it might
also mediate efflux rather than influx as a primary function,
particularly since it would normally be exposed to very high
Mg2+ concentrations in seawater. However, if the primary
function of the MJ-CorA was as an efflux system, it would likely leak
Mg2+ extensively from the S. typhimurium cell,
thus inhibiting growth markedly. This phenotype is seen with at least
one mutant of ST-CorA (16a). In addition, the quite high
affinity of MJ-CorA for the various other cations required for influx
argues strongly that this protein is not a primary Mg2+
efflux system in M. jannaschii. This further suggests that
M. jannaschii possesses a separate active efflux mechanism
for Mg2+.
Second, even with such relatively weak sequence conservation between
the two polypeptides, their highly similar functions imply that their
overall tertiary structures may be very similar.
Finally, these data confirm the ubiquity of this class of
Mg2+ transport system. An archaeal CorA homolog is able to
function as an Mg2+ transporter in a cell type from another
kingdom of life. The two other archaeal organisms whose genomes have
been completely sequenced, Archeoglobus fulgidus and
Pyrococcus horikoshii ot3, both carry a clear CorA homolog
according to genome sequence data (8, 9). Our previous data
demonstrated the ubiquity of corA-like genes in virtually
all gram-positive and gram-negative bacterial species tested
(14). Thus, we suggest that a CorA-like Mg2+
transporter is the major constitutive Mg2+ uptake system of
both the Bacteria and the Archaea. It remains to
be seen whether similar proteins will be found in eukaryotes.
This work was supported by PHS grant GM39447 to M.E.M. R.L.S.
was supported by a Metabolism Training Grant (DK07319).
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