Potassium channels are the most diverse group of the ion channel family [, ]. They are important in shaping the action potential, and in neuronal excitability and plasticity []. The potassium channel family is composed of several functionally distinct isoforms, which can be broadly separated into 2 groups []: the practically non-inactivating 'delayed' group and the rapidly inactivating 'transient' group.These are all highly similar proteins, with only small amino acid changes 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 or other second messengers []. In eukaryotic cells, K+channels are 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 []. In prokaryotic cells, they play a role in the maintenance of ionic homeostasis [].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 has been 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) []. 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.KCNQ channels (also known as KQT-like channels) differ from other voltage-gated 6 TM helix channels, chiefly in that they possess no tetramerisation domain. Consequently, they rely on interaction with accessory subunits, or form heterotetramers with other members of the family []. Currently, 5 members of the KCNQ family are known. These have been found to be widely distributed within the body, having been shown to be expressed in the heart, brain, pancreas, lung, placenta and ear. They were initially cloned as a result of a search for proteins involved in cardiac arhythmia. Subsequently, mutations in other KCNQ family members have been shown to be responsible for some forms of hereditary deafness []and benign familial neonatal epilepsy [].KCNQ1 was the first member of the KCNQ channel family to be isolated, and has been found to be the most common cause of the disease `long QT syndrome' a cardiac arhythmia resulting in a prolonged QT interval. In exceptional cases, this can lead to sudden death, triggered by extreme stress [, ]. KCNQ1 is expressed in the stria vascularis of the inner ear, and may be the cause of hereditary deafness [].
KCNE1 (Potassium voltage-gated channel subfamily E member 1, also known as Mink) subunits associate with KCNQ1 alpha subunits to form channels that are responsible for the IkS currents that determine the duration of the action potential in cardiac muscle []. Mutations in both of the genes encoding these subunits cause an inherited disorder that increases the risk of death from cardiac arrhythmia (long QT syndrome type 1) and Jervell and Lange-Nielsen syndrome, associated with congenital deafness [].Two types of beta subunit (KCNE and KCNAB) are presently known to associate with voltage-gated alpha subunits (Kv, KCNQ and eag-like). However, not all combinations of alpha and beta subunits are possible. The KCNE family of K+ channel subunits are membrane glycoproteins that possess a single transmembrane (TM) domain. They share no structural relationship with the alpha subunit proteins, which possess pore forming domains. The subunits appear to have a regulatory function, modulating the kinetics and voltage dependence of the alpha subunits of voltage-dependent K+ channels. KCNE subunits are formed from short polypeptides of ~130 amino acids, and are divided into five subfamilies: KCNE1 (MinK/IsK), KCNE2 (MiRP1), KCNE3 (MiRP2), KCNE4 (MiRP3) and KCNE1L (AMMECR2). Potassium channels are the most diverse group of the ion channel family [, ]. They are important in shaping the action potential, and in neuronal excitability and plasticity []. The potassium channel family is composed of several functionally distinct isoforms, which can be broadly separated into 2 groups []: the practically non-inactivating 'delayed' group and the rapidly inactivating 'transient' group.These are all highly similar proteins, with only small amino acid changes 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 or other second messengers []. In eukaryotic cells, K+channels are 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 []. In prokaryotic cells, they play a role in the maintenance of ionic homeostasis [].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 has been 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) []. 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.
Potassium channels are the most diverse group of the ion channel family [, ]. They are important in shaping the action potential, and in neuronal excitability and plasticity []. The potassium channel family is composed of several functionally distinct isoforms, which can be broadly separated into 2 groups []: the practically non-inactivating 'delayed' group and the rapidly inactivating 'transient' group.These are all highly similar proteins, with only small amino acid changes 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 or other second messengers []. In eukaryotic cells, K+channels are 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 []. In prokaryotic cells, they play a role in the maintenance of ionic homeostasis [].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 has been 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) []. 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.Two types of beta subunit (KCNE and KCNAB) are presently known to associate with voltage-gated alpha subunits (Kv, KCNQ and eag-like). However, not all combinations of alpha and beta subunits are possible. The KCNE family of K+ channel subunits are membrane glycoproteins that possess a single transmembrane (TM) domain. They share no structural relationship with the alpha subunit proteins, which possess pore forming domains. The subunits appear to have a regulatory function, modulating the kinetics and voltage dependence of the alpha subunits of voltage-dependent K+ channels. KCNE subunits are formed from short polypeptides of ~130 amino acids, and are divided into five subfamilies: KCNE1 (MinK/IsK), KCNE2 (MiRP1), KCNE3 (MiRP2), KCNE4 (MiRP3) and KCNE1L (AMMECR2). KCNE3 is known to associate with the pore forming subunits KCNQ1, KCNQ4,HERG and Kv3.4. KCNE3 forms complexes with Kv3.4 in skeletal muscle -KCNE3 mutations have been identified in families with skeletal muscledisorders []. In the intestine, KCNE3 associates with KCNQ1 to formchannels that are stimulated by cAMP and are thought to be involved insecretory diarrhoea and cystic fibrosis [].
