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J. Biol. Chem., Vol. 276, Issue 9, 6473-6478, March 2, 2001
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From the Cystic Fibrosis Research Laboratory, Stanford University,
Stanford, California 94305-2130
Received for publication, October 12, 2000, and in revised form, November 29, 2000
Cystic fibrosis transmembrane conductance regulator
(CFTR), which causes cystic fibrosis when nonfunctional, is an anion
channel and a member of the ATP binding cassette superfamily. After
phosphorylation, CFTR gates by binding and hydrolyzing ATP. We show
that CFTR open probability (Po) also depends on
the electrolyte concentration of the cytosol. Inside-out patches from
Calu-3 cells were transiently exposed to solutions of 160 mM salt or solutions in which up to 90% of the salt was
replaced by nonionic osmolytes such as sucrose. In lowered salt
solutions, CFTR Po declined within 1 s to a
stable lower value that depended on the electrolyte concentration,
(K1/2 Cystic fibrosis is caused by mutations that cause loss of function
in CFTR,1 an anion channel
primarily found in the apical membranes of epithelial cells. CFTR is an
unusual ion channel in several respects. It is a member of the ATP
binding cassette superfamily of transporters but is the only known ion
channel among at least 39 other human ABC transporters (1). Regulation
of CFTR channel gating is unusually complex. It requires
phosphorylation by protein kinase C (2) and protein kinase A (3) and
binding and hydrolysis of ATP in the presence of Mg2+ by
the two nucleotide binding domains (4). The exact way in which ATP
binding and hydrolysis facilitate CFTR gating is still being
investigated (5-11). CFTR gating is also influenced by cytosolic pH
(12), the species and concentrations of divalent cations (10, 13, 14),
and the redox status (14-17). At least some of these effects are
secondary to effects on enzymes that control the phosphorylation state
of CFTR.
We have discovered an unsuspected and powerful influence on CFTR
gating. Our data indicate that CFTR senses the concentrations of
cytosolic electrolytes (apparently without regard to species) and
requires high levels of electrolytes (~160 mM) for full activity.
Cells and Cell Culture--
Experiments were conducted using
Calu-3 cells grown as previously described by Shen et al.
(18) and CFTR-transfected C127 cells (2WT2 cells) provided by Seng
Cheng (Genzyme Corp.) and grown as described by Haws et al.
(19).
Patch Clamp Recording and Solutions--
Inside-out
single-channel patch clamp recordings were made at ~23 °C. CFTR
channels were identified by their signature: unitary conductance of
6.1 ± 0.2 pS (n = 8, mean ± S.E.), linear
current-voltage relations, appeared in clusters, showed no voltage
sensitivity, and required PKA and ATP for activity. They were the only
anion channels recorded in cell-attached patches from surrounded cells in confluent islands of Calu-3 cells and were by far the most prevalent
channels in transfected C127 cells (2WT2). Excised patches were
alternately exposed to different flowing cytosolic solutions via
solution changers (see below). CFTR activity was maintained with a
perfusate containing 160 mM NaCl, 2 mM
Mg2+-ATP, and 10 units of PKA and was eliminated with a
perfusate that lacked ATP and PKA. Other solutions contained the same
activation ingredients and various concentrations of electrolytes or
nonelectrolytes. Pipettes were pulled from Prism LA16/SA16 glass (Dagan
Corp.) on a Brown-Flaming puller. Currents were amplified and filtered at 100 Hz with an Axopatch 1C amplifier. Data acquisition was controlled by pClamp 8.0 (Axon Instruments). Data were digitized at 1 kHz and stored on disk. Membrane voltage was held at 60 mV unless
otherwise indicated.
The standard bath and cytosolic control solution was (in
mM) 150 NaCl, 5 KCl, 0.5 EGTA, 2.5 MgCl2, 0.227 CaCl2, and 10 HEPES titrated to pH 7.3 with ~6.2
mM NaOH. Osmolarity was adjusted to 330-350 mosmol. The
free calcium level was 100 nM as computed by MaxChelator.
This formulation gives an electrolyte concentration of ~334
mM; the main anion is Cl
Because most test solutions contained reduced levels of
Cl Chemicals--
Reagents were from Sigma unless otherwise noted.
Forskolin was stored as a 10 mM stock solution in ethanol
at Solution-changing Apparatus--
A rapid solution-switching
system was fabricated using a piezoelectric translator (21) connected
to an array of three parallel perfusion tubes, each ~100
µM in diameter and containing a different solution. The
excised patch was positioned in relation to the array such that any of
three computer-controlled voltages moved the array to bathe the patch
in one of the streams. Switching between streams occurred in <50 ms.
Patches were alternately exposed to an activating solution containing
PKA, 1-5 mM Mg2+-ATP, and normal bath
electrolytes; to a minimal solution that lacked PKA and ATP; and to
test solutions that were equivalent except that varying proportions of
cytosolic NaCl were replaced with nonelectrolytes or other electrolytes.
The valve-controlled solution changer consisted of a manifold with
seven inlet ports, each connected to a different solution reservoir,
and one outlet port connected to a perfusion pipette. The manifold was
located ~2 mm from the tip of the perfusion pipette. Solutions were
switched manually in <2 s. In this setup the patch and perfusion
pipette have fixed positions, which assures identical access to each solution.
Data Analysis--
All-point amplitude histograms were
constructed for selected traces to determine the amplitude of the
unitary current. Histograms were least squares fit with (N + 1) Gaussian functions (N, number of active channels in the
patch; resolution N < 10). The resulting average
peak-to-peak interval represented the unitary current (i).
Po was determined through least squares fits of
binomial distributions of the multiple Gaussians or by
Po = I/iNT, where I is the
integrated current, and T is the total recording time. Mean
closed time (tC) was estimated by multiplying
N times the mean closed time of full closures (the record
contained at least 10 full closures). Mean open time
(tO) was derived from the equation Po = tO/(tO + tC). N was also determined by the
peak current observed in a patch, divided by the unitary current
obtained as above.
When cytosolic NaCl was rapidly switched from 160 to 15 mM with replacement by an iso-osmolar amount of sucrose,
the Po of CFTR declined to <20% of control
values within 1 s. Po returned to normal
with a similar time course after switching back to the control
cytosolic solution (Fig. 1a). An
equivalent reduction of Po was observed
regardless of the species of nonelectrolyte that we used for
replacement, including the cyclic sugar sucrose, the linear sugar
mannitol, or the amino acids L-proline and taurine (Fig.
1b). (Proline is uncharged at neutral pH, and only ~3% of taurine is predicted to be charged at pH 7.4.)
This surprising result could be explained by many variables, including
increased molar concentration of a nonelectrolyte, decreased molar
concentration of Na+, Cl
Cystic Fibrosis Transmembrane Conductance Regulator Gating
Requires Cytosolic Electrolytes*
ABSTRACT
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MATERIALS AND METHODS
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DISCUSSION
REFERENCES
80 mM NaCl).
Po was rapidly restored in normal salt
concentrations without regard to the electrolyte species. Reducing
external electrolytes did not affect CFTR Po.
The same results were obtained when CFTR was stably phosphorylated with
adenosine 5'-O-(thiotriphosphate). The decrease in
Po resulted entirely from an increase in mean closed time. Increasing ATP levels up to 20-fold did not counteract the
effect of low electrolytes. The same effect was observed for CFTR
expressed in C127 cells but not for a different species of anion
channel. Cytosolic electrolytes are an unsuspected, essential cofactor
for CFTR gating.
INTRODUCTION
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ABSTRACT
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MATERIALS AND METHODS
RESULTS
DISCUSSION
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MATERIALS AND METHODS
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ABSTRACT
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MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
(160 mM). Mg2+-ATP or Na2ATP (0.5-10
mM) was added from stock, and the pH was adjusted to 7.3 by
adding ~1.55 mM NaOH/mM ATP. To avoid changes in Po caused by dephosphorylation via
patch-associated phosphatases, all solutions in most experiments
contained both Mg2+-ATP and 10 units of the catalytic
subunit of PKA with 50 mg/ml dithiothreitol. Test solutions were
identical, except that NaCl and KCl were replaced with NaCl of the
desired concentration plus the osmotically balanced replacement solute.
The standard pipette solution was (in mM) 150 NMDG-Cl, 2.5 CaCl2, 2.5 MgCl2, and 10 HEPES, with pH
adjusted to 7.3 with NaOH and osmolarity adjusted to 320-340 mosmol.
, recordings were usually made at positive cell
voltages to provide the clearest channel recordings. Poor channel
resolution was seen at negative voltages because of the unfavorable
gradient for chloride and a fast block by anions (20). With reduced
cystosolic Cl, the channel amplitude was increased less
at positive voltages than the Nernstian prediction. This effect
presumably resulted from channel saturation and block by the osmolytes
(20).
20 °C. PKA stock was made from 1000 units of solid dissolved in
1 ml of deionized water containing 50 mg/ml dithiothreitol and stored
at 20 °C. ATP (Mg2+ and Na+ salt) and
ATP
S (BioChemika) were dissolved in deionized water and stored at
10 °C. Test solutions were made by replacing part of the NaCl with
2 parts of the nonelectrolytes sucrose, mannitol (Mallinckrodt),
L-proline and or taurine or 1 part of the electrolytes Na-gluconate and Na-isethionate. In experiments in which
Na2SO4 replaced NaCl, the electrolyte
concentrations were balanced.
RESULTS
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MATERIALS AND METHODS
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DISCUSSION
REFERENCES
View larger version (42K):
[in a new window]
Fig. 1.
CFTR Po was decreased
by reduction of cytosolic NaCl. a, reduction of
cytosolic electrolytes with sucrose reduced Po
to ~15% of normal. The dashed line shows estimated full
closure; the large shift between the two solid lines is the
offset due to differing Cl activities. Currents were
recorded at 40 mV. b, effects of different nonelectrolytes.
c, addition of 320 mM mannitol to normal bath
electrolytes did not affect Po. d,
replacing Na+ or Cl with other electrolytes
did not affect CFTR Po. Currents were recorded
at 60 mV unless otherwise indicated. Nonstationary transitions
resulting from the solution change were omitted from the display and
analysis.
, Na+ + Cl, or electrolytes, or decreased ionic strength. Adding
320 mM mannitol to 160 mM NaCl did not reduce
Po (Figs. 1c and
2a), indicating that the
nonelectrolytes were not themselves inhibitory. (The current amplitude
in the presence of 300 mM mannitol was reduced from 0.47 to
0.43 pA. This result is consistent with that of Linsdell and Hanrahan
(20), who found that internal 220 mM sucrose or urea caused
a 5-10% reduction in unitary conductance.)
View larger version (53K):
[in a new window]
Fig. 2.
Summary of CFTR Po
(mean ± S.D.) in solute substitution experiments.
a, electrolyte replacements with nonelectrolytes.
Po in high and low electrolytes differed
significantly (t test, p < 0.01).
b, replacement with different electrolytes. N for
each group is shown above bars.
CFTR Po was also unaffected by solutions in
which Na+, Cl, or both were replaced with
different ions, including the impermeant anions gluconate, sulfate, and
isethionate and the impermeant cation
N-methyl-D-glucamine (Figs. 1d and
2b). Taken together, these experiments appear to eliminate
all variables except the concentrations of electrolytes or ionic
strength. Experiments with divalent ions will be needed to discriminate
between those possibilities. In the remainder of the paper we use the
term "electrolyte concentration" as an inclusive term for either
molar concentrations of electrolytes or ionic strength.
To determine the quantitative dependence of CFTR
Po on electrolyte concentration, the cytosolic
solution bathing a single excised patch was rapidly switched between
control solution with 160 mM NaCl and different test
solutions containing 5, 15, 30, 60, 80, or 320 mM NaCl. For
most of these experiments, the nonelectrolyte was proline. Open
probability in these experiments was a sigmoid function of log salt
concentration, with the steepest slope occurring between 60 and 100 mM and a half-maximal value of ~80 mM NaCl (Fig. 3).
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In experiments to this point, the extracellular (pipette) solution was 150 mM NMDG-Cl. To determine whether the concentration of external electrolytes or the electrolyte gradient across the patch influenced CFTR Po, we measured Po in a series of experiments in which ~1 mm of the pipette tip was filled with 150 mM NMDG-Cl and then back-filled with 15 mM NaCl + 300 mM proline, while the bath remained constant at 160 mM NaCl. In prior experiments with the nonpermeant anion ATP, we found that the tip solution equilibrates with the back-fill solution over a few minutes (22). In our present experiments, when tips were filled with 150 NMDG-Cl and back-filled with 15 Cl, the unitary current at 60 mV decreased by 25% within 1 min and remained stable. Under these conditions CFTR Po remained constant (Po = 0.35-0.42; < 3.8% reduction; n = 3) for >15 min. We also studied two patches in which the extracellular solution was 60 mM NaCl and the cytosolic solution was switched among 160, 80, 30, and 15 mM NaCl. We saw the same decline as when the external solution was 160 mM NMDG-Cl. These experiments indicate that Po depends on the electrolyte concentration of the cytosol and not on external electrolyte concentration or the gradient of electrolytes across the membrane.
To determine the kinetic basis of the electrolyte dependence of CFTR
activity (NPo), we first needed to determine
whether channel number (N) was altered in lower electrolyte
solutions. Because of the smooth functions relating
NPo to electrolyte concentration and the rapid
onset and reversal of the changes in activity, we suspected that
N did not change, and that was confirmed when N was estimated for each patch as described under "Materials and Methods." To assess N more directly, we searched our
records for instances of multiple channel openings during periods when
the reduction of electrolytes caused a reduction in
NPo. As shown by the example in Fig.
4, by this criterion we concluded that N was essentially unchanged in the two conditions. We also
considered the alternate possibility that reduction in
NPo was caused entirely by a change in
N. For that to be true, the ratio of apparent
Po in solutions of high and low electrolyte
concentrations should be directly related to the ratio of N
in high- and low-salt concentrations. As shown Fig. 4C, the
ratio of N was almost invariant and was close to 1 for
>10-fold change in Po.
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Based on this evidence that N was constant across
electrolyte concentrations, we proceeded to estimate channel open and
closed times in solutions of different electrolyte concentrations using the strategy outlined under "Materials and Methods." This analysis, (shown for a single patch containing four channels in Fig.
5, a and b), indicated
that the low Po observed in 15 mM
NaCl solutions resulted from a >10-fold increase in mean closed time
with no detectable effect on mean open time. For this patch, control
versus 15 mM NaCl values were
Po, 0.30 versus 0.021; open time (s),
0.591 versus 0.512; and closed time, 1.52 versus
23.87.
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The same results were obtained after analysis of four additional patches exposed to 160 and 30 mM NaCl. For those four patches, the mean open time in low-electrolyte solution was 0.99 ± 0.20 of the control value, whereas the closed time was 3.95 ± 0.9 times longer than the control value. We conclude that electrolyte levels affect channel closed time and not N or open time.
In addition to this multichannel analysis, we searched for single-channel patches. We eventually obtained one example of a "pseudo" single-channel patch, in which two CFTR channels were active in the cell-attached configuration, but after excision one channel became nearly silent, so that double openings constituted only 0.0005 of the total open time. We exposed the patch to control solution and to 60 mM NaCl balanced osmotically with taurine and analyzed the dominantly active channel as a single channel. Probability density distributions (Fig. 5, c and d) were computed for open and closed time in the control (531 events) and 60 mM NaCl (368 events) solutions. The distributions were best fit with two exponentials with the time constants shown in the figures. This analysis confirmed that open time was unchanged and also revealed that only the slow component of the closed time was changed in 60 mM NaCl, increasing from 0.6 to 1.4 s.
CFTR activity is affected by the cytosolic pH, which alters
phosphatases and kinases and hence the phosphorylation and
dephosphorylation of CFTR (12). To determine whether electrolytes might
also affect the phosphorylation state of CFTR, we studied CFTR after
stable phosphorylation with ATPS and found that the
Po of CFTR was still decreased and then restored
to normal when switching between control and low-electrolyte
concentrations in the absence of PKA. For these experiments,
Po values in 160 and 15 NaCl were 0.31 ± 0.05 and 0.05 ± 0.02, respectively (n = 5;
p < 0.01; Fig. 6). The
Po and mean open and mean closed times were
derived for one patch phosphorylated with ATPS and indicate that the
effect was again entirely due to a change in the mean closed time. For
160 mM NaCl the values were: Po,
0.19; tO, 0.22 s; and
tC, 0.91 s. For 15 mM NaCl the
values were Po, 0.01; tO,
0.21 s; and tC, 27.63 s. (Note that this
30-fold change in closed time occurred in a patch estimated to have
only four channels.) These results suggest that the electrolyte effect
is most likely mediated via postphosphorylation gating events.
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The constellation of effects observed so far distinguishes the
electrolyte effect from most other factors known to affect CFTR gating
but is similar (although scaled by a concentration factor of ~1000)
to the dependence of Po on ATP concentration, which also decreases the mean closed time with no effect on mean open
time (23). To determine whether the reduced opening rate of CFTR in low
cytosolic electrolytes resulted from reduced binding of ATP, we varied
the ATP level 20-fold, from 0.5 to 10 mM, and measured
Po in 15 mM NaCl. Na2ATP
was used, with Mg2+ held constant at 2.5 mM and
pH adjusted to 7.3 with NaOH. (This process caused Mg-ATP levels to
vary only 5-fold, from 0.5 to 2.5.) Higher levels of Mg2+
were not used because of known effects on CFTR gating (13). Higher ATP
levels did not restore Po to the control level
but did cause a modest increase in relative Po
(from 11 ± 7 to 33 ± 21% of control; n = 3; mean ± S.E.). However, this increase is predicted simply by
the added electrolytes; i.e. 10 mM
Na2ATP + 15.5 mM NaOH moved the salt
concentration from 15 mM to 38 mM NaCl
equivalents (see Fig. 3). These results do not support the
interpretation that the electrolyte effect is mediated by altering ATP binding.
To determine whether the electrolyte effect on CFTR required factors
specific to Calu-3 membranes, we excised patches from mouse mammary
epithelial (C127) cells exogenously expressing CFTR (2WT2; Refs. 24,
25) and exposed them to reduced electrolyte concentrations. The
responses were equivalent to those observed for CFTR channels in Calu-3
cells (Fig. 7a),
indicating that the effect is either intrinsic to CFTR or relies on
cellular components common to these two cell types.
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To determine whether the gating dependence on cytosolic electrolytes
might be common to all channels, we studied an outwardly rectifying
anion channel that was sometimes activated by strong depolarization of
patches excised from cells at the edges of islands of Calu-3 cells
(26). Excised patches were subjected to strong, sustained
depolarization until rectifying anion channel activity was induced.
Patches were then switched between normal and low levels of
electrolytes. In contrast with CFTR channels, the
Po of the rectifying channels was unaffected by
the cytosolic electrolyte concentration (Fig. 7, b and
c). This is additional evidence suggesting that the effect
might be intrinsic to CFTR although by no means exclusive to CFTR.
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DISCUSSION |
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The discovery that CFTR gating depends on the electrolyte
concentration of the cytosol was completely unexpected. In two prior studies using reduced electrolytes with sucrose substitution, no
effects on Po were described (20, 27). In those
studies the electrolyte level was decreased to 75 mM, a
value that in our studies reduced Po only to
approximately half of control values. Because of inherent variations in
CFTR Po (13, 28), it is unlikely that a decline
of that magnitude would be noticed unless a rapid solution switching
system were used. Although no effect on Po was
noted, it is interesting that a reduction in CFTR channel conductance
by intracellular sucrose was observed (20, 27). We also observed a
reduction in channel conductance when using uncharged intracellular
osmolytes (see "Materials and Methods"), which tended to offset the
expected increase in channel current predicted by the greater driving
force for Cl.
We found no other reports in which channel gating was shown to require cytosolic electrolytes. Many prior studies show influences of electrolytes on gating, but these are all distinct from the present phenomenon. For example, volume-regulated channels are affected by ionic strength (29), but the sign is opposite from what we observe. The gating of many channels is influenced by specific ions. Examples include a strong affect of extracellular K+ concentration on the voltage dependence of an inwardly rectifying cardiac K+ current (30) and the gating of an outward-rectifying K+ channel in plant stomatal guard cells (31). Gating (NPo) of the rat epithelial Na+ channel was significantly decreased when intracellular [Na+] was increased from 0 to 50 mM in inside-out patches (32), and CLC channel gating is influenced in a complex manner by the concentration and species of internal and external anions (33). These phenomena are distinct in their features from the present findings. They all depend on specific ions, two are voltage-dependent, and the epithelial Na+ channel effect has a sign opposite to what we observed. Also, in these examples ion levels modulate gating, whereas CFTR gating has an absolute dependence on electrolytes. We also found only rare examples in which the transport or ATPase activity of ABC transporters was shown to be affected by electrolyte concentration. For example, ATPase activity and histidine transport of histidine permease are salt-sensitive, but the pattern differed markedly from what we observed, being high at 0 NaCl, maximal at ~50 mM, and then declining at higher concentrations (34, 35).
Is electrolyte concentration a physiological regulator of CFTR? It is conceivable that the steep dependence of CFTR Po on the electrolyte concentration of the cytosol could regulate CFTR-mediated conductance under some physiological conditions, such as in the sweat duct, where it has been hypothesized that a salt sensor closes CFTR at low luminal NaCl values (12, 36). The electrolyte dependence of CFTR Po could allow it to be the salt sensor if the extremely high conductance of the duct apical membrane (~125 mS/cm2; Ref. 37) coupled the NaCl concentrations of cytosol and lumen. However, the sweat duct is exceptional in having a very high apical membrane conductance, a very low concentration (~15 mM) of luminal ions, and low water permeability. In most cells and conditions, the cytosolic electrolyte concentration should be sufficient to maintain high CFTR activity, and alterations in external electrolyte concentration would probably affect only cell volume. These considerations suggest that electrolytes are, like Mg2+ and ATP, essential and typically invariant cofactors for CFTR activity.
How might electrolytes alter CFTR gating? Kinetic analysis distinguishes the electrolyte effect from other factors known to affect CFTR gating, such as the species and concentrations of divalent cations (10, 13, 14) and the redox status (14). Our data also suggest that an altered phosphorylation state does not explain the electrolyte effect, although more work needs to be done on that point. Low electrolytes did not affect channel open time. Because of considerable evidence that the open time is determined by hydrolysis at NBD2 (6, 38), the unchanged open time indicates that hydrolysis of ATP at NBD2 and all subsequent steps leading to closing are not directly affected by electrolyte levels. The remaining possibilities are that the electrolyte concentration affects either ATP binding at the NBD, ATP hydrolysis at NBD1, or the linkage between these events and channel opening. Our preliminary results do not support an electrolyte effect on ATP binding, although those experiments do not definitively rule out such an effect, because when we varied the ATP levels, we also varied the electrolyte concentration. Although the possibility of an electrolyte effect on ATP binding is still open, it appears to be more likely that electrolytes are affecting some aspect of ATP hydrolysis at NBD1 or the consequences following from hydrolysis.
How might electrolytes have such effects? Two general kinds of explanations seem salient. First, electrolytes may be required for the structural integrity of CFTR. An effect on CFTR structure might arise, for example, if solution electrolytes are needed for screening of Coulomb repulsion (e.g. Ref. 39) or attraction as occurs in salt bridges (40) between portions of CFTR. This explanation is consistent with the lack of any ion specificity to the effect. A physical effect of this nature might be expected to respond to ionic strength rather than electrolyte concentration (41), so it will be important, in future work, to establish definitively whether ionic strength is the critical variable contributing to changed Po.
Alternatively, electrolytes may be required to allow CFTR to complete a hydrolysis cycle because they participate in that process directly, for example, by being a substrate for transport, or a necessary cofactor in transport. This possibility is made less attractive by the lack of any evidence that CFTR is a transporter and by the lack of specificity in the ionic species that can support activity. However, other members of the ABC transport family, such as MDR1 (P-glycoprotein), transport an extremely diverse set of substrates, which have little in common except hydrophobicity (42). That the dependence of CFTR gating on cytosolic electrolytes is most similar to the dependence on ATP itself raises the possibility that CFTR might retain either a transport function or at least components of transport that might now serve to regulate gating.
Although we found no prior example of an electrolyte effect on "typical" channels, transporters with requirements for specific electrolytes are commonplace, and some electrolyte-dependent transporters also function as ion channels (43, 44). CFTR is the only known ion channel among at least 39 human ABC transporters (1), many of which transport lipids, bile salts, conjugates with glutathione or sulfates, nucleoside-based compounds, or simply hydrophobic compounds (1). ABC1, which transports cholesterol (45), also appears to be a cAMP-dependent and sulfonylurea-sensitive anion transporter, although it is not itself a channel (46). The presence of multiple functions within ABC transporters and the coexistence of channel and transport functions in other proteins suggest that a possible transport function for CFTR should not be dismissed. If CFTR gating does depend on the transport of a substrate, that substrate would need to be present within the bilayer (47) or the bath solution.
Regardless of the merits of these speculations, the discovery that CFTR
gating depends on the electrolyte concentration of the cytosol provides
a new tool for probing gating and ATPase hydrolysis cycles in CFTR and
perhaps in other ABC transporters.
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ACKNOWLEDGEMENTS |
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We thank Valerie Baldwin for help with cell culture, Richard Aldrich, Ron Kopito, and Rabin Tirouvanziam for criticisms and advice, and Seng Cheng (Genzyme Corp.) for the generous gift of C127 cells stably expressing CFTR.
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FOOTNOTES |
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* This work was supported by National Institutes of Health Grant DK51817 and the Cystic Fibrosis Foundation.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.
To whom correspondence should be addressed:
Cystic Fibrosis Research Laboratory, Bldg. 420, Stanford University,
Stanford, CA 94305-2130. Tel.: 650-725-2462; Fax: 650-725-5699; E-mail: wine@stanford.edu.
Published, JBC Papers in Press, December 8, 2000, DOI 10.1074/jbc.M009305200
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ABBREVIATIONS |
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The abbreviations used are:
CFTR, cystic
fibrosis transmembrane conductance regulator;
ABC, ATP binding
cassette;
NMDG, N-methyl-D-glucamine;
NBD, nucleotide binding domain;
Po, open probability;
NPo, open probability of channel number;
PKA, protein kinase A;
ATPS, adenosine
5'-O-(thiotriphosphate).
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REFERENCES |
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