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DESCRIPTIONSelective Permeability. Jackie Bonds January 23, 2012 Neus 586. Selective Permeability What is it?. Selective permeability means that the cell membrane has some control over what can cross it, so that only certain molecules either enter or leave the cell. . - PowerPoint PPT Presentation
Selective PermeabilityJackie BondsJanuary 23, 2012Neus 586Selective Permeability What is it?Selective permeability means that the cell membrane has some control over what can cross it, so that only certain molecules either enter or leave the cell.
Regulation of Membrane PermeabilityIonic composition inside neurons is tightly regulated and must be able to be rapidly modified in response to various stimuliSpecialized transmembrane channels allow ions to rapidly enter the cell while still maintaining high selelctivityVoltage-gated channelsLigand-gated channelsMechano-sensitive channelsNeural SignalingResting potential is the result of an equilibrium between the electrical and chemical forces acting upon Na+ and K+Depolarization is result of a stimulus large enough to cause voltage-gated Na+ channels to openRepolarization is the closing of Na+ channels, increasing the relative permeability to K+
What properties govern the specificity of channels? How can this be studied? How can we understand this in terms of dysfunction and disease? TechniquesX-ray crystallographyElectron Density MapsPharmacologyGenetics SequencingSite-directed mutagenesisPatch-clampingVoltage-Gated Ion ChannelsVoltage-gated ion channels are like the transistors of logical circuits, detecting, amplifying, and reshaping electrical messagesHodgkin and Huxley succeeded in describing the permeability changes as a set of chemical reactions whose rate constants are a function of voltage
changes in ionic permeability depend on the movement of some component of the membrane which behaves as though it had a large charge or dipole movementto allow ions to pass when they occupy particular sites in the membrane.
Predicted the gating current which is a small charge movement generated by the voltage-driven conformational changes that open the channels
7Visualization of the channelStructure and function has been studied using X-ray crystallographic studiesElucidated an important feature of the selectivity filter to create a queue of cation binding sites that mimic the waters of hydration surrounding that specific cation Allows the observation of the channel structure in various chemical environmentsAllowed the visualization of ion position within the filter
Reduction of K+ to 3mM in the presence of 150mM Na+ causes the selectivity filter to undergo a conformational change to a collapsed state
8General Features of a VGIC
SF = Selectivity FilterVS = Voltage SensorOV = Outer VestibuleIV = Inner Vestibule Armstrong et al. 1998Voltage Gated: Pore opening is energetically coupled to the movement of a charged voltage sensor within the membrane electric fieldConformational changes underlying the voltage sensing property are large and involve movements of arginine residues through the membrane near the protein-lipid interface
9Voltage Sensor (S4 Segment)Cysteine mutagenesis and cysteine labeling with thiol-reactive compounds has provided good evidence that the S4 segments in both Na+ and K+ channels move as expected following voltage changesState dependent labeling a specific cysteine may be labeled only from the outside when the channel is open and only from the inside when the channel was closed. The labeling patterns are consistent with the outward movement of S4 (the voltage sensor) in response to a positive change of membrane potential Potassium ChannelPotassium conduction through their specific channels is critical for cell volume regulation, hormone secretion, and electrical impulse formation (neural signaling)Signature Sequence forms the selectivity filter which prevents the passage of sodium ions but allows potassium ions to conduct across the membrane at rates approaching the diffusion limit.
MacKinnon. 2003Channel ArchitectureSince the inner helices are tilted slightly inward and kinked with respect to the membrane, the subunits open like the petals of a flower facing the outside of the cell. Gives the appearance of an inverted teepee This architecture is likely a general feature of cation channels with four inner helices, four pore helices, and a selectivity filter each tuned to select the specific ion at the extracellular surface
MacKinnon et al. 1998The channel pore is made up of four subunits, each of which contain two fully transmembrane -helices (an inner and outer) and a tilted pore helix that runs half-way through the membrane and points its negative end charged C-terminus toward the ion pathway
MacKinnon et al. 1998Ion Selectivity and MovementA K+ ion could move through the internal pore and cavity without shedding its waters of hydration but the selectivity pore is so narrow that it must shed its hydrating waters to enter. The ion is stabilized in the selectivity pore because it is lined exclusively with polar main chain atoms found in the signature sequence. The configuration of these ions within the selectivity pore allows the channel to exploit the electrostatic repulsive forces to overcome the attractive forces between K+ ions and the selectivity filter
As the ion encounters the selectivity filter, it interacts with 4 evenly spaced layers of carbonyl oxygen atoms and a single layer of threonine hydroxyl oxygen atoms this creates 4 binding sites where K+ binds in a dehydrated state and is surrounded by 8 oxygens that essentially mimic the arrangement of water molecules around a hydrated ion that is observed in the central cavity
MacKinnon. 2003Visualization of Ion PositionAll K+ channels show a selectivity sequence of K+ Rb+ > Cs+. Permeability for the smaller ions such as Na+ and Li+ are immeasurably lowRb+ (1.48) is almost a perfect analog to K+ (1.33) due to the similarity in size and permeability characteristics. Rb+ (as well as Cs+) is more electron dense than K+ making it very suitable for use in visualizing the locations of permeant ions in the pore using electron density maps
MacKinnon et al. 1998Isolating the ChannelAll K+ channels can be blocked by tetraethylammonium (TEA)TEA interacts with the amino acids that define the entryway into the pore and blocks the K+ channel at both sides of the membrane at distinct sites. The amino acids that TEA cations interact with are located just external to and internal to the structure that forms the selectivity filter. TEA cannot enter the selectivity filter, only block it.
TEA ions can only traverse ~20% of the transmembrane voltage difference which suggests that ~80% of the transmembrane voltage is imposed across the relatively short distance through the selectivity filter which is ~12 (Length of pore = 45) the rate-limiting steps for a K+ to travel through the channel occur in the short span of the selectivity filter
17Functional ExperimentsThe transmembrane voltage is imposed across the relatively short distance through the selectivity filter which is ~12 (Length of pore = 45) the rate-limiting steps for a K+ to travel through the channel occur in the short span of the selectivity filterTEA ions can only traverse ~20% of the transmembrane voltage Summary of K+ ChannelThe pore is constructed like an inverted teepee, with the selectivity filter held at its wide end. This is most likely applicable to other cation channels such as Ca2+ and Na+The narrow selectivity filter is ~12 long and has a polar lining, whereas the remainder of the pore is wider and has a relatively inert hydrophobic lining. This favors high K+ throughput by minimizing the distance over which K+ interacts strongly with the channelThe large water filled cavity and helix dipoles help to overcome the high electrostatic energy barrier in the low dielectric membrane centerThe K+ selectivity filter is lined with carbonyl oxygen atoms which provide 4 closely spaced binding sites and is constrained to an optimal geometry that will only fit a dehydrated K+ ion Two K+ ions in close proximity within the filter exude repulsive forces on each other, which helps to overcome the otherwise strong interaction between ion and protein. This further allows for rapid conduction while still favoring high selectivity.
InactivationInactivation gating is a second gating factor that is mechanistically simpler and different from activation after the activation gate opens, a portion of the channel peptide diffuses into the mouth of the inner vestibule of the pore and blocks conduction (a foot in the door sort of mechanism)
Inactivation (cont.)Site-directed mutagenesis by Aldrich et al. found that deletions within the first 20 amino acids of the N-terminus completely destroy fast inactivation. Even more fascinating was the finding that deletions in the next 63 amino acids resulted in speeding inactivationCutting off the inactivation particle with pronase removed the inactivation ability CNS PathologiesMany of the ion channel diseases are known as paroxysmal disorders in which the principal symptoms occur intermittently in some individuals who otherwise may be healthy and active
EpilepsyEpileptic seizures are behavioral attacks resulting from the overly synchronized and excessive activity of large groups of neurons. Symptoms: Alterations/loss of consciousness, sustained or rhythmic muscle contraction, stereotyped gestural movements, and visual/somatosensory hallucinationsAltered Na+ channel function has been linked to epilepsy
Epilepsy (cont.)The SCN1B (the 1 auxiliary subunit of the voltage gated Na+ channel) genes of epileptic individuals contained a single base-pair substitution which caused a single amino acid change in the protein sequence The 1 subunit is important for the timing of channel open and closeA mutation in this region results in a loss of function cells expressing the mutant exhibit slow channel openings and closings
On a larger scaleThe blood-brain barrier (BBB) provides a stable environment for neural function thr