Conserved tyrosine-147 plays a critical role in the ligand-gated current of the epithelial cation/amino acid transporter/channel CAATCH1
1 The Department of Physiology and Functional Genomics, University of
Florida College of Medicine, Gainesville, FL 32652, USA
2 The Whitney Laboratory, University of Florida, 9505 Ocean Shore Boulevard,
St Augustine, FL 32080, USA
* Present address: Shriners Hospital for Children of Northern California,
Sacramento, CA 98517, USA
e-mail:
wharvey{at}whitney.ufl.edu
Accepted 21 May 2002
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Summary |
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Key words: ion channel, potassium, sodium, amino acid, transporter, ionic current, Xenopus laevis, Manduca sexta
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Introduction |
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The present study concerns the conserved equivalent position at
Tyr147 in CAATCH1 (Fig.
1) and focuses on the role that a site-directed mutation,
Tyr147 (wild type, WT) to Phe147 (Y147F), plays in
CAATCH1 ionic channel gating by a variety of free amino acid ligands. This
mutation removes the OH group from tyrosine, leaving the non-polar phenyl
ring. The ligand-modulating channel role of CAATCH1 is distinct from its other
functional role as a cation-catalyzed, but non-symporting, amino acid
transporter that is thermodynamically uncoupled from alkali cation fluxes
(Quick and Stevens, 2001).
We previously demonstrated that, in Na+-containing medium, free
methionine binds to CAATCH1 as an inhibitory ligand, potently blocking its
ionic current; conversely, in K+-containing medium, methionine acts
as an agonist, increasing cationic current
(Feldman et al., 2000;
Quick and Stevens, 2001
).
Given that the conserved tyrosine residue of the SNF family affects neural
membrane protein functions, we hypothesized that a Y147F mutation would alter
the ligandgating specificity of the epithelial CAATCH1 ionic channel. The
results implicate a critical role for Tyr147 in the channel
conductance of CAATCH1 and its gating by amino acids.
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Materials and methods |
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Electrophysiology
Injected oocytes were superperfused (22°C) using a peristaltic pump.
The Na+-containing medium was 100 mmol l-1 NaCl, 1 mmol
l-1 MgCl2, 1 mmol l-1 CaCl2 and 10
mmol l-1 Taps/N-methyl-D-glucamine (NMG+), pH
8.0. To determine cation specificity, Na+ was completely replaced
by K+, Li+ or NMG+; to determine anion
specificity, Cl+ was completely replaced by gluconate.
Transmembrane currents were measured in intact oocytes using a two-electrode
voltage-clamp with agar-bridged bath electrodes. Current/voltage
(I/V) relationships were generated using voltage steps or ramps (36
mV s-1, 1.8 mV per point) between -150 and +30 mV from a holding
potential of -60 mV. CAATCH1-associated net ionic currents (I)
were obtained by subtracting control current values measured in the absence of
amino acids from currents measured in their presence. It should be noted that,
for ligands that inhibit constitutive currents, this procedure produces the
inverse of the inhibited current, resulting in I/V relationships that
have negative slopes and have current values with inverted signs compared with
conventional I/V curves. This phenomenon has been described by for
cocaine inhibition of dopamine transporter (DAT) currents
(Sonders et al., 1997
).
Immunolocalization
CAATCH1 WT and Y147F expressed in Xenopus laevis oocytes was
immunolocalized using a polyclonal primary antibody (a gift from Dr P. Linser)
prepared in rabbits against the N terminus of CAATCH1, which is common to both
the wild type and the mutant. Oocytes in which transport activity had been
confirmed electrically were fixed at 22°C for 2h in PBSAT buffer
(phosphate-buffered saline, 1.5% bovine serum albumin, 0.1% Triton X-100)
containing 4% paraformaldehyde. Oocytes were washed three times with PBSAT
(22°C), then incubated at 4°C for 2 days in PBSAT containing 0.1%
sodium azide buffer, which was changed periodically. The affinity-purified
primary antibody (0.2 mg ml-1, diluted 1:100 in PBSAT) was
incubated with fixed oocytes for 2h at 22°C. Oocytes were washed with
PBSAT three times at 22°C, then twice for 1h each at 4°C. The oocytes
were then incubated for 2h at 22°C with Texas-Red-conjugated,
affinity-purified, secondary antibody (goat anti-rabbit, 1.4mg
ml-1, diluted 1:100 in PBSAT; Jackson ImmunoResearch, West Grove,
PA, USA). Finally, stained oocytes were washed three times with PBSAT at
22°C, then incubated with periodic buffer changes at 4°C for 2 days.
Texas Red images were obtained with a BioRad confocal microscope (model 1024
ES) equipped with Olympus IX70 inverted optics. Images were visualized in
2µm steps with 4X Kalman filtering using the same gain and laser power
settings for all oocytes.
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Results |
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Cation-dependent, I/V relationships were drastically altered in
Y147F compared with WT (Figs
3,4,5).
In the absence of amino acids (Fig.
3), the slopes of the I/V relationships for WT for total
current in Na+-, K+- and Li+-containing media
were similar to each other and steeper than in NMG+-containing
medium (Fig. 3A). The WT
reversal potentials were approximately -5 mV for Li+-, -15 mV for
Na+-, -45 mV for K+- and -80 mV for
NMG+-containing media. The net cation-induced CAATCH1-specific
inward currents in WT (Fig. 3B)
were subsequently obtained by subtracting the outward K+ current
component of the total current in NMG+-containing medium; the
resulting Na+-, K+- and Li+-specific currents
were strictly inwardly rectifying at positive potentials. The NMG+
curve in Y147F (Fig. 3C) was
the same as that in water-injected controls (data not shown) and similar to
that in WT (Fig. 3A), with a
reversal potential of approximately -60 mV. Notably, in Y147F, the
Na+, K+ and Li+ curves were all very similar
to that of NMG+, each yielding reversal potentials of approximately
-50 mV. The positive net current values observed at depolarizing potentials
are due primarily to outward K+ current
(Quick and Stevens 2001), and
subtracting the currents obtained in NMG+-containing medium
effectively reduced net cation-specific Y147F currents in the absence of
ligands to zero at all potentials (Fig.
3D). These data collectively indicate that, in the absence of any
amino acid ligand, the Na+, K+ and Li+
conductances observed in WT are reduced to nearly zero in Y147F.
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The effects of various free amino acids on I/V relationships were
also investigated in both Na+- and K+-containing media
by subtracting the small control values from values obtained with each ligand
(Figs 4,
5). As we have shown previously
(Feldman et al., 2000;
Quick and Stevens, 2001
) for
WT in Na+-containing medium
(Fig. 4A), the proline-evoked
inward currents were the largest elicited by any test ligand at all
potentials. In contrast, methionine elicited a potent blockade of inward
Na+ current (Fig.
4A), as shown by inverted I/V curves with negative slopes
(due to the subtraction protocol), representing the algebraic inverse of the
inhibited current (Feldman et al.,
2000
; Quick and Stevens,
2001
). The methionine block of WT inward current appears only in
Na+-containing medium and is not observed in K+-
(Fig. 5A) or
Li+-containing medium, as shown previously
(Feldman et al., 2000
).
In WT in Na+-containing medium, leucine produced inhibitory effects similar to methionine, whereas threonine, phenylalanine and glycine all produced complex I/V curves characterized by negatively sloping domains at depolarized voltages and positively sloping domains at hyperpolarized voltages, to produce inverted U-shaped I/V relationships. These complex I/V relationships suggest a voltage-dependent modulation of gating such that, at depolarized potentials, these amino acids inhibit currents, but at hyperpolarized potentials they increase currents. The WT relative inward current inhibition ranking for the ligands that have pronounced negatively sloping I/V domains was: Met>Leu>Phe>Gly>Thr. These effects were observed for WT in 100 mmol l-1 Na+-containing medium completely lacking K+ (Fig. 4A); addition of as little as 2 mmol l-1 K+ attenuated the net outward current elicited by these ligands (data not shown). For WT in K+-containing medium lacking Na+ (Fig. 5A), all test amino acids, except leucine, yielded inwardly rectifying currents, with threonine eliciting the greatest current. In some oocytes expressing WT, leucine in K+-containing medium (Fig. 5A) elicited an apparently inverted, negatively sloping I/V relationship (produced by the subtraction protocol) with reversal potentials near -100 mV, suggesting a possible current inhibition.
In stark contrast to WT, there was no current block in the Y147F mutant by any test amino acid in Na+-containing medium, with all ligands instead eliciting solely inwardly rectifying currents (Fig. 4B). Unlike the complex I/V curves observed in WT (Fig. 4A), the curves for Y147F were linear with all test ligands in Na+-containing medium (Fig. 4B). Also in contrast to WT, proline failed to elicit currents in either Na+- or K+-containing media beyond NMG+ values in Y147F (Figs 3B, 4B, 5B) or water-injected control oocyte currents. For Y147F, the relative potency ranking for permissive gating of inward current in Na+-containing medium (Met>Thr>Phe>Leu>Gly>Pro) (Fig. 4B) was similar to the ranking order of WT inward current inhibition (Fig. 4A). It is notable that, in Na+-containing medium, methionine elicited inward currents in Y147F (Fig. 4B), rather than blocking it as in WT (Fig. 4A). Furthermore, the magnitude of methionine-elicited inward currents in Y147F (Fig. 4B) was the largest measured for any test amino acid at hyperpolarizing potentials in all trial oocytes expressing CAATCH1 WT or Y147F.
In K+-containing medium, methionine permitted large inwardly rectifying currents in Y147F (Fig. 5B), much like in WT (Fig. 5A). In contrast to WT in K+-containing medium, none of the test amino acids blocked Y147F inward currents (Fig. 5B). For WT in K+-containing medium, leucine demonstrated a mild inward blocking effect with a considerable negative reversal potential in some oocytes (Fig. 5A), whereas for Y147F in K+-containing medium leucine also yielded both inward and outward currents (Fig. 5B), but the I/V relationships were complex and reversed at approximately -25 mV.
We have previously demonstrated that amino acid ligands bind CAATCH1 with
varying affinities, thereby modifying the degree of charge transfer across the
membrane (Feldman et al.,
2000; Quick and Stevens,
2001
). In Fig. 6,
we depict the ligand-binding selectivity of WT and Y147F, as revealed by the
evoked inward or blocked inward (i.e. data reported as apparently outward as a
result of the subtraction protocol) currents with 0.5 mmoll-1 test
amino acids. Current was measured at -60 mV, and data were normalized to the
value for the preferred ligand in Na+-containing medium, proline
(Fig. 6A), or to the value for
threonine in K+-containing medium
(Fig. 6B). Three striking
results emerge in Na+-containing medium. First, all relative inward
currents elicited by all test amino acids were much greater in Y47F than in
WT. Second, the inward current block phenomenon was eliminated in Y147F for
all ligands. Third, the inhibition of inward current by methionine in WT was
converted in Y147F to a strictly inward current that was nearly 30 times
greater than the inward current elicited by proline. Thus, the largest
currents in Na+-containing medium were produced by methionine in
Y147F, but by proline in WT. In K+-containing medium, notable
differences also exist between the WT and Y147F variants
(Fig. 6B). First, glycine,
proline and phenylalanine in K+-containing medium were ineffective
or poor gating ligands in Y147F compared with WT. Second, in
K+-containing medium, methionine and leucine are by far the
preferred ligands in Y147F.
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Although CAATCH1 WT and Y147F display quite different current-gating behavior, they are alike in at least one respect, the absence of Cl- does not influence amino-acid-gated currents (Fig. 7). When Cl- was completely replaced by gluconate in all media, there were no significant effects on (i) the WT proline-elicited current in Na+-containing medium; (ii) the Y147F methionine-elicited current in Na+-containing medium or (iii) the Y147F leucine-elicited current in K+-containing medium.
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Discussion |
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SNF proteins expressed in vitro clearly exhibit ionic currents,
but the interpretation of these currents in the literature varies with the
experimental evidence for a particular protein. A range of models
(DeFelice et al., 2001;
Amara, 1996
;
Cao et al., 1998
;
DeFelice and Blakely, 1996
;
DeFelice and Galli, 1998
;
Fairman and Amara, 1999
; Galli
et al., 1996
,
1997
,
1998
;
Kanner et al., 2001
;
Kavanaugh, 1998
;
Mager et al., 1996
;
Petersen and DeFelice, 1999
;
Sonders and Amara, 1996
;
Sonders et al., 1997
;
Su et al., 1996
;
Zhu et al., 1997
) has been put
forward for various channel/transporters. The current has been interpreted as
(i) a transport-associated ionic current that arises from secondary active
cotransport of organic solutes that are stoichiometrically coupled with
activator ions, (ii) ion leakage through a saturable cotransporter as the
result of slippage (uncoupling), (iii) organic ligand-gating of constitutive
leak channels exclusively serving inorganic ions, (iv) channel movement of
charged organic solutes singly or in clusters, (v) transient currents from the
protein's conformational shifts and/or (vi) multiple conductance states of one
or more of the above. CAATCH1 fits the last category. It is a dual-function
digestive epithelial membrane protein that operates as both (i) an
amino-acid-gated alkali cation channel and (ii) a stoichiometrically uncoupled
non-symporting, but Na+-dependent, proline-preferring amino acid
transporter (Feldman et al.,
2000
; Quick and Stevens,
2001
).
Although the ionic channel and transport functions of CAATCH1 are not
thermodynamically coupled (Quick and
Stevens, 2001), they are interdependent. Thus, alkali metal
cations modulate specific amino acid transport and vice versa. This
interdependence is likely to be complex. Our earlier steady-state
electrophysiological studies with CAATCH1
(Feldman et al., 2000
)
initially demonstrated that methionine unmasked constitutive cationic current
pathways(s). Our subsequent transient current/radiotracer studies
(Quick and Stevens, 2001
)
confirmed that CAATCH1 WT possesses non-saturating Na+ and
K+ channel activity which behaves according to the
NernstPlanck relationship and displays voltage-dependent transient
currents with symmetrical ON/OFF rates.
It is not known whether CAATCH1 possesses one or more than one channel
serving Na+ and K+. A variety of free amino acids bind
to CAATCH1 ligand-gating site(s), thereby modulating its channel conductance
(Quick and Stevens, 2001);
e.g. proline binding increases the conductance of both Na+ and
K+, whereas methionine and other amino acids inhibit cation
conductance in Na+-containing medium but potentiate it in
K+-containing medium. Ligand-binding affinities and ON/OFF
transient rate constants are both dependent on the concentrations of
Na+ and K+. However, the maximal transient charge
transfer by CAATCH1 is affected neither by organic solute ligands nor by
Na+ (Quick and Stevens,
2001
), unlike `prototypical' cotransporters such as the
Na+/glucose cotransporter SGLT1
(Parent et al., 1992
;
Quick et al., 2001
) or the
-aminobutyric acid transporter GAT1
(Hilgemann and Lu, 1999
).
Thus, the ionic conductance of CAATCH1 is thermodynamically uncoupled from
amino acid transport, but the physical relationship between ionic channel and
amino acid transport events is unknown.
The results of the present study are summarized in the schematic model of Fig. 8. In the absence of any gating amino acid ligand, Y147F conductance in Na+-, K+- or Li+-containing media is effectively reduced to zero (Fig. 3B), compared with the open-channel I/V relationships with these alkali cations of WT (Fig. 3A). The absence of measurable Y147F constitutive currents in the absence of gating ligands (Fig. 3B) represents an inherent property of the mutated protein because protein translation and trafficking to the membrane in the heterologous oocyte expression system appear to be the same for both Y147F and WT (Fig. 2). Moreover, free methionine strongly stimulated inward currents of Y147F in both Na+- and K+-containing media, whereas it strongly inhibited inward currents of WT in Na+-containing medium (Figs 4,5,6). Furthermore, although proline increased inward currents of WT strongly in Na+-containing medium and moderately in K+-containing medium, it failed to elicit currents of Y147F in either Na+- or K+-containing media beyond values in NMG+-containing medium or in water-injected control oocytes (Figs 3,4,5,6). Thus, the Y147F mutation alters the modulation of ionic conductance induced by the binding of particular amino acid ligands so that antagonists of conductance in the wild type become agonists and wild-type agonists lose efficacy in the mutant. One property shared by both WT and Y147F is the Cl--independence of CAATCH1 channel activity (Fig. 7), in contrast to KAAT1 and neural SNF family members, which are Cl--dependent.
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It is remarkable that, in contrast to WT, there is no current block in
Y147F by the test amino acids. This is especially notable since the Y147F
mutation consists simply of removing the polar OH group from tyrosine, leaving
the non-polar phenyl ring of phenylalanine
(Fig. 8). The functional
effects of a single mutation of the conserved tyrosine residue could be
attributed (i) to the binding of ligand amine nitrogen to the protein's
hydroxyl side chain itself, as suggested for other SNF members
(Bismuth et al., 1997), (ii) to
steric alterations of the site or pore geometry as a result of hydrophobic
shifts within the transmembrane pore or, more likely, (iii) to a complex
combination of many biophysical/structural perturbations. Considered together
with other SNF mutagenesis studies, it is noteworthy that this conserved
tyrosine residue is required for appropriate physiological behavior among such
a wide spectrum of SNF proteins. Because the residue is highly conserved, it
is possible that naturally occurring nucleotide substitutions, even single
substitutions, elsewhere in the sequence could give rise to completely
different functional phenotypes for a particular SNF protein isoform, either
genetically programmed by design as a splice variant or by an unintended
mutation.
Amino acid uptake studies using Manduca sexta midgut brush-border
membrane vesicles (for reviews, see
Giordana et al., 1989;
Wolfersberger, 2000
) indicated
the presence of a variety of amino acid transporters with widely overlapping
specificities. This pattern is consistent with the possible existence of
multiple isoforms of CAATCH1-like transporter/channels representing simple
splice variants (Z. Liu and W. R. Harvey, unpublished data). The only other
amino acid transporter cloned from Manduca sexta midgut is KAAT1,
which is approximately 90% identical to CAATCH1 but exhibits a battery of
properties distinct from those of CAATCH1
(Castagna et al., 1998
). The
effects of several KAAT1 mutations (Z. Liu and W. R. Harvey, unpublished data)
were similar to those of the equivalent GAT1 mutations, with one notable
exception: mutation of conserved Tyr140 to Phe140
(Y140F) in GAT1 led to a complete loss of GABA uptake activity and prevented
the non-transportable GABA analog SKF 100330A from blocking transient currents
in Na+-containing medium
(Bismuth et al., 1997
).
However, the equivalent Y147F mutation in KAAT1 led to a fivefold increase in
steady-state currents in Na+-containing medium and to changes in
the substrate amino acid specificity (Z. Liu and W. R. Harvey, unpublished
data). These differential effects of amino acids on KAAT1 currents in either
Na+- or K+-containing media are therefore quite
different from those in CAATCH1 Y147F and WT
(Fig. 6). Nonetheless, in some
respects, CAATCH1 Y147F resembles KAAT1 WT
(Fig. 6). Notably, both differ
from CAATCH1 WT in that they demonstrate only inward currents with no current
block for the test amino acid ligands. Also, in both cases, in
K+-containing medium, methionine and leucine are the favored
current-gating ligands whereas proline is relatively inert.
CAATCH1 shares several biophysical properties with carrier/channel proteins
such as the serotonin transporter (SERT), the dopamine transporter (DAT), the
norepinephrine transporter (NET) and the -aminobutyric acid transporter
(GAT1). However, whereas neural ionic channels are blocked by pharmacological
agents such as cocaine, CAATCH1 is a digestive epithelial channel that is
blocked by nutrient amino acids, exemplified by methionine. Our previous
studies with CAATCH1 (Quick and Stevens,
2001
) clearly demonstrated that the transport of amino acids is
thermodynamically uncoupled from cation fluxes, although amino acid substrate
binding is catalyzed by Na+, with the ion channel aspect being
differentially gated by amino acids. Thus, both neural tissue SNF members and
epithelial CAATCH1 can regulate current movement by organic ligands while also
transporting organic substrates. CAATCH1 may be an evolutionary transition
protein at a branch point separating neurotransmitter transporters and
nutrient transporters, implying the existence of undiscovered endogenous,
naturally occurring inhibitors/modulators of SNF proteins in neural
tissues.
Several other epithelial membrane, cation-dependent amino acid and
cation-independent cotransporters displaying ionic channel activity have been
cloned (Avissar et al., 2001;
Broer et al., 2000
;
Kekuda et al., 1996
;
Wagner et al., 2001
;
Zerangue and Kavanaugh, 1996
).
However, B0,+ (Sloan and Mager,
1999
), CAATCH1 and KAAT1 appear to be the only epithelial members
of the SNF family to be cloned to date. B0,+ transports amino acids
in a digestive/absorptive epithelium
(Nakanishi et al., 2001
;
Sloan and Mager, 1999
), but
there is no evidence that any ligand gates currents in ATB0,+ in
the manner of CAATCH1. It is reasonable to hypothesize that, like the neural
SNF transporter/channels and CAATCH1, pharmacological or natural antagonistic
ligands that block an ionic current in ATB0,+ may exist.
What do the channel properties of CAATCH1 and their drastic alteration by a point mutation contribute to our understanding of the physiology of insect larvae that normally possess alkaline (pH 11) midguts? This information sheds light on two processes, namely amino acid uptake in posterior midgut and lumen alkalization in anterior midgut.
In the posterior midgut of living caterpillars such as Manduca
sexta, from which CAATCH1 was cloned, the epithelial intracellular
compartment is 240 mV negative with respect to the lumen. Luminal
[K+] is nearly 300 mmol l-1, whereas [Na+] is
less than 3 mmol l-1 (Dow et
al., 1984; Harvey et al.,
1975
). Under these conditions, a flow of K+ through the
CAATCH1 channel would be inward and would compete electrically with the
transport of K+ through a K+-coupled amino acid
cotransporter such as KAAT1. Blockade of the channel function of CAATCH1 by
certain amino acids (e.g. methionine inhibition of Na+ conductance
in WT) could conceivably prevent this competition while allowing the solute
transporter aspect to remain functional in promoting nutrient amino acid
uptake. With respect to gating ionic currents, CAATCH1 is unlike KAAT1 in that
KAAT1 appears to operate in an uncoupled conductance mode without ligands, but
then switches to a thermodynamically coupled ion-cotransport current in the
presence of amino acid co-substrates (Bossi
et al., 1999
).
In larval anterior midgut, the channel aspect of CAATCH1 may play a role in
midgut alkalization. Bicarbonate/chloride exchange has been suggested as a
mechanism for accumulating bicarbonate in the lumen and accounting for
alkalization up to pH 8.5 (Moffett and
Cummings, 1994). However, to account for measured luminal pH
values approaching 11 (Dow,
1984
), a proton would have to be removed from bicarbonate to yield
carbonate. H+ flow from the lumen to the cell through the CAATCH1
channel, driven by the apical transmembrane potential of -240 mV
(Dow and Harvey, 1988
), would
accomplish this end. We have shown that CAATCH1 is at least partially
conductive to H+ (Feldman et
al., 2000
), and future studies with CAATCH1 should directly
address the proton conductance and its gating by amino acid ligands.
To conclude, we have shown that Phe147 substitution of SNF-family-conserved Tyr147 differentially alters the amino acid ligand selectivity for gating CAATCH1 channel conductance in Na+- and K+-containing media, while leaving Cl--independence unaffected. The removal of a hydroxyl group at this critical residue alters the association between amino acid binding and ionic conductance modulation, such that amino acids that normally inhibit the conductance become agonists of conductance. In the absence of any amino acid ligands, the Y147F mutant constitutive currents are virtually eliminated, in contrast to the freely conducting ionic channel(s) of WT. Given the tissue milieu in which CAATCH1 resides in situ, CAATCH1 probably plays an important physiological role in ion homeostasis in vivo. The results support the idea that methionine inhibition of ionic conductance in CAATCH1 parallels the phenomenon of organic solute ligand pharmacological inhibition of ionic conductance in neural SNF family members.
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Acknowledgments |
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