Calcium Channels Flashcards
List the properties of L-type Cavs.
High-threshold.
Large-conductance.
Slowly inactivating.
List the properties of N-, P-, Q- and R-type Cavs.
Medium conductances.
Intermediate inactivation kinetics.
Medium to high activation thresholds.
List the properties of T-type Cavs.
Low-threshold.
Low-conductance.
Describe the currents produced by L- and Q-type Cavs.
Two-exponential function with the early fast phase of current decay governed by calcium dependent inactivation (CDI).
Describe the structure of the a1 subunit of Cav.
Consists of 24 transmembrane a-helices, organised into 4 homologous repeats (Domains I-IV) that each consist of 6 transmembrane a-helices (S1-S6).
What forms the voltage-sensing domain of a Cav?
The fourth transmembrane segment of each repeat (S4) (has 5 positively charged amino acids).
S1, S2 and S3.
What forms the pore-forming domain of a Cav?
S5 and S6.
What is the overall structure of a low voltage-activated Cav?
Tetramer-mimicking.
Comprised of a single a1 subunit.
How do auxiliary subunits affect Cavs?
Modulate membrane trafficking, current kinetics, and gating properties of Cavs.
Mediate the regulation of the a1 subunit by a variety of signals.
Describe the overall structure of a high voltage-activated Cav.
Tetramer-mimicking.
Heteromultimer of a1 and auxiliary subunits: beta subunit, alpha2delta dimer, and gamma subunit.
How is the activation gate of a Cav formed?
Formed from the bundle-crossing region at the lower third of the S6 segments.
How is the negatively-charged selectivity filter vestibule of a Cav formed?
Formed by the side chains of the essential EEEE residues (Glu292, Glu614, Glu1014 and Glu1323).
How does the voltage-sensing domain of a Cav sense depolarisation?
A signature motif of positively-charged arginine or lysine at every third position of helix S4 rearranges in response to depolarisation.
How does the gate of a Cav close?
At rest, voltage sensors are pulled into a down position by the electrical field, which locks the channel in its closed state.
How does the gate of a Cav open?
Membrane depolarisation releases the voltage sensors, so they move upwards and release the closed channel gates.
Describe voltage-dependent inactivation (VDI).
Membrane depolarisation triggers a conformational change in the channel that opens the pore. Prolonged depolarisation triggers a further conformational change that repositions the inactivation shield to expose a docking site for the inactivation gate.
Describe calcium-dependent inactivation (CDI).
Ca2+ influx creates a Ca2+ domain by the inner pore. Ca2+ binds to calmodulin, which in turn causes it to change configuration leading to an inactivated conformation of the channel.
How might the domain I-II linker contribute to VDI of Cavs?
Might act as an inactivation particle that physically occludes the pore of the channel by interacting with the domain II and III S6 regions of the channel.
Why does the beta subunit associate with the domain I-II linker?
To modify inactivation properties.
How does CDI depend on CaM interactions with Cavs?
At rest, the amino-terminal lobe of CaM is tethered to the 1-8-14-binding motif, whereas both the amino- and the carboxy-terminal lobes are tethered to the CB region. Following the influx of Ca2+, the Ca2+-binding sites of the carboxy-terminal lobe of CaM become occupied, and the carboxy-terminal lobe binds the IQ motif. Processes that lead to channel closure are induced.
Why is CDI important?
It is essential for Cav autoregulation.
What is the possible mechanism for Cav autoregulation via CDI?
The Cav selectivity filter forms the CDI gate, suggesting an SF-based inactivation paradigm shared with other voltage-gated ion channel (VGIC) superfamily members, e.g. Kvs.
How are L-type Cavs modulated?
A-kinase anchoring proteins (AKAPs) direct PKA to the channel. L-type Cavs interact directly with AKAP79/150, which binds both PKA and calcineurin (CaN). CaN strongly opposes L channel phosphorylation by PKA.
Which Cav types are located presynaptically?
N- and P/Q-types.
Why is the amount of NT release dependent on presynaptic calcium concentrations?
Calcium entry through presynaptic Cavs links membrane depolarisation to the exocytosis of synaptic vesicles at the presynaptic terminal.
Describe the four steps involved in the exocytosis of NT.
Actin helps move vesicles to the active zone.
Several proteins are involved with attaching the vesicle to the presynaptic membrane.
Complex of SNARE proteins docks vesicles to membrane.
Fusion between vesicle and membrane requires an increase of calcium in the cytosol.
Which proteins are known to be involved in anchoring Cavs to the presynaptic membrane?
Rab3.
Synaptotagmin (major calcium sensor).
Synaptobrevin.
How do dihydropyridines block L-type Cavs?
Bind to the pore domain formed by domains I S6, III S5, and IV S6.
How do phenylalkylamines and benzothiazepines block Cavs?
Bind to the central cavity of the pore through the open activation gate.
How do w-conotoxins block N-type Cavs?
Bind to the outer vestibule and block ion conductance through pore.
How do w-agatoxins block P/Q-type Cavs?
Bind reversibly to the outside of the pore region to block the channel and reduce release of NT.