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      Protein Rearrangements Underlying Slow Inactivation of the Shaker K + Channel

      research-article
      ,
      The Journal of General Physiology
      The Rockefeller University Press
      inactivation, fluorescence, K+ channel, pore, S4

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          Abstract

          Voltage-dependent ion channels transduce changes in the membrane electric field into protein rearrangements that gate their transmembrane ion permeation pathways. While certain molecular elements of the voltage sensor and gates have been identified, little is known about either the nature of their conformational rearrangements or about how the voltage sensor is coupled to the gates. We used voltage clamp fluorometry to examine the voltage sensor (S4) and pore region (P-region) protein motions that underlie the slow inactivation of the Shaker K + channel. Fluorescent probes in both the P-region and S4 changed emission intensity in parallel with the onset and recovery of slow inactivation, indicative of local protein rearrangements in this gating process. Two sequential rearrangements were observed, with channels first entering the P-type, and then the C-type inactivated state. These forms of inactivation appear to be mediated by a single gate, with P-type inactivation closing the gate and C-type inactivation stabilizing the gate's closed conformation. Such a stabilization was due, at least in part, to a slow rearrangement around S4 that stabilizes S4 in its activated transmembrane position. The fluorescence reports of S4 and P-region fluorophore are consistent with an increased interaction of the voltage sensor and inactivation gate upon gate closure, offering insight into how the voltage-sensing apparatus is coupled to a channel gate.

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          Most cited references28

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          Transmembrane movement of the shaker K+ channel S4.

          We have probed internal and external accessibility of S4 residues to the membrane-impermeant thiol reagent methanethiosulfonate-ethyltrimethlammonium (MTSET) in both open and closed, cysteine-substituted Shaker K+ channels. Our results indicate that S4 traverses the membrane with no more than 5 amino acids in the closed state, and that the distribution of buried residues changes when channels open. This change argues for a displacement of S4 through the plane of the membrane in which an initially intracellular residue moves to within 3 amino acids of the extracellular solution. These results demonstrate that the putative voltage-sensing charges of S4 actually reside in the membrane and that they move outward when channels open. We consider constraints placed on channel structure by these results.
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            Rapid and efficient site-specific mutagenesis without phenotypic selection.

            T Kunkel (1985)
            Several single-base substitution mutations have been introduced into the lacZ alpha gene in cloning vector M13mp2, at 40-60% efficiency, in a rapid procedure requiring only transfection of the unfractionated products of standard in vitro mutagenesis reactions. Two simple additional treatments of the DNA, before transfection, produce a site-specific mutation frequency approaching 100%. The approach is applicable to phenotypically silent mutations in addition to those that can be selected. The high efficiency, approximately equal to 10-fold greater than that observed using current methods without enrichment procedures, is obtained by using a DNA template containing several uracil residues in place of thymine. This template has normal coding potential for the in vitro reactions typical of site-directed mutagenesis protocols but is not biologically active upon transfection into a wild-type (i.e., ung+) Escherichia coli host cell. Expression of the desired change, present in the newly synthesized non-uracil-containing covalently closed circular complementary strand, is thus strongly favored. The procedure has been applied to mutations introduced via both oligonucleotides and error-prone polymerization. In addition to its utility in changing DNA sequences, this approach can potentially be used to examine the biological consequences of specific lesions placed at defined positions within a gene.
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              Gating currents from a nonconducting mutant reveal open-closed conformations in Shaker K+ channels.

              In voltage-dependent ion channels, a voltage sensor region is responsible for channel activation and an aqueous pore is responsible for ion conduction. These two processes have been traditionally considered to be independent. We describe here a mutation in the putative pore region (W434F) that completely abolishes ion conduction without affecting the gating charge of the channel. Gating currents in the nonconductive mutant were found to be identical in their kinetic and steady-state properties to those in conductive channels. Gating current measurements could be performed without subtracting pulses and in the presence of normal physiological solutions. Application of internal tetraethylammonium (an open channel blocker) induced Off charge immobilization for large depolarizations, suggesting that the internal tetraethylammonium-binding site becomes available upon depolarization. We concluded that for this mutant, although the conduction pathway is not functional, the channel can still undergo the closed-open conformation in response to voltage changes.
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                Author and article information

                Journal
                J Gen Physiol
                The Journal of General Physiology
                The Rockefeller University Press
                0022-1295
                1540-7748
                1 October 1998
                : 112
                : 4
                : 377-389
                Affiliations
                From the Department of Molecular & Cell Biology, University of California, Berkeley, Berkeley, California 94720-3200
                Author notes

                Address correspondence to E.Y. Isacoff, Department of Molecular & Cell Biology, University of California, Berkeley, 271 Life Science Addition, Berkeley, CA 94720-3200. Fax: 510-643-6791; E-mail: ehud@ 123456uclink.berkeley.edu

                Article
                2229423
                9758858
                19122265-604a-459e-adde-a6ea94f51a34
                Copyright @ 1998
                History
                : 12 May 1998
                : 3 August 1998
                Categories
                Article

                Anatomy & Physiology
                inactivation,fluorescence,k+ channel,pore,s4
                Anatomy & Physiology
                inactivation, fluorescence, k+ channel, pore, s4

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