ion channels in plants

2. Ion channel gating in plants
 INTRODUCTION:
 Cells must be ready to respond to essential signals in
their environment. These are often chemicals in the
extracellular fluid (ECF).
 Long-range allostery is often a significant component
of cell signaling events.

3.  A precise control of ion channel opening is essential
for the physiological functioning of plant cells. This
process is termed gating. This may be effected by
ligand-binding, fuctuations in membrane potential,
membrane stretch and light quality.
 gating mechanisms are complex and that individual
ion channels can be regulated by a number of factors

4.  Ion channels can be divided into four `historically-based'
groups according to gating mechanism:
 ligand-gated
 voltage-gated
 stretch-activated
 light-activated.
 Ligand-gated ion channels bind intracellular secondary
messengers which provide the essential links between
external stimuli and specific intracellular responses.

5.  They are also involved in membrane voltage stabilization,
which is critical for maintaining ionic gradients and
nutritional ion fluxes.
 . Stretch-activated ion channels serve as additional specifc
transmembrane `receptors' co-existing with other cellular
volume-sensing mechanisms.
 Light-activated channels are in fact ligand-gated, although
a precise indication of the ligands is not yet possible
because the process of light signal transduction remains
unclear. These channels are distinguished particularly
because of a special importance of light stimuli in plant
signalling processes

6.  The rate and direction of ion movement is
governed by the electrochemical gradient. The rate of
ion transport through the channel is very high 107
ions/sec.
 Transport is always down the gradient.

7. selectivity
 Permit ions of a specific size and charge.
 The permeating ions will lose their dissociated water
molecules and pass through the hole in the channel
which is known as SELECTIVITY FILTER.
 This limits their rate of passage.
 Two discrete states – open(conducting) or
closed(nonconducting)

9.  Voltage gated ion channels consist of a highly
processed α subunit, associated with auxiliary β
subunits.
 The pore-forming α subunit is sufficient for functional
expression, but the kinetics and voltage dependence of
channel gating are modified by the β subunits.

10.  The α subunits are organized in four homologous
domains (I-IV) each with six transmembrane segments
(S1-S6) - 24 transmembrane segments in total. The
pore forming segments are formed by S5 and S6.

13.  Each of these segments coils is called a
transmembrane domain, and within a transmembrane
domain the side chains necessarily face outward where
they readily interact with the lipids of the membrane
are known as Polypeptide chain.

14. Conformational changes
 change in the voltage across the membrane (voltage-
gated channels)
 binding of a ligand (ligand-gated channels)

15. Voltage gated ion channel
 TRANSMEMBRANE ION CHANNEL that are
activated by changes in electrical membrane potential.
structure and function
 Voltage Gated ion channels are made of three basic
parts: 1) The transmembrane pore 2) Voltage sensor 3)
Selectivity filter Contains different subunits: α subunit
and other auxillary Subunits.

17. Ligand gated ion channel
 TRANSMEMBRANE PROTEIN which open to allow
ions such as Na+, K+, Ca2+, or Cl− to pass through the
membrane in response to the binding of a chemical
messenger (i.e. a ligand).
 They are all receptors.

18. Types of ligand gated ion channels
 (1) Extracellularly activated ligand - gated ion channel :
 The receptors of the cys-loop family (nicotinic receptors,
5-HT3, GABAA and GABAC, glycine and serotonin)
 The glutamate activated cationic channels (NMDA, AMPA,
kainate receptors)
 (2) Intracellularly activated ligand-gated ion channel:
 ATP sensitive potassium pump
 Calcium activated-potassium pump, chloride pump
 G-protein activated potassium pump
 Aquaporin (cGMP gated ion channels)

19.  Ion channels are pore-forming membrane proteins
whose function is establishing a resting membrane
potential, shaping action potentials and other
electrical signals by gating the flow of ions across the
cell membrane, controlling the flow of ions across
membranes, and regulating cell volume.

20.  They are often described as narrow, water-filled
tunnels that accept only specific type of ions. This
characteristic is called selective permeability. Ion
channels are integral membrane proteins, formed as
assemblies of several proteins. Such "multi-subunit"
assemblies usually make a circular arrangement of
identical or homologous proteins closely packed
around a water-filled pore through the plane of the
lipid bilayer membrane.

21.  Ion channels are different from other transporter
proteins: The rate of ion transport through the
channel is very high (often 107 ions per second or
above). Ions pass through channels down their
electrochemical gradient, which is a function of ion
concentration and membrane potential, "downhill",
without the input of metabolic energy (e.g. Adenosine
triphosphate, active transport mechanisms, co-
transport mechanisms).

22.  2 major types:
 Voltage gated ion channels
 Ligand gated ion channels

23.  a single transmembrane domain is shown as the
voltage sensor that operates the gate. The segment of
voltage gated channel is the voltage sensor that is
responsible for changing conformation as the voltage
changes. All voltage gated channels have this segment.

24.  Changes in the membrane potential modulate the
channel's opening or closing, as changing the
membrane potential changes the relative amounts of
positive and negative charges on the inside and outside
of the membrane. Like charges repel, so the positively
charged segment will be pushed away from a positive
intracellular fluid towards the negative extra cellular
fluid, changing the protein's conformation and
opening the channel.

25. Types of V.G Ion Channels
major types:
 V.G Sodium Channels
 V.G Calcium Channels

26. Voltage Gated Sodium Channels
 The founding member of the ion channel superfamily
in terms of its discovery as a protein is the voltage
gated sodium channel. These channels are responsible
for the rapid influx of sodium ions that underlies the
rising phase of the action potential in cells.
 Sodium channel composed of one principal alpha
subunit and one or two auxiliary beta subunits.

27.  The a subunits of sodium channels are composed of
four homologous domains that each contain six
transmembrane segments.
 Different distinct binding sites have been identified
within the Na channel protein, with different effects
on ion permeation and gating resulting in either
inhibition or enhancement of Na currents.

28. Voltage Gated Calcium Channels
 Voltage-gated calcium channels mediate calcium
influx in response to membrane depolarization and
regulate intracellular processes.
 Like sodium channels, the α1 subunit of voltage gated
calcium channels is organized in four homologous
domains (I-IV), with six transmembrane segments (S1-
S6) in each.
 An intracellular β subunit and a transmembrane,
disulfide-linked α2β subunit complex are components
of most types of calcium channels.

30. conclusions
 Ion channels are mainly originate from multiple gating
mechanisms that can sense the energy status of the
cell and thus make the cell responsive to various
stimuli in a very effe•cient way.
 Calcium represents a central element in transduction
of a large variety of signals.
 With out knowing the molecular nature of calcium
channels in plasma membrane and endo membrane
systems, the knowledge of most signaling cascades will
remain incomplete.