GPCR: Phosphoinositides

GPCR Side notes: Phosphoinositides Phosphatidylinositol (PI) is a negatively charged phospholipid It sits at the cytosolic part of the plasma membrane Can be phosphorylated at positions 3, 4, and 5 to form phosphoinositides: Phosphatidylinositol phosphate (PIP) (3, 4, or 5) Phosphatidylinositol biphosphate (PIP2) Phosphatidylinositol triphosphate (PIP3) Some G-proteins activate inositol phospholipid signaling pathway G protein activates phospholipase C-beta Phospholipase cleaves PIP2 PIP2 —> DG + IP3 Which of the following is water-soluble? • • • • • PIP2 PIP3 IP3 All of the above None of the above Inositol 1,4,5-triphosphate IP3 Water-soluble small intracellular mediator Opens IP3-gated Ca2+-release channels in endoplasmic reticulum Increase of Ca2+ concentration in cytosol Ca2+ influences activity of some proteins Response terminated by lipid phosphatases IP3 —> IP2, or lipid kinases IP3 —> IP4. Ca2+ is pumped out of the cell Diacylglycerol and Ca2+ activate protein kinase C (PKC) Roles of PKC Contraction of smooth muscle Vasoconstriction Secretion (salivary gland, sweat gland, etc.) Other functions Ca2+ functions as a ubiquitous intracellular mediator Many extracellular signals increase Ca2+ concentration Ca2+ concentration is normally low in the cytosol, but high in the extracellular fluid and in the lumen of endoplasmic reticulum The concentration is kept low by pumps in the plasma membrane, ER, and mitochondria, and by Ca2+ -binding proteins Signals can transiently open the channels in plasma membrane or ER High Ca2+ concentrations activate Ca2+-responsive proteins Calmodulin activates other proteins by ______ • Inducing conformational change • Phosphorylation • Dephosphorylation Calmodulin The most important protein, which helps to relay Ca2+ signal Found in all eukaryotic cells Constitute up to 1% of cell protein mass Intracellular Ca2+ receptor Highly conserved Does not have enzymatic activity; acts by binding to and activating other proteins Calmodulin has four Ca2+ binding sites Ca2+ binding induces conformational change Calmodulin goes through conformational change upon binding calcium GPCR Side notes: Calmodulin activates: Plasma membrane Ca2+ pump, that pumps Ca2+ out of the cell Many enzymes and membrane proteins Ca2+/calmodulin-dependent protein kinases (CaM-kinases) Some CaMKs phosphorylate gene regulatory proteins, including CREB The role of CaMKII CaMKII is enriched in nervous system, can constitute up to 2% of the cell protein mass in some brain cells Highly concentrated at synapses CaMKII can stay active for a long time after Ca2+ impulse and serve as a memory device Nice which lack a brain-specific form of CaMKs have defects in their ability to remember objects locations CaMKII strengthens synapses, which accounts for long term memory Glutamate is one of the main neurotransmitters in the mammalian central nervous system CaMKII helps in addition of glutamate receptors to the membrane of postsynaptic neurons It also aids in phosphorylation of glutamate receptors (ion channels) for enhanced permeability Both of the above strengthens synapses promoting long term memory Which of the following explains long-term memory? • CaMKII stays activated by calmodulin • CaMKII is autophosphorylated Ca2+/calmodulin-dependent protein kinases mediate many responses to Ca2+ signals A domain pops out of CaMK II kinase complex Calmodulin binds Ca2+ This enables calmodulin to bind to CaMK II domain and activate it by promoting a conformational change This results in a partially active kinase When another domain pops out, it gets activated as well, and then CaMKII domains phosphorylate each other Phosphorylated CaMK stays phosphorylated and active even after calmodulin dissociates This mechanisms is a molecular form of memory, important in memory and learning in nervous system The same signaling molecule, acetylcholine, can bind to GPCR or an ion channel receptor Nicotinic acetylcholine receptors is an ion channel receptor Can also be opened by nicotine, which causes its addictive effects Muscarinic acetylcholine receptor is a GPCR Can also be activated by muscarine; poisoning with muscarine may end in convulsions and death. Associated G protein can open ion channels Smell depends on GPCRs Olfactory receptors are responsible for smell They are GPCRs Olfactory GPCRs activate G proteins, which activate adenylyl cyclase, which increases cAMP GPCR Side notes: A possible role of cAMP is • Activation of a kinase • Opening an ion channel • Both A and B Glutamate is always an inhibitory neurotransmitter • True • False Which of the following is mostly based on cAMP increase? • Smell • Vision cAMP opens cAMP -gated ion channel, which allow the influx of Na+ in the cell It depolarizes the olfactory neuron and initiates nerve impulse, which travels along the axon to the brain Vision depends on GPCR Visual responses are the fastest G-protein-mediated responses Light activates GPCR receptor rhodopsin inside the photoreceptor cells in the retina (rods and cones) A signal transduction pathway for rods Rods are responsible for night vision When rhodopsin absorbs a photon, it’s conformation changes The alpha subunit of G protein activates cGMP phosphodiesterase. cGMP concentration falls cGMP-sensitive channels close This causes hyper-polarization of the neuron This inhibits release of glutamate Glutamate inhibits the next neuron in the chain (bipolar cell) Bipolar cell loses its inhibition and transmits the signal Light decreases the glutamate release and so inhibits the bipolar cells Dark GPCR rhodopsin inactive High level of cGMP Neuron depolarized High rate of neurotransmitter glutamate release Glutamate inhibits bipolar cells No signal Light Stimulates GPCR Low level of cGMP Na-channels close Low rate of glutamate release Bipolar cell gets activated Signal gets transmitted to the brain Comparison of the light and the smell signal transduction pathways Light Rhodopsin GPCRs Activation leads to decrease in cGMP Na-channel closes Glutamate level decreases Intermediate neurons are activated Signal is transmitted Smell Olfactory GPCRs Activation leads to increase in cAMP Na-channel opens Neural cell transmits the signal to the brain Gas NO is a signal molecule in animals and plants NO relaxes smooth muscles in animals Nerves release neurotransmitter acetylcholine GPCR Side notes: Which enzyme does viagra block? • It blocks guanylyl cyclase that makes cGMP, which is a signal to relax a smooth muscle • It blocks cGMP phosphodiesterase • It blocks GPCR NO synthase in smooth muscle relaxation pathway is in ____ • Smooth muscle cells • Nerve cells • Epithelial cells Endothelial cell responses by releasing NO Blood vessel dilates Gas NO only acts locally Gas NO lifetime is 5-10 sec Oxygen and water eventually convert it in nitrites and nitrates