Ion Channels The plasma membrane is 6-8nm thick, and consists of a mosaic of lipids and proteins. The lipid is hydrophobic, and will not allow ions through. Ions are surrounded by waters of hydration. To move through the hydrophobic lipid bilayer, water molecules would need to be stripped off the ion. This takes too much energy.

Ion Channels The solution is to provide ions with specialized pathways such as ion channels that will permit ions to cross with most or all of their water molecules. Ion channel pores therefore provide ions with a polar environment.

Structure of Ion Channels Ion channels are large assemblies of proteins, which make up subunits, which combine to form functional channels.

K+ Channels:

Shaker K+ Channel: Each Channel is made of 4 Subunits. Each Subunit is made up of a large protein having 6 trans-membrane segments (S1-S6). Between S5 and S6 there is a loop (red) that, along with the S6 segment, lines the conduction pore.

Shaker K+ Channel:

What they look like:

Important features: 1.Gating (opening and closing) What are the features that cause the channel to open and close? 2.Ion selectivity Why does a K+ channel not allow Na+ through? 3.Molecular features related to their function. What are the structural features that determine the function of the channel?

Gating What are the features that cause the channel to open and close?

Gating: Controlled by different types of stimuli: Stretch Ligands Voltage Phosphorylation

Gating Involves conformational changes in the ion channel protein. Each channel protein has two or more conformational states (e.g., open & closed) that are relatively stable. Each stable conformation represents a different functional state. 3 modes of gating:

Voltage-Gated Ion Channels CLOSEDOPEN

Voltage-Gated Ion Channels A class of ion channels gated (opened and closed) by the trans-membrane potential difference (voltage). There are many, many, types. Among these are: -Na + Channels -K + Channels -Ca 2+ Channels -Cl - Channels. There are actually many types of Na, K, Cl, and Ca Channels, classified according to pharmacology, physiology, and more recently- molecular structure.

Voltage-gated ion channels Involved in: 1.Initiation and propagation of action potentials 2.Control of synaptic transmission 3.Intracellular ion homeostasis 4.Other aspects of intracellular function Acting as activators of intracellular enzymes Coordinating signals between cell membrane and internal organelles (e.g., mitochondria).

Ligand-Gated Ion Channels Typically, these are ion channels located on the postsynaptic (receiving) side of the neuron Some act in response to a secreted (external) ligand- typically a neurotransmitter such as Acetylcholine (Ach) GABA Glycine Glutamate Some act in response to internal ligands such as cGMP and cAMP, and are also regulated by internal metabolites such as phosphoinositides, arachidonic acid, calcium.

Ligand-Gated Ion Channels Among the first ligand-gated channels to be thoroughly characterized and cloned is the Ach channel. 5 subunits, each made of 4 membrane-spanning components (M1-M4) 2 Ach molecules need to bind in order to open the channel pore. Fluxes Na and K.

Glutamate Receptors: An important class of ligand-gated receptors because: Glutamate is the main excitatory neurotransmitter in the CNS Glutamate receptors include Ionotropic receptors  Receptors gating a channel whose ligand is glutamate Are the NMDA, AMPA & Kainate receptors as defined by these ligands (more later). Metabotropic receptors  glutamate receptors that trigger intracellular 2 nd messenger systems.

Ion Selectivity: Why does a K+ channel not allow Na+ through? nm nm

Ion Selectivity: Why does a K+ channel not allow Na+ through? Concept of waters of hydration: Increase the effective diameter of the Na+ ion. Thus: Pore Size is 1 mechanism for selectivity.

Ion Selectivity: So why does an Na channel exclude K? Channels have a specialized region that acts as a molecular sieve  The SELECTIVITY FILTER. This is where an ion sheds its waters of hydration & forms a weak chemical bond with charged or polar amino acid residues that line the walls of the channel.

Molecular features related to function What are structural features that determine the function of the channel? Gating Ion Selectivity

Ion Selectivity: K Channel Pore structure: Ionic interactions that “stabilize” certain ions preferentially

K + Channel Two mechanisms by which the K+ channel stabilizes a cation in the middle of the membrane. First, a large aqueous cavity stabilizes an ion (green) in the otherwise hydrophobic membrane interior. Second, oriented helices point their partial negative charge (carboxyl end, red) towards the cavity where a cation is located.

Ion selectivity: Interaction of GluR2 subunit with kainate Is sometimes dependent on a single amino acid located in the critical channel-forming segment. In AMPA receptor GluR2 subunits: Glutamine –permeability to Ca Arginine – No Ca Permeability

Ion selectivity & human disease: An understanding of the principles of ion selectivity has led to some molecular theories of human disease. FOR EXAMPLE: The GluR2 Hypothesis of delayed neuronal death in cerebral ischemia Whereby certain brain neurons exposed to transient ischemic challenges die because they fail to express the GluR2 AMPA receptor subunit, which governs Ca- permeability. Increased Ca permeability causes neurons to die (much more about this later)

Gating: K+ Channel Changes in voltage are sensed by charged groups in transmembrane segments. Most of the sensing is done by the outermost positive charges (blue) of the S4 segment although one negative charge (red, E293) of the S2 segment is also contributing

Gating The green shaded region represents the low dielectric region of the channel. The charges are accesible to the inside or to the outside, depending on the membrane potential. At resting (negative) membrane potential, the positive charges of S4 are attracted to the interior At more positive potentials, they are attracted to the extracellular side.