Beyond the diffusion limit: Water flow through the empty bacterial potassium channel

Saparov, Sapar M.; Pohl, Peter

doi:10.1073/pnas.0308309101

Cited by 75 publications

(110 citation statements)

References 44 publications

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“…This, too, is in contrast to potassium channels, where the rate of ionic flux limits the maximum water flux through the channel (12,21). The key difference is that the SF of the sodium channel is much wider than that of the potassium channel.…”

Section: Discussionmentioning

confidence: 97%

Molecular dynamics of ion transport through the open conformation of a bacterial voltage-gated sodium channel

Ulmschneider

Bagnéris

McCusker

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

155

248

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The crystal structure of the open conformation of a bacterial voltage-gated sodium channel pore from Magnetococcus sp.(NaVMs) has provided the basis for a molecular dynamics study defining the channel's full ion translocation pathway and conductance process, selectivity, electrophysiological characteristics, and ion-binding sites. Microsecond molecular dynamics simulations permitted a complete time-course characterization of the protein in a membrane system, capturing the plethora of conductance events and revealing a complex mixture of single and multiion phenomena with decoupled rapid bidirectional water transport. The simulations suggest specific localization sites for the sodium ions, which correspond with experimentally determined electron density found in the selectivity filter of the crystal structure. These studies have also allowed us to identify the ion conductance mechanism and its relation to water movement for the NavMs channel pore and to make realistic predictions of its conductance properties. The calculated single-channel conductance and selectivity ratio correspond closely with the electrophysiology measurements of the NavMs channel expressed in HEK 293 cells. The ion translocation process seen in this voltage-gated sodium channel is clearly different from that exhibited by members of the closely related family of voltage-gated potassium channels and also differs considerably from existing proposals for the conductance process in sodium channels. These studies simulate sodium channel conductance based on an experimentally determined structure of a sodium channel pore that has a completely open transmembrane pathway and activation gate. V oltage-gated cation channels are proteins that produce electrical signals in neurons and other excitable cells to regulate muscle contraction, gene expression, and release of hormones and neurotransmitters among other functions. In response to a change in transmembrane electrical potential, the channels open pores through which ions move passively across the membrane. The large family of cation channels includes those selective for sodium, potassium, or calcium. The opening and closing of these ion-specific channels is carefully choreographed to produce the electrical signals required by the nervous system for rapid signal transduction (1).Voltage-gated sodium channels have been causally linked with a wide range of neurological and cardiovascular diseases and hence are important pharmaceutical drug-development targets (2, 3). Eukaryotic voltage-gated sodium channels are large, singlechain polypeptides, consisting of 24 transmembrane (TM) helices that form four homologous repeats, each contributing both a voltage sensor and a pore domain; the latter are arranged to form a central Na + -selective transmembrane pathway. Bacterial voltagegated sodium channels are far simpler, consisting of four polypeptide chains, each of which is composed of six TM segments, with segments TM1-TM4 forming the voltage sensors and TM5-TM6 forming the pore domains. The TM5-TM6 segment...

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Section: Discussionmentioning

confidence: 97%

Molecular dynamics of ion transport through the open conformation of a bacterial voltage-gated sodium channel

Ulmschneider

Bagnéris

McCusker

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

155

248

View full text Add to dashboard Cite

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“…This hypothesis is supported by the observation that KcsA provides an extremely efficient water pathway. The reported single-channel water permeability p f = 4.8 × 10 −12 cm 3 ·s −1 (8) exceeds that of channels specialized in water transport by more than an order of magnitude (9). For example, the p f values for aquaporin-1 and aquaporin-4 amount to 0.6-1.1 and 2.4 × 10 −13 cm…”

mentioning

confidence: 89%

“…5). K + concentrations of 300-400 mM are required for complete blockage of water flux through the KcsA pore (8). We added 200 mM K + to the remainder of the proteoliposome population (Fig.…”

mentioning

confidence: 99%

Filter gate closure inhibits ion but not water transport through potassium channels

Hoomann

Jahnke

Hörner

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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The selectivity filter of K + channels is conserved throughout all kingdoms of life. Carbonyl groups of highly conserved amino acids point toward the lumen to act as surrogates for the water molecules of K + hydration. Ion conductivity is abrogated if some of these carbonyl groups flip out of the lumen, which happens (i) in the process of C-type inactivation or (ii) during filter collapse in the absence of K + . Here, we show that K + channels remain permeable to water, even after entering such an electrically silent conformation. We reconstituted fluorescently labeled and constitutively open mutants of the bacterial K + channel KcsA into lipid vesicles that were either C-type inactivating or noninactivating. Fluorescence correlation spectroscopy allowed us to count both the number of proteoliposomes and the number of protein-containing micelles after solubilization, providing the number of reconstituted channels per proteoliposome. Quantification of the per-channel increment in proteoliposome water permeability with the aid of stopped-flow experiments yielded a unitary water permeability p f of (6.9 ± 0.6) × 10 −13 cm 3 ·s −1 for both mutants. "Collapse" of the selectivity filter upon K + removal did not alter p f and was fully reversible, as demonstrated by current measurements through planar bilayers in a K + -containing medium to which K + -free proteoliposomes were fused. Water flow through KcsA is halved by 200 mM K + in the aqueous solution, which indicates an effective K + dissociation constant in that range for a singly occupied channel. This questions the widely accepted hypothesis that multiple K + ions in the selectivity filter act to mutually destabilize binding. membrane channels | protein reconstitution | knock-on mechanism | aquaporin | brain water homeostasis

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“…3), the L p value for the empty selectivity filter (L 0 ) is necessary, while f 0 is calculated from the fitted rate constants as the probability of the empty pore (state 8) under the given conditions. For the KcsA channel, the L p value is available (Saparov and Pohl, 2004), and the L p value represents the selectivity filter. However, an extremely high L p value, overwhelming the rest of the pore, does not satisfy the requirement of Equation 1, in which a rate-limiting L p value is required.…”

mentioning

confidence: 99%

Counting Ion and Water Molecules in a Streaming File through the Open-Filter Structure of the K Channel

Iwamoto¹,

Oiki²

2011

J. Neurosci.

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The mechanisms underlying the selective permeation of ions through channel molecules are a fundamental issue related to understanding how neurons exert their functions. The "knock-on" mechanism, in which multiple ions in the selectivity filter are hit by an incoming ion, is one of the leading concepts. This mechanism has been supported by crystallographic studies that demonstrated ion distribution in the structure of the Streptomyces lividans (KcsA) potassium channel. These still pictures under equilibrium conditions, however, do not provide a snapshot of the actual, ongoing permeation processes. To understand the dynamics of permeation, we determined the ratio of the ion and water flow [the water-ion coupling ratio (CR w-i )] through the KcsA channel by measuring the streaming potential (V stream ) electrophysiologically. The V stream value was converted to the CR w-i value, which reveals how individual ion and water molecules are queued in the narrow and short filter during permeation. At high K ϩ concentrations, the CR w-i value was 1.0, indicating that turnover between the alternating ion and water arrays occurs in a single-file manner. At low K ϩ , the CR w-i value was increased to a point over 2.2, suggesting that the filter contained mostly one ion at a time. These average behaviors of permeation were kinetically analyzed for a more detailed understanding of the permeation process. Here, we envisioned the permeation as queues of ion and water molecules and sequential transitions between different patterns of arrays. Under physiological conditions, we predicted that the knock-on mechanism may not be predominant.

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Beyond the diffusion limit: Water flow through the empty bacterial potassium channel

Cited by 75 publications

References 44 publications

Molecular dynamics of ion transport through the open conformation of a bacterial voltage-gated sodium channel

Molecular dynamics of ion transport through the open conformation of a bacterial voltage-gated sodium channel

Filter gate closure inhibits ion but not water transport through potassium channels

Counting Ion and Water Molecules in a Streaming File through the Open-Filter Structure of the K Channel

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