发布时间 : 星期四 文章G蛋白及G蛋白偶联受体(来自维基百科整理)更新完毕开始阅读
G proteins, also known as guanine鸟嘌呤 nucleotide-binding proteins, are a family of proteins that act as molecular switches inside cells, and are involved in transmitting signals from a variety of stimuli outside a cell to its interior. When they are bound to GTP, they are 'on', and, when they are bound to GDP, they are 'off'. G proteins belong to the larger group of enzymes called GTPases.
G proteins located within the cell are activated by G protein-coupled receptors (GPCRs) G蛋白偶联受体 that span the cell membrane. The human genome encodes roughly 800 G protein-coupled receptors（Approximately 150 of the GPCRs found in the human genome have still-unknown functions.）which detect signals, e.g., light and olfactory嗅觉 stimulatory molecules; adenosine腺苷, bombesin, bradykinin, endothelin, γ-aminobutyric acid (GABA), hepatocyte growth factor (HGF), melanocortins, neuropeptide Y, opioid peptides, opsins, somatostatin, GH, tachykinins, members of the vasoactive intestinal peptide family, and vasopressin; biogenic amines (e.g., dopamine, epinephrine, norepinephrine, histamine, glutamate (metabotropic effect), glucagon, acetylcholine (muscarinic effect), and serotonin); chemokines; lipid mediators of inflammation (e.g., prostaglandins, prostanoids, platelet-activating factor, and leukotrienes); and peptide hormones (e.g., calcitonin, C5a anaphylatoxin, follicle-stimulating hormone (FSH), gonadotropin-releasing hormone (GnRH), neurokinin, thyrotropin-releasing hormone (TRH), cannabinoids, and oxytocin.
Signaling molecules bind to a domain of the GPCR located outside the cell, and an intracellular GPCR domain then in turn activates a particular G protein. G proteins can regulate metabolic enzymes, ion channels, transporter proteins, and other parts of the cell machinery, controlling transcription, motility, contractility, and secretion, which in turn regulate diverse systemic functions such as embryonic development, learning and memory, and homeostasis. G protein can refer to two distinct families of proteins. (1) Heterotrimeric异三聚体 G proteins, sometimes referred to as the \, are activated by G protein-coupled receptors and are made up of alpha (α), beta (β), and gamma (γ) subunits. (2)\G proteins (20-25kDa) belong to the Ras superfamily of small GTPases. These proteins are homologous to the alpha (α) subunit found in heterotrimers, but are in fact monomeric consisting of only a single unit. Like their larger relatives, they also bind GTP/GDP and are involved in signal transduction.
Heterotrimeric G proteins
Different types of heterotrimeric G proteins share a common mechanism.
When a ligand binds to the GPCR, it causes a conformational change in the GPCR, which allows it to act as a guanine nucleotide exchange factor (GEF). The GPCR can then activate an associated G protein by exchanging its bound GDP for a GTP. At this point, the subunits of the G-protein dissociate from the receptor, as well as each other, to yield a Gα-GTP monomer and a tightly interacting Gβγ dimer, which are now free to further modulate the activity of other intracellular proteins.
The Gα subunit will eventually hydrolyze the attached GTP to GDP with its inherent GTPase, allowing it to re-associate with Gβγ and to form the \GPCR and await next activation. A group of proteins called Regulator of G protein signalling (RGSs), act as GTPase-activating proteins (GAPs), can accelerate the hydrolysis of GTP to GDP, turning the G protein off thus terminating终止 the transduced signal. (G proteins have an inherent GTPase hydrolytic activity that is slow. However, in the presence of GAP, this hydrolytic activity is fast)
Gαs activates the cAMP-dependent pathway by stimulating the production of cyclic AMP (cAMP) from ATP. This is accomplished by direct stimulation of the membrane-associated enzyme adenylate cyclase腺苷酸环化酶. cAMP can then act as a second messenger that goes on to interact with and activate protein kinase A (PKA). PKA can phosphorylate a myriad downstream targets.
Gαi inhibits the production of cAMP from ATP.
Small GTPases
They are a type of G-protein found in the cytosol that are homologous to the alpha subunit of heterotrimeric G-proteins, but unlike the alpha subunit of G proteins, small GTPase exist as monomers and can function independently as a hydrolase enzyme to bind to and hydrolyze a GTP to form GDP. The most well-known members are the Ras GTPases and hence they are sometimes called Ras superfamily GTPases. Small GTPases regulate a wide variety of processes in the cell, including growth, cellular differentiation, cell movement and lipid vesicle transport.
GPCR signaling
If a receptor in an active state encounters a G protein, it may activate it. Some evidence suggests that receptors and G proteins are actually pre-coupled. For example, binding of G proteins to receptors affects the receptor's affinity for ligands. Activated G proteins are bound to GTP. Currently, GPCRs are considered to utilize two primary types of transducers: G-proteins and β-arrestins. Because β-arr’s have high affinity only to the phosphorylated form of most GPCRs, the majority of signaling is ultimately dependent upon G-protein activation. However, the possibility for interaction does allow for G-protein-independent signaling to occur.
G-protein-dependent signaling
Gα signaling
1. The effector of both the Gαs and Gαi/o pathways is the cAMP-generating enzyme adenylate cyclase(AC). AC catalyze the conversion of cytosolic ATP to cAMP, and all are directly stimulated by G-proteins of the Gαs class. In contrast, however, interaction with Gα subunits of the Gαi/o type inhibits AC from generating cAMP. Thus, a GPCR coupled to Gαs counteracts the actions of a GPCR coupled to Gαi/o, and vice versa. The level of cytosolic cAMP may then determine the activity of various ion channels as well as members of the ser/thr-specific protein kinase A (PKA) family. Thus cAMP is considered a second messenger and PKA a secondary effector.
PS: 【Adenylate cyclases may also be activated or inhibited in other ways (e.g., Ca2+/Calmodulin（钙调蛋白CaM） binding), which can modify the activity of these enzymes in an additive or synergistic fashion along with the G proteins】
2. The effector of the Gαq/11 pathway is phospholipase C-β (PLCβ), which catalyzes the cleavage of membrane-bound phosphatidylinositol 4,5-biphosphate (PIP2) into the second messengers inositol (1,4,5) trisphosphate (IP3) and diacylglycerol (DAG). IP3 acts on IP3 receptors found in the membrane of the endoplasmic reticulum (ER) to elicit Ca2+ release from the ER, while DAG diffuses along the plasma membrane where it may activate any membrane localized forms of a second ser/thr kinase called protein kinase C (PKC). Since many isoforms of PKC are also activated by increases in intracellular Ca2+, both these pathways can also converge on each other to signal through the same secondary effector. Elevated intracellular Ca2+ also binds and allosterically activates proteins called calmodulins, which in turn go on to bind and allosterically activate enzymes such as Ca2+/calmodulin-dependent kinases (CAMKs).
3. The effectors of the Gα12/13 pathway are three RhoGEFs (p115-RhoGEF, PDZ-RhoGEF, and LARG), which, when bound to Gα12/13 allosterically activate the cytosolic small GTPase, Rho. Once bound to GTP, Rho can then go on to activate various proteins responsible for cytoskeleton regulation such as Rho-kinase (ROCK). Most GPCRs that couple to Gα12/13 also couple to other sub-classes, often Gαq/11.
Gβγ signaling
The above descriptions ignore the effects of Gβγ–signalling, which can also be important, in particular in the case of activated Gαi/o-coupled GPCRs. The primary effectors of Gβγ are various ion channels, such as G-protein-regulated inwardly rectifying K+ channels (GIRKs), P/Q- and N-type voltage-gated Ca2+ channels, as well as some isoforms of AC and PLC, along with some phosphoinositide-3-kinase (PI3K) isoforms.
G-protein-independent signaling
Although they are classically thought of working only together, GPCRs may signal through G-protein-independent mechanisms, and heterotrimeric G-proteins may play functional roles independent of GPCRs. GPCRs may signal independently through many proteins already mentioned for their roles in G-protein-dependent signaling such as β-arrs, GRKs, and Srcs. In addition, further scaffolding proteins involved in subcellular localization of GPCRs (e.g., PDZ-domain-containing proteins) may also act as signal transducers. Most often the effector is a member of the MAPK family.