Potassium channel, voltage dependent, Kv1.4, tandem inactivation domain (K_chnl_volt-dep_Kv1.4_TID)
Type: Domain

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.

The Kv family can be divided into several subfamilies on the basis of sequence similarity and function. Four of these subfamilies, Kv1 (Shaker), Kv2 (Shab), Kv3 (Shaw) and Kv4 (Shal), consist of pore-forming alpha subunits that associate with different types of beta subunit. Each alpha subunit comprises six hydrophobic TM domains with a P-domain between the fifth and sixth, which partially resides in the membrane. The fourth TM domain has positively charged residues at every third residue and acts as a voltage sensor, which triggers the conformational change that opens the channel pore in response to a displacement in membrane potential . More recently, 4 new electrically-silent alpha subunits have been cloned: Kv5 (KCNF), Kv6 (KCNG), Kv8 and Kv9 (KCNS). These subunits do not themselves possess any functional activity, but appear to form heteromeric channels with Kv2 subunits, and thus modulate Shab channel activity . When highly expressed, they inhibit channel activity, but at lower levels show more specific modulatory actions.

The first Kv1 sequence (also known as Shaker) was found in Drosophila melanogaster (Fruit fly). Several vertebrate potassium channels with similar amino acid sequences were subsequently found and, together with the D. melanogaster Shaker channel, now constitute the Kv1 family. The family consists of at least 6 genes (Kv1.1, Kv1.2, Kv1.3, Kv1.4, Kv1.5 and Kv1.6) which each play distinct physiological roles. A conserved motif found towards the C terminus of these channels is required for efficient processing and surface expression . Variations in this motif account for the differences in cell surface expression and localisation between family members. These channels are mostly expressed in the brain, but can also be found in non-excitable cells, such as lymphocytes .

This entry features the tandem inactivation domain found at the N terminus of the Kv1.4 potassium channel. It is composed of two subdomains. Inactivation domain 1 (ID1, residues 1-38) consists of a flexible N terminus anchored at a 5-turn helix, and is thought to work by occluding the ion pathway, as is the case with a classical ball domain. Inactivation domain 2 (ID2, residues 40-50) is a 2.5 turn helix with a high proportion of hydrophobic residues that probably serves to attach ID1 to the cytoplasmic face of the channel. In this way, it can promote rapid access of ID1 to the receptor site in the open channel. ID1 and ID2 function together to bring about fast inactivation of the Kv1.4 channel, which is important for the role of the channel in short-term plasticity .