Potassium channels are the most diverse group of the ion channel family. They are important in shaping the action potential.nd in neuronal excitability and plasticity . The potassium channel family iscomposed of several functionally distinct isoforms.hich can be broadlyseparated into 2 groups : the practically non-inactivating delayed group and the rapidly inactivating transient group.These are all highly similar proteins.ith only small amino acidchanges causing the diversity of the voltage-dependent gating mechanism.hannel conductance and toxin binding properties. Each type of K+ channel is activated by different signals and conditions depending on their type of regulation: some open in response to depolarisation of the plasma membrane; others in response to hyperpolarisation or an increase in intracellular calcium concentration; some can be regulated by binding of a transmitter.ogether with intracellular kinases; and others are regulated by GTP-binding proteins orother second messengers . In eukaryotic cells.+ channelsare involved in neural signalling and generation of the cardiac rhythm.ct as effectors in signal transduction pathways involving G protein-coupled receptors (GPCRs) and may have a role in target cell lysis by cytotoxic T-lymphocytes . In prokaryotic cells.hey play a role in themaintenance of ionic homeostasis . All K+ channels discovered so far possess a core of alpha subunits.ach comprising either one or two copies of a highly conserved pore loop domain (P-domain). The P-domain contains the sequence (T/SxxTxGxG).hich hasbeen termed the K+ selectivity sequence.In families that contain one P-domain.our subunits assemble to form a selective pathway for K+ across the membrane.However.t remains unclear how the 2 P-domain subunits assemble to form a selective pore. The functional diversity of these families can arise through homo- or hetero-associations of alpha subunits or association with auxiliary cytoplasmic beta subunits. K+ channel subunits containing one pore domain can be assigned into one of two superfamilies: those that possess six transmembrane (TM) domains and those that possess only two TM domains. The six TM domain superfamily can be further subdivided into conserved gene families: the voltage-gated (Kv) channels; the KCNQ channels (originally known as KvLQT channels); the EAG-like K+ channels; and three types of calcium (Ca)-activated K+ channels (BK.K and SK). The 2TM domain family comprises inward-rectifying K+ channels. In addition.here are K+ channel alpha-subunits that possess two P-domains. These are usually highly regulated K+ selective leak channels. Ca2+-activated K+ channels are a diverse group of channels that are activated by an increase in intracellular Ca2+ concentration. They are found in the majority of nerve cells.here they modulate cell excitability and action potential. Three types of Ca2+-activated K+ channel have been characterised.ermed small-conductance (SK).ntermediate conductance (IK) and large conductance (BK) respectively .BK channels (also referred to as maxi-K channels) are widely expressed in the body.eing found in glandular tissue.mooth and skeletal muscle.s well as in neural tissues. They have been demonstrated to regulate arteriolar and airway diameter.nd also neurotransmitter release. Each channel complex is thought to be composed of 2 types of subunit: the pore- forming (alpha) subunits and smaller accessory (beta) subunits. The alpha subunit of the BK channel was initially thought to share the characteristic 6TM organisation of the voltage-gated K+ channels. However.he molecule is now thought to possess an additional TM domain.ith an extracellular N-terminus and intracellular C-terminus. This C-terminal region contains 4 predominantly hydrophobic domains.hich are also thought to lie intracellularly. The extracellular N-terminus and the first TM region are required for modulation by the beta subunit. The precise location of the Ca2+-binding site that modulates channel activation remains unknown.ut it is thought to lie within the C-terminal hydrophobic domains. The beta subunit (which is thought to possess 2 TM domains) increases the Ca2+ sensitivity of the BK channel . It does this by enhancing the time spent by the channel in burst-like open states. However.t has little effect on the durations of closed intervals between bursts.r on the numbers of open and closed states entered during gating .