|
PWS Articles PWS Research
Other |
[ Printable Page | Edit ]
Research Notes: G-proteinsG Protein Receptors (from http://web.mit.edu/esgbio/www/cb/membranes/gp.html) The body uses 7 membrane-spanning serpentine receptors for an astounding variety of biological signalling functions. Receptors on the cells lining our tongue convey taste. Hundreds of distinct receptor species in the cells of our olfactory bulbs in our nose convey information about the presence of odors (odorant ligands). A carotenoid molecule related to vitamin A is bound in the ligand position of rhodopsin in the rods and cones of our eyes where it serves to pick up photons, alter its conformation, and cause the receptor to which it is bound to release signals into the rod/cone cytoplasm that result in our perception of light. These serpentine receptors are of very ancient lineage. Baker's yeast cells communicate their sexual identity to each other by release of polypeptide mating factors. The cell surface receptors that recognize these mating factors are once again 7 membrane-spanning serpentine receptors! We will explore another well-studied example of a ligand/serpentine receptor pair. Epinephrine This one involves the ligand epinephrine also known as adrenaline, which is released by the adrenal glands above the kidneys in response to very stressful stimuli. Once released, epinephrine courses throughout our blood stream and adsorbs to specific receptors on the surfaces of cells in various tissues throughout the body. The result is the establishment of the primitive mammalian fight / flight reaction. This reaction increases heart rate, decreases blood flow to gut, increases blood flow to skeletal muscles, and increases blood glucose by causing liver and muscle cells to break down glycogen and release resulting glucose into the circulation. How does epinephrine/adrenaline evoke all these responses? Acting as a ligand, it binds to its own receptor displayed on the surface of a variety of cell types throughout the body. This beta adrenergic receptor is a 7 membrane-spanning, serpentine receptor embedded in the plasma membranes of these cells. As is the case with the growth factor receptors described earlier, the epinephrine ligand is not internalized into the cell. Instead, while bound for a short period of time to its receptor, it causes the latter to release biochemical signals into the cell cytoplasm. Serpentine receptors like the beta adrenergic receptor do not depend upon receptor dimerization (as described above) in order to transduce signals across the plasma membrane. Instead, single receptor molecules will change their 3 dimensional steric configuration in response to ligand binding. This steric shift affects the configuration of the cytoplasmic domains of the receptor, that is the loops of receptor protein that protrude into the cytoplasm. Cytoplasmic signal transduction The beta adrenergic receptor communicates with the cytoplasm by stimulating a second protein, which is known as a G protein for reasons that will become clear. The G protein normally lies near the receptor in an inactive, quiet state. When the receptor gets activated by ligand binding, it will rapidly poke the G protein. The G protein responds by switching itself on into an active state. Once in the active state, the G protein will send signals further into the cell. However, the G protein will remain in the active state for only a brief period of time, after which it will shut itself off. In effect, the G protein acts like a binary switch, a light switch which,once turned on, will remain on for a limited period of time before it flips itself off. The G protein's two states (ON or OFF) are determined by the guanine nucleotide that it binds (whence the term G protein). When it is inactive it binds GDP; when active, it binds GTP. Accordingly, the resting, OFF form of the G protein sits around with its bound GDP. When a ligand- activated receptor pokes it, the G protein releases its bound GDP and allows a GTP molecule to jump aboard. This GTP-bound form of the G protein represents the active ON configuration of the G protein. While in the ON state, it releases downstream signals. After a short period of time (seconds or less), the G protein will then hydrolyze its own GTP down to GDP, thereby shutting itself off. This hydrolysis represents a negative feedback mechanism which ensures that the G protein is only in the active, signal-emitting ON mode for a short period of time. Signalling Cascades We will make a brief excursion into the downstream signalling pathway (often called a signal cascade) that is triggered by the active G protein. In fact the G protein is formed from 3 distinct protein subunits, termed alpha, beta, and gamma. When in its inactive OFF state, 3 subunits are bound together; the a subunit has the job of binding the guanine nucleotide, in this case GDP. When the beta adrenergic receptor activates the G protein, the alpha subunit releases GDP, binds GTP and falls away from the beta and gamma subunits. Once this happens, the GTP-bound a subunit also loses affinity for the receptor, dissociates from it, and moves over and pokes yet another nearby protein, the enzyme adenylate cyclase, which until this time has been inactive. Once it is poked by the active, GTP-binding oc subunit of the G protein, the adenylate cyclase enzyme gets activated and does its job: it cyclizes ATP into 3'5' cyclic AMP. This reaction involves the release of the beta and gamma phosphates from the ATP and the linking of the surviving a phosphate (still attached to the 5' hydroxyl of ribose) to the 3' hydroxyl as well, forming a circular or cyclic structure, whence the term cyclic adenosine monophosphate or simply cAMP. After a several second encounter with the adenyl cyclase enzyme, the alpha subunit of the G protein will hydrolyze its bound GTP and release the adenyl cyclase, thereby reverting to an inactive OFF signalling state. It will then rejoin the beta and gamma subunits that it deserted earlier in the game. The adenyl cyclase, no longer being poked by the activated a subunit of the G protein, will shut down and stop making cAMP from ATP. The whole cycle has resulted in only a brief pulse of signalling, in this case the production of several hundred cAMP molecules made by the adenylate cyclase during its brief period of activity. Once made, the cAMP molecules act as intracellular glycogen, the high cAMP concentrations enable A kinase to phosphorylate and thereby activate an enzyme that activates glycogen phosphorylase which in turn breaks down glycogen into glucose-l-phosphate molecules; and it phosphorylates glycogen synthase, and in this way turns it off, thereby preventing the reconversion of the released glucose to glycogen. These two changes together ensure the mobilization of glucose through the breakdown of glycogen stored in the liver. A number of other reactions are triggered as well that together contribute to the fight/flight response. There is enormous signal amplification in this cascade. A single epinephrine molecule (present at 1O-10M) may cause the activation of dozens of alpha subunits of proteins. Each of these in turn will activate the synthesis of a single adenylate cyclase, and each of these in turn will synthesize hundreds of cAMP molecules. Each of these in turn can activate a cAMP-dependent kinase that will on its own right modify hundreds of target molecules in the cell. (from Expasy - updated 04/06) G-protein coupled receptors [1 to 4] (also called R7G) are an extensive group of hormones, neurotransmitters, odorants and light receptors which transduce extracellular signals by interaction with guanine nucleotide-binding (G) proteins. The receptors that are currently known to belong to this family are listed below.
Trends Pharmacol Sci. 2006 May. Heterotrimeric G proteins couple receptors for diverse extracellular signals to effector enzymes or ion channels. Each G protein comprises a specific alpha-subunit and a tightly bound betagamma dimer. Several human disorders that result from genetic G-protein abnormalities involve the imprinted GNAS gene, which encodes Gs alpha, the ubiquitously expressed alpha-subunit that couples receptors to adenylyl cyclase and cAMP generation. Loss-of-function and gain-of-function mutations, in addition to imprinting defects, of this gene lead to diverse clinical phenotypes. Mutations of GNAT1 and GNAT2, which encode the retinal G proteins (transducins), are rare causes of specific congenital visual defects. Common polymorphisms of the GNAS and GNB3 (which encodes Gbeta3) genes have been associated with multigenic disorders (e.g. hypertension and metabolic syndrome). To date, no other G proteins have been implicated directly in human disease. Science. 2002 May 31. The heterotrimeric guanine nucleotide-binding proteins (G proteins) are signal transducers that communicate signals from many hormones, neurotransmitters, chemokines, and autocrine and paracrine factors. The extracellular signals are received by members of a large superfamily of receptors with seven membrane-spanning regions that activate the G proteins, which route the signals to several distinct intracellular signaling pathways. These pathways interact with one another to form a network that regulates metabolic enzymes, ion channels, transporters, and other components of the cellular machinery controlling a broad range of cellular processes, including transcription, motility, contractility, and secretion. These cellular processes in turn regulate systemic functions such as embryonic development, gonadal development, learning and memory, and organismal homeostasis. Biochem Pharmacol. 2002 Mar 15. It is widely believed that guanine nucleotide-binding regulatory proteins (G-proteins) play central roles as "molecular switches" in a variety of cellular processes ranging from signal transduction to protein and vesicle trafficking. To achieve these regulatory functions, G-proteins form complexes with a wide range of effector molecules whose activities are altered upon interaction with the G-protein. These effector molecules can be either soluble or membrane bound, and it is likely that some are localized to secretory granules where they direct the movement, docking, and fusion of granules during exocytosis. The effector molecules regulated by G-proteins are diverse and include phospholipases, protein kinases, protein phosphatases, ion channels, adenylate cyclases, cytoskeletal elements, as well as secretory vesicle and plasma membrane-associated fusion-proteins. The majority of studies performed in the pancreatic beta-cell have focused on the role of G-proteins in the regulation of insulin secretion, whereas very little attention has been focused on their potential involvement in other cellular processes. Such studies have identified and implicated both heterotrimeric (comprising alpha, beta, and gamma subunits) and monomeric (low molecular mass) G-proteins in the regulation of insulin secretion, but intriguing recent evidence has also begun to emerge which favors the view that they may be involved in the maintenance of beta-cell viability. In the present commentary, we will review this evidence and discuss the current understanding of the role of G-proteins in the life and death of the beta-cell. Brain Res Dev Brain Res. 2002 Jan 31. Developmental changes in the distribution of guanine nucleotide-binding regulatory proteins (G proteins) were investigated in the rat brain during postnatal development. Using a standard or high-resolution urea-SDS-PAGE and specific polyclonal antipeptide antibodies oriented against G(i)alpha1/G(i)alpha2, G(i)alpha3, G(s)alpha, G(o)alpha1/G(o)alpha2, G(q)alpha/G(11)alpha and Gbeta subunit, all these proteins were determined by quantitative immunoblotting in homogenates prepared from cortex, thalamus, hippocampus and pituitary of 1-, 7-, 12-, 18-, 25- and 90-day-old animals. The levels of the majority of G protein alpha subunits, namely G(i)alpha1, G(i)alpha2, G(i)alpha3, G(o)alpha1, G(o)alpha2, G(q)alpha, G(11)alpha and Gbeta, were high already at birth. Whereas the short variant of G(s)alpha, G(s)alphaS, rose sharply in all tested brain regions between postnatal day (PD) 1 and 90, the long variant of G(s)alpha, G(s)alphaL, was unchanged in cortex and thalamus and slightly increased in hippocampus. An increase was observed also in expression of G(i)alpha1/G(i)alpha2 and G(o)alpha1 protein, while G(o)alpha2 remained constant. Minority protein G(o)alpha* dramatically increased in cortex and thalamus, was unchanged in hippocampus and not detectable in pituitary. By contrast, the highest levels of G(i)alpha3 and G(q)alpha/G(11)alpha were detected as early as at PD 1. During the next 90 days, the immunological signal of G(i)alpha3 almost disappeared and G(q)alpha/G(11)alpha continuously declined to the levels corresponding to 50% of the levels determined at birth. Expression of Gbeta subunit was basically unchanged during postnatal development. Our present analysis indicates that G(s)alpha, G(i)alpha/G(o)alpha and G(q)alpha/G(11)alpha proteins are differently expressed in the course of brain development. Differential expression of the individual alpha subunits of trimeric G proteins during postnatal development suggests their different roles in maturation of the brain tissue. Oncogene. 2001 Mar 26. Heterotrimeric guanine nucleotide binding proteins, commonly known as G proteins form a super-family of signal transduction proteins. They are peripherally associated with the plasma membrane and provide signal coupling to seven transmembrane surface receptors. G proteins are composed of monomers of alpha, beta, and gamma subunits. The beta- and gamma-subunits are tightly associated. The receptors activated by the appropriate "signal", interact catalytically with specific G-proteins to mediate guanine nucleotide exchange at the GDP/GTP binding site of the G-protein alpha-subunits, thus displacing the bound GDP for GTP. The GTP bound form of the g-protein alpha-subunit and in some cases the free betagamma-subunits initiate cellular response by altering the activity of specific effector molecules. Recent studies have indicated that the asyncronous activation of these proteins can lead to the oncogenic transformation of different cell types. The mechanism by which G-proteins regulate the various cell functions appear to involve a complex net-working between different signaling pathways. This review summarizes the signaling mechanisms involved in the regulation of cell proliferation by these transforming G proteins. Oncogene. 1998 Sep 17. Heterotrimeric G-proteins are important signalling proteins which function in all cells of the mammalian organism. Inactivating mutations in a variety of G-protein alpha-subunit genes in mice resulted in mostly unexpected phenotypes and have provided interesting new insight into their biological roles. Whereas the inactivation of some G alpha genes led to mild phenotypes suggesting the presence of redundant or compensatory mechanisms, other G-proteins appear to play highly specific biological or developmental roles. The purpose of this review is to summarize the current knowledge about G-protein functions based on gene-inactivation studies. Biochem Pharmacol. 1997 Jan 10. Heterotrimeric G-proteins are well-known transducers of signaling from a populous class of heptihelical, membrane receptors to a smaller group of effector molecules that includes adenylylcyclases, cyclic GMP phosphodiesterases, phospholipases (type C beta), and various ion channels. Dramatic changes in specific G-protein subunits that coincide with commitment to highly-specialized cell types suggest a key role for these extrinsic membrane proteins in cell differentiation and development. Through analysis of the effects of gain-of-function and loss-of-function mutants, it has been possible to explore this new dimension in G-protein biology, intimately linking specific G-proteins to development. G-protein subunits are shown to be important molecular switches in the complex biological processes controlling both cellular differentiation and early development. FASEB J. 1989 Aug. Hormones, neurotransmitters, and autacoids interact with specific receptors and thereby trigger a series of molecular events that ultimately produce their biological effects. These receptors, localized in the plasma membrane, carry binding sites for ligands as diverse as peptides (e.g., glucagon, neuropeptides), lipids (e.g., prostaglandins), nucleosides and nucleotides (e.g., adenosine), and amines (e.g., catecholamines, serotonin). These receptors do not interest directly with their respective downstream effector (i.e., an ion channel and/or an enzyme that synthesizes a second messenger); rather, they control one or several target systems via the activation of an intermediary guanine nucleotide-binding regulatory protein or G protein. G proteins serve as signal transducers, linking extracellularly oriented receptors to membrane-bound effectors. Traffic in these pathways is regulated by a GTP (on)-GDP (off) switch, which is regulated by the receptor. The combination of classical biochemistry and recombinant DNA technology has resulted in the discovery of many members of the G protein family. These approaches, complemented in particular by electrophysiological experiments, have also identified several effectors that are regulated by G proteins. We can safely assume that current lists of G proteins and the functions that they control are incomplete. |