User:Bellamen/G protein-gated ion channel rough

Introduction

G-protein-gated ion channels are a family of proteins involved in the direct activation of specific ion channels located in the plasma membrane of neurons. Ion channels allow for the selective movement of certain ions across the plasma membrane. Along with ion transporters, they are responsible for maintaining the electrochemical gradient across the cell.

G-proteins are a family of intracellular proteins capable of mediating signal transduction pathways by binding receptor molecules which have binded their respective ligands. Once bound to its ligand, a transmembrane receptor protein, a GCPR, usually binds to the α-subunit of the hetero-trimeric g-proteins (which consist of α-, β-, and γ- subunits). The conformational change this binding caused in the g-protein allows the α-subunit to bind GTP, which leads to another conformational change which results in the departure of the βγ-subunit complex (Gβγ). The α-subunit is then able to continue the signal transduction pathway. G-protein gated ion channels are transmembrane proteins that have a g-protein binding site, and an ion channel with a selectivity filter. Unlike most effectors, not all g-protein gated ion channels have their activity mediated by the α-subunit of their corresponding g-proteins. For instance, the opening of inwardly rectifying K+ (GIRK) channels is mediated by the binding of the -βγ subunit (Gβγ) complex.

G-protein gated ion channels are primarily found in CNS neurons and atrial myocytes , and affect the flow of potassium (K+), calcium (Ca+2), sodium (Na+), and chloride (Cl-) across the plasma membrane. Mutations in g-proteins associated with g-protein gated ion channels have been shown to be involved in diseases such as epilepsy, muscular diseases, neurological diseases, and chronic pain, among others.

Types

G-protein gated inwardly-rectifying potassium channels

Structure

Four G-protein gated inwardly-rectifying potassium (GIRK) channel subunits have been identified in mammals: GIRK1, GIRK2, GIRK3, and GIRK4. These ion channels, once activated, allow for the flow of potassium ions from the extracellular space surrounding the cell across the plasma membrane and into the cytoplasm. Each channel consists of domains which span the plasma membrane, providing the K+-selective pore region through which the K+ ions will flow. Both the N-and C-terminal ends of the GIRK channels are located within the cytoplasm. These domains interact directly with the βγ-complex of the G protein, leading to activation of the K+ channel. The GIRK ion channel family is the only group which is known to directly interact with G-proteins. These domains which interact with the G-proteins contain certain residues which are critical for the proper activation of the GIRK channel. In GIRK4, the N-terminal residue is His-64 and the C-terminal residue is Leu-268; in GIRK1 they are His-57 and Leu-262. Mutations in these domains lead to the channel's desensitivity to the βγ-complex and therefore reduce the activation of the GIRK channel.

The four GIRK subunits show similarity in their structures and sequences. The four GIRK subunits show a similarity of 80-90% in their pore-forming and transmembrane domains. GIRK2, GIRK3, and GIRK4 share an overall identity of 62% with eachother, while GIRK1 only shares 44% identity with the others.

Because of their similarity, the GIRK channel subunits can come together easily to form heteromultimers. GIRK1, GIRK2, and GIRK3 show abundant and overlapping distribution in the central nervous system (CNS) while GIRK1 and GIRK4 are found mainly in the heart. GIRK1 combines with GIRK2 in the CNS and GIRK4 in the atrium to form heterotetramers; each final heterotetramer contains two GIRK1 subunits and two GIRK2 or GIRK4 subunits. GIRK2 subunits can also form homotetramers in the brain, while GIRK4 subunits can form homotetramers in the heart. GIRK1 subunits have not been shown to be able to form functional homotetramers. Though GIRK3 subunits are found in the CNS, their role in forming functional ion channels is still unknown.

Subtypes

• GIRKs found in the heart

One G-protein gated potassium channel is the inward rectifying potassium channel (IKACh) found in cardiac muscle (the sinoatrial node and atria), which contributes to the regulation of heart rate. Activation of the IKACh channels begins with release of acetylcholine (ACh) from the vagus nerve. ACh binds to the M2 muscarinic acetylcholine receptors, which interact with G proteins and promote the dissociation of the G\alpha subunit and Gβγ-complex. IKACh is composed of two homologous inward rectifying K+ channel subunits: GIRK1 and GIRK4. The Gβγ-complex binds directly and specifically to the IKACh channel with interactions on both the GIRK1 and GIRK4 subunits. The ion channel is activated and K+ ions flow into the cell causing the cell to hyperpolarize. The neuron, in its hyperpolarized state, cannot fire action potentials as quickly, which slows the heartbeat of muscle cells.

• GIRKs found in the brain

The g-protein inward rectifying K+ channel found in the CNS is a heterotetramer comprised of GIRK1 and GIRK2 subunits and is responsible for maintaining the resting membrane potential and excitability of the neuron.

Ca2+ Channels

Structure and Importance

In addition to the subset of potassium channels that are directly gated by G proteins, G proteins can also directly gate some calcium ion channels in neuronal cell membranes. Although membrane ionic channels and protein phospohylation are typically indirectly affected by G protein-coupled receptors using effector proteins such as phospholipase C and adenylyl cyclase and second messengers such as inositol triphosphate, diacylglycerolm and cAMP (see picture part I), G-proteins can short circuit the second-messenger pathway and gate the ion channels directly. Such bypassing of the second-messenger pathways is observe in mammalian cardiac myocytes and associated sarcolemmal vesicles, where calcium channels are able to survive and function in the absence of cAMP, ATP or protein kinase C when in the presence of the activated alpha subunit of the G-protein, Gs. The G protein, Gs, which is stimulatory to adenylyl cyclase, as an effector acts on the Ca2+ channel directly. This short circuit is membrane-delimiting, allowing direct gating of calcium channels by G proteins to produce effects more quickly than the cAMP cascade(see picture part II). This direct gating has been found also in specific Ca2+ channels in the heart and skeletal muscle T tubules.

Function

Several high-threshold, slowly inactivating calcium channels in neurons are regulated by G proteins. The activation of alpha subunits of G proteins has been shown to cause rapid closing of voltage-dependent Ca2+ channels, which causes difficulties in the firing of action potentials. This inhibition of voltage-gated Calcium channels by G protein-coupled receptors has been demonstrated the dorsal root ganglion of a chick among other cell lines. Further studies have indicated roles for both Gaplha and Gbetagamma subunits in inhibition of calcium ion channels, and but much of the research to define the involvement of each subunit have not uncovered the specificity or mechanisms by which Ca2+ channels are regulated.

Other channels

• Na+

Patch clamp measurements suggest a direct role for Galpha in the inhibition of fast Na+ current within cardiac cells, while other studies show evidence of a second-messanger pathway indirectly controlling this channel. In this case, indirect or direct activation of sodium ion channels has not been defined with complete certainty.

• Cl-

Cl channel activity in epethelial and cardiac cells has been shown to be G protein-dependent, but the cardiac channel that has been shown to be directly gated by the Galpha subunit has not yet been identified. As with Na channel inhibition, second-messanger pathways cannot be discounted in Cl- channel activation.

Roles of G Protein-gated Ion Channels in Disease

Alcohol intoxication has been shown to be directly connected to the actions of GIRK channels in the presence of alcohol. GIRK channels have a hydrophobic pocket that is capable of binding ethanol. When ethanol acts as an agonist, GIRK channels in the brain experience prolonged opening. This causes decreased neuronal activity, the result of which manifests as the symptoms of alcohol intoxication. The discovery of the hydrophobic pocket capable of binding ethanol is significant in the field of clinical pharmacology. Agents that can act as agonists to this binding site can be potentially useful in the creation of drugs for the treatment of neurological disorders such as epilepsy.

Studies have shown that a link exists between GIRK1 channels and the beta-adrenergic receptor pathway in breast cancer cells responsible for growth regulation of the cells. Approximately 40% of primary human breast cancer tissues have been found to carry the mRNA which codes for GIRK1 channels. Treatment of breast cancer tissue with alcohol have been shown to trigger increased growth of the cancer cells. The mechanism of this activity is still a subject of research.