SW Bulletin Report - Ion Channels

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Wed Sep 8 09:59:55 EST 1999


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SW BULLETIN - September 6, 1999
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This Week's Topic:

NEUROBIOLOGY: ION CHANNELS IN CELL MEMBRANES

[The following report appeared in ScienceWeek 13 Aug 99]

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STRUCTURAL REARRANGEMENTS IN POTASSIUM CHANNEL GATING
Ion channels are protein channels in cell membranes that allow
ions to pass from extracellular solution to intracellular
solution and vice versa. Most ion channels are selective,
allowing only certain ions to pass, and an individual cell has
ion channels with various ion selectivities. The selectivity of
an ion channel can be "gated", the channel effectively opened or
closed, and ion channels are said to voltage-gated or ligand-
gated, depending on how the change in selectivity is provoked.
The opening of a previously closed ion channel produces a sudden
increase in transmembrane conductance for that ion, and the
process is called "activation". The gating of the movements of
ions through ion channels is of considerable importance for
various processes in all living systems, and forms the basis of
the electrical activity of all nervous systems. Recently (see
background material below), an important advance in ion-channel
research occurred with the experimental determination of the
crystal structure of a potassium channel (KcsA) in the bacterial
species Streptomyces lividans. The structure involves a
tetrameric complex with a centrally located pore framed by the
apposition of individual subunits, each subunit with 2
transmembrane helices (TM1 and TM2) flanking a "selectivity
filter". Intensive studies of this potassium channel in *planar
lipid bilayers have been in progress in a number of laboratories.
... ... E. Porozo et al (3 authors at University of Virginia, U)
now report a study of the structural rearrangements underlying
activation gating in this potassium channel, the study using
*spin-labeling methods and *electron paramagnetic resonance
spectroscopy. The authors report that a comparison of the closed
and open conformations of the channel revealed periodic changes
in spin-label mobility and intersubunit *spin-spin interaction
consistent with rigid-body movements of the two transmembrane
helices TM1 and TM2. These changes involve translations and
counterclockwise rotations of both helices relative to the center
of symmetry of the channel. The movement of TM2 apparently
increases the diameter of the permeation pathway along the point
of convergence of the four subunits, thus opening the pore.
Although the extracellular residues flanking the selectivity
filter remained immobile during gating, small movements were
detected at the *C-terminal end of the pore helix, and the
authors suggest this has possible implications for the gating
mechanism.
-----------
E. Perozo et al: Structural rearrangements underlying
K(+)-channel activation gating.
(Science 2 Jul 99 285:73)
QY: Eduardo Perozo [eperozo at virginia.edu]
-----------
Text Notes:
... ... *planar lipid bilayers: The cell membrane consists of a
lipid bilayer and associated proteins, the ensemble approximately
75 to 100 angstroms in thickness. Similar membranes are also
found within a cell surrounding various organelles. Lipid
bilayers are spontaneously forming self-organizing bimolecular
layers of certain molecules (lipids) with long nonpolar chains
terminated by a polar group. In addition to their presence in
cell membranes, such molecules (surfactants) are also found in
soaps. A variety of artificial lipid bilayer membrane systems can
be investigated in the laboratory.
... ... *spin-labeling methods: A "spin-label" is a synthetic
paramagnetic organic free radical incorporated in a macromolecule
or assemblage of macromolecules and used, in particular, in
electron paramagnetic resonance spectroscopy.
... ... *electron paramagnetic resonance spectroscopy: (ESR) This
technique is used to investigate paramagnetic centers in a
molecular system. Only electrons whose spin is not paired with
the oppositely directed spin of another electron give an ESR
signal. With this technique, information can be obtained about
certain transitional ions, free radicals, and free electron
centers. A probe giving an ESR signal can be incorporated into
membrane lipids or attached to proteins to enable otherwise
inaccessible systems to be studied. Through analysis of ESR
spectra, rates of molecular motion and relative orientation of
spin-labeled molecules whose motion is restrained by surrounding
molecules can be determined. Measurements of rates of molecular
motion and molecular orientation have proved to be important in
the study of a variety of biological problems.
... ... *spin-spin interaction: In this context, an interaction
of two neighboring paramagnetic entities, the interaction
producing a change in ESR signal.
... ... *C-terminal end: In general, this refers to the end of
any polypeptide chain at which the 1-carboxy function of a
constituent alpha-amino acid is not attached in peptide linkage
to another amino acid residue.
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Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 13Aug99
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Related Background:
ANALYSIS OF POTASSIUM ION MEMBRANE CHANNEL STRUCTURE
... The potassium ion channel from the prokaryotic soil bacterium
Streptomyces lividans is an integral membrane protein with
sequence similarity to all known potassium ion channels,
particularly in the pore region. ... ... Doyle et al (8 authors
at Rockefeller University, US) report an x-ray analysis (data to
3.2 angstroms) of the Streptomyces lividans potassium channel
reveals four identical subunits create an inverted cone cradling
the selectivity filter of the pore in its outer end. The narrow
selectivity filter is only 12 angstroms long, whereas the
remainder of the pore is wider and lined with hydrophobic amino
acids. The selectivity filter is apparently held open by
structural constraints to coordinate potassium ions but not
smaller sodium ions. The authors suggest the architecture of the
pore establishes the physical principles underlying selective
potassium ion conduction.
QY: Roderick MacKinnon (mackinn at rockvax.rockefeller.edu)
(Science 3 Apr 98) (Science-Week 17 Apr 98)
-------------------
Related Background:
SIMILAR STRUCTURE OF PROKARYOTIC VS. EUKARYOTIC K(+) CHANNELS
Toxins from scorpion venom are known to interact with potassium
ion channels in eukaryotic cell membranes. Mackinnon et al (5
authors at Rockefeller University, US) report the use of resin-
attached mutant potassium ion channels from the bacterium
Streptomyces lividans to screen scorpion venom, and the toxins
that interact with the channel were identified by mass
spectrometry. The authors suggest their results indicate that the
prokaryotic potassium ion channel, whose structure has now been
revealed, has the same pore structure as eukaryotic potassium ion
channels, and that this structural conservation, through the
application of their techniques, offers a new approach to
potassium ion channel pharmacology.
QY: Roderick MacKinnon (mackinn at rockvax.rockefeller.edu)
(Science 3 Apr 98) (Science-Week 17 Apr 98)

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