David C. Gadsby

Cystic fibrosis transmembrane conductance regulator (CFTR) channel opening and closing are driven by cycles of adenosine triphosphate (ATP) binding–induced formation and hydrolysis-triggered disruption of a heterodimer of its cytoplasmic nucleotide-binding domains (NBDs). Although both composite sites enclosed within the heterodimer interface contain ATP in an open CFTR channel, ATP hydrolysis in the sole catalytically competent site causes channel closure. Opening of the NBD interface at that site then allows ADP–ATP exchange. But how frequently, and how far, the NBD surfaces separate at the other, inactive composite site remains unclear. We assessed separation at each composite site by monitoring access of nucleotide-sized hydrophilic, thiol-specific methanothiosulfonate (MTS) reagents to interfacial target cysteines introduced into either LSGGQ-like ATP-binding cassette signature sequence (replacing equivalent conserved serines: S549 and S1347). Covalent MTS-dependent modification of either cysteine while channels were kept closed by the absence of ATP impaired subsequent opening upon ATP readdition. Modification while channels were opening and closing in the presence of ATP caused macroscopic CFTR current to decline at the same speed as when the unmodified channels shut upon sudden ATP withdrawal. These results suggest that the target cysteines can be modified only in closed channels; that after modification the attached MTS adduct interferes with ATP-mediated opening; and that modification in the presence of ATP occurs rapidly once channels close, before they can reopen. This interpretation was corroborated by the finding that, for either cysteine target, the addition of the hydrolysis-impairing mutation K1250R (catalytic site Walker A Lys) similarly slowed, by an order of magnitude, channel closing on ATP removal and the speed of modification by MTS reagent in ATP. We conclude that, in every CFTR channel gating cycle, the NBD dimer interface separates simultaneously at both composite sites sufficiently to allow MTS reagents to access both signature-sequence serines. Relatively rapid modification of S1347C channels by larger reagents—MTS-glucose, MTS-biotin, and MTS-rhodamine—demonstrates that, at the noncatalytic composite site, this separation must exceed 8 Å.

A single Na⁺/K⁺-ATPase pumps three Na⁺ outwards and two K⁺ inwards by alternately exposing ion-binding sites to opposite sides of the membrane in a conformational sequence coupled to pump autophosphorylation from ATP and auto-dephosphorylation. The larger flow of Na⁺ than K⁺ generates outward current across the cell membrane. Less well understood is the ability of Na⁺/K⁺ pumps to generate an inward current of protons. Originally noted in pumps deprived of external K⁺ and Na⁺ ions, as inward current at negative membrane potentials that becomes amplified when external pH is lowered, this proton current is generally viewed as an artifact of those unnatural conditions. We demonstrate here that this inward current also flows at physiological K⁺ and Na⁺ concentrations. We show that protons exploit ready reversibility of conformational changes associated with extracellular Na⁺ release from phosphorylated Na⁺/K⁺ pumps. Reversal of a subset of these transitions allows an extracellular proton to bind an acidic side chain and to be subsequently released to the cytoplasm. This back-step of phosphorylated Na⁺/K⁺ pumps that enables proton import is not required for completion of the 3 Na⁺/2 K⁺ transport cycle. However, the back-step occurs readily during Na⁺/K⁺ transport when external K⁺ ion binding and occlusion are delayed, and it occurs more frequently when lowered extracellular pH raises the probability of protonation of the externally accessible carboxylate side chain. The proton route passes through the Na⁺-selective binding site III and is distinct from the principal pathway traversed by the majority of transported Na⁺ and K⁺ ions that passes through binding site II. The inferred occurrence of Na⁺/K⁺ exchange and H⁺ import during the same conformational cycle of a single molecule identifies the Na⁺/K⁺ pump as a hybrid transporter. Whether Na⁺/K⁺ pump–mediated proton inflow may have any physiological or pathophysiological significance remains to be clarified.

Na⁺/K⁺ pumps move net charge through the cell membrane by mediating unequal exchange of intracellular Na⁺ and extracellular K⁺. Most charge moves during transitions that release Na⁺ to the cell exterior. When pumps are constrained to bind and release only Na⁺, a membrane voltage-step redistributes pumps among conformations with zero, one, two or three bound Na⁺, thereby transiently generating current. By applying rapid voltage steps to squid giant axons, we previously identified three components in such transient currents, with distinct relaxation speeds: fast (which nearly parallels the voltage-jump time course), medium speed (τ_m=0.2–0.5 ms) and slow (τ_s=1–10 ms). Here we show that these three components are tightly correlated, both in their magnitudes and in the time courses of their changes. The correlations reveal the dynamics of the conformational rearrangements that release three Na⁺ to the exterior (or sequester them into their binding sites) one at a time, in an obligatorily sequential manner.

CFTR is the only member of the ABC (ATP-binding cassette) protein superfamily known to function as an ion channel. Most other ABC proteins are ATP-driven transporters, in which a cycle of ATP binding and hydrolysis, at intracellular nucleotide binding domains (NBDs), powers uphill substrate translocation across the membrane. In CFTR, this same ATP-driven cycle opens and closes a transmembrane pore through which chloride ions flow rapidly down their electrochemical gradient. Detailed analysis of the pattern of gating of CFTR channels thus offers the opportunity to learn about mechanisms of function not only of CFTR channels but also of their ABC transporter ancestors. In addition, CFTR channel gating is subject to complex regulation by kinase-mediated phosphorylation at multiple consensus sites in a cytoplasmic regulatory domain that is unique to CFTR. Here we offer a practical guide to extract useful information about the mechanisms that control opening and closing of CFTR channels: on how to plan (including information obtained from analysis of multiple sequence alignments), carry out, and analyze electrophysiological and biochemical experiments, as well as on how to circumvent potential pitfalls.

Interactions of the three transported Na ions with the Na/K pump remain incompletely understood. Na/K pump crystal structures show that the extended C terminus of the Na,K–adenosine triphosphatase (ATPase) α subunit directly contacts transmembrane helices. Deletion of the last five residues (KETYY in almost all Na/K pumps) markedly lowered the apparent affinity for Na activation of pump phosphorylation from ATP, a reflection of cytoplasmic Na affinity for forming the occluded E1P(Na3) conformation. ATPase assays further suggested that C-terminal truncations also interfere with low affinity Na interactions, which are attributable to extracellular effects. Because extracellular Na ions traverse part of the membrane’s electric field to reach their binding sites in the Na/K pump, their movements generate currents that can be monitored with high resolution. We report here electrical measurements to examine how Na/K pump interactions with extracellular Na ions are influenced by C-terminal truncations. We deleted the last two (YY) or five (KESYY) residues in Xenopus laevis α1 Na/K pumps made ouabain resistant by either of two kinds of point mutations and measured their currents as 10-mM ouabain–sensitive currents in Xenopus oocytes after silencing endogenous Xenopus Na/K pumps with 1 µM ouabain. We found the low affinity inhibitory influence of extracellular Na on outward Na/K pump current at negative voltages to be impaired in all of the C-terminally truncated pumps. Correspondingly, voltage jump–induced transient charge movements that reflect pump interactions with extracellular Na ions were strongly shifted to more negative potentials; this signals a several-fold reduction of the apparent affinity for extracellular Na in the truncated pumps. Parallel lowering of Na affinity on both sides of the membrane argues that the C-terminal contacts provide important stabilization of the occluded E1P(Na3) conformation, regardless of the route of Na ion entry into the binding pocket. Gating measurements of palytoxin-opened Na/K pump channels additionally imply that the C-terminal contacts also help stabilize pump conformations with occluded K ions.

The Na+,K+-ATPase pump achieves thermodynamically uphill exchange of cytoplasmic Na+ ions for extracellular K+ ions by using ATP-mediated phosphorylation, followed by autodephosphorylation, to powe...

Ions move across cell membranes through either ion channels or ion pumps. Recently, atomic-resolution structures and high-resolution functional measurements of examples from both channels and pumps have begun to suggest that these molecules need not be as different as was once thought. The incessant traffic of ions across cell membranes is controlled by two kinds of border guards: ion channels and ion pumps. Open channels let selected ions diffuse rapidly down electrical and concentration gradients, whereas ion pumps labour tirelessly to maintain the gradients by consuming energy to slowly move ions thermodynamically uphill. Because of the diametrically opposed tasks and the divergent speeds of channels and pumps, they have traditionally been viewed as completely different entities, as alike as chalk and cheese. But new structural and mechanistic information about both of these classes of molecular machines challenges this comfortable separation and forces its re-evaluation.

In principle, an ion channel needs no more than a single gate, but a pump requires at least two gates that open and close alternately to allow ion access from only one side of the membrane at a time. In the Na ⁺ ,K ⁺ -ATPase pump, this alternating gating effects outward transport of three Na ⁺ ions and inward transport of two K ⁺ ions, for each ATP hydrolysed, up to a hundred times per second, generating a measurable current if assayed in millions of pumps. Under these assay conditions, voltage jumps elicit brief charge movements, consistent with displacement of ions along the ion pathway while one gate is open but the other closed. Binding of the marine toxin, palytoxin, to the Na ⁺ ,K ⁺ -ATPase uncouples the two gates, so that although each gate still responds to its physiological ligand they are no longer constrained to open and close alternately, and the Na ⁺ ,K ⁺ -ATPase is transformed into a gated cation channel. Millions of Na ⁺ or K ⁺ ions per second flow through such an open pump–channel, permitting assay of single molecules and allowing unprecedented access to the ion transport pathway through the Na ⁺ ,K ⁺ -ATPase. Use of variously charged small hydrophilic thiol-specific reagents to probe cysteine targets introduced throughout the pump's transmembrane segments allows mapping and characterization of the route traversed by transported ions.

P-type ATPases are molecular machines that use energy derived from ATP hydrolysis to pump ions across membranes. Recent work on the Na+, K+ ATPase has identified a pathway for ions from the extracellular side of the protein to its ion binding sites. Here, Takeuchi et al. employ the marine toxin palytoxin to 'hold open' the gates of the pump, allowing direct measurements of ion flow. They define the entire route for ions from one side of the membrane to the other. P-type ATPases are molecular machines that use energy derived from ATP hydrolysis to pump ions across membranes. Recent work on the Na+, K+ ATPase has identified a pathway for ions from the extracellular side of the protein to its ion binding sites. This paper employs the marine toxin palytoxin to 'hold open' the gates of the pump, allowing direct measurements of ion flow and defines the entire route for ions from one side of the membrane to the other. P-type ATPases pump ions across membranes, generating steep electrochemical gradients that are essential for the function of all cells. Access to the ion-binding sites within the pumps alternates between the two sides of the membrane1 to avoid the dissipation of the gradients that would occur during simultaneous access. In Na+,K+-ATPase pumps treated with the marine agent palytoxin, this strict alternation is disrupted and binding sites are sometimes simultaneously accessible from both sides of the membrane, transforming the pumps into ion channels (see, for example, refs 2, 3). Current recordings in these channels can monitor accessibility of introduced cysteine residues to water-soluble sulphydryl-specific reagents4. We found previously5 that Na+,K+ pump-channels open to the extracellular surface through a deep and wide vestibule that emanates from a narrower pathway between transmembrane helices 4 and 6 (TM4 and TM6). Here we report that cysteine scans from TM1 to TM6 reveal a single unbroken cation pathway that traverses palytoxin-bound Na+,K+ pump-channels from one side of the membrane to the other. This pathway comprises residues from TM1, TM2, TM4 and TM6, passes through ion-binding site II, and is probably conserved in structurally and evolutionarily related P-type pumps, such as sarcoplasmic- and endoplasmic-reticulum Ca2+-ATPases and H+,K+-ATPases.