Potassium channel, calcium-activated, BK, alpha subunit

Potassium channel, calcium-activated, BK, alpha subunit(InterPro Protein Domain record)

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Potassium channel, calcium-activated, BK, alpha subunit

InterPro Protein Domain record

description http://metadb.riken.jp/db/SciNetS_rib124i/prib124u1i

Potassium channels are the most diverse group of the ion channel family[<cite idref="PUB00001055"/>, <cite idref="PUB00001622"/>]. They are important in shaping the action potential, and in neuronal excitability and plasticity [<cite idref="PUB00004020"/>]. The potassium channel family iscomposed of several functionally distinct isoforms, which can be broadlyseparated into 2 groups [<cite idref="PUB00006577"/>]: the practically non-inactivating 'delayed' group and the rapidly inactivating 'transient' group.These are all highly similar proteins, with only small amino acidchanges causing the diversity of the voltage-dependent gating mechanism,channel 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, together with intracellular kinases; while others are regulated by GTP-binding proteins orother second messengers [<cite idref="PUB00004011"/>]. In eukaryotic cells, K+ channelsare involved in neural signalling and generation of the cardiac rhythm, act 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 [<cite idref="PUB00002771"/>]. In prokaryotic cells, they play a role in themaintenance of ionic homeostasis [<cite idref="PUB00009378"/>]. All K+ channels discovered so far possess a core of alpha subunits, each comprising either one or two copies of a highly conserved pore loop domain (P-domain). The P-domain contains the sequence (T/SxxTxGxG), which hasbeen termed the K+ selectivity sequence.In families that contain one P-domain, four subunits assemble to form a selective pathway for K+ across the membrane.However, it 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, IK and SK)[<cite idref="PUB00009378"/>]. The 2TM domain family comprises inward-rectifying K+ channels. In addition, there 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, where they modulate cell excitability and action potential. Three types of Ca2+-activated K+ channel have been characterised, termed small-conductance (SK), intermediate conductance (IK) and large conductance (BK) respectively [<cite idref="PUB00008285"/>].BK channels (also referred to as maxi-K channels) are widely expressed in the body, being found in glandular tissue, smooth and skeletal muscle, as well as in neural tissues. They have been demonstrated to regulate arteriolar and airway diameter, and 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, the molecule is now thought to possess an additional TM domain, with an extracellular N terminus and intracellular C terminus. This C-terminal region contains 4 predominantly hydrophobic domains, which 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, but it is thought to lie within the C-terminal hydrophobic domains.

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