Signal Transduction In Biological Membranes Lecture Notes PDF

Session 7 Lectures 1 Signal transduction in biological membranes Receptor Effector Signaling via Proteins Dr. Ruwiada Wahab Salman References 1. Introduction to General, Organic and Biochemistry Frederick A. Bettelheim 2. Alberts, et al., Molecular Biolo...

Session 7 Lectures 1 Signal transduction in biological membranes Receptor Effector Signaling via Proteins Dr. Ruwiada Wahab Salman References 1. Introduction to General, Organic and Biochemistry Frederick A. Bettelheim 2. Alberts, et al., Molecular Biology of the Cell: Fourth Edition, New York: Garland Science, 2002 Objectives To understand how signaling molecules are recognized by cells to evoke an appropriate cellular response. To understand how activated G protein-coupled receptors initiate a linear sequence of signalling events involving G proteins and enzyme or ion channel effector proteins. To understand how signalling molecules are recognized by cells to evoke an appropriate cellular response. What is signal transduction? For cells respond to extracellular signaling molecules (e.g. hormones, neurotransmitters, growth factors, etc….) they must possess the appropriate receptor. Receptors can be intracellular for (steroid and thyroid hormones, etc…), or extracellular, the majority of signaling molecules do not readily cross the plasma membrane, therefore, more receptors are located at the cell surface.  Although some receptors can directly alter cellular activity, many require (transduction) of the initial ligand binding event via other intracellular signaling components to generate a response, (e.g. contraction, secretion, proliferation, differentiation, etc…). There are 3 ‘’superfamilies’’ of cell surface receptor 1. Ligand gated (receptor operated) ion channels (e.g. nicotinic acetylcholine receptors). 2. Receptors with intrinsic enzymatic activity (receptor tyrosine kinases) (e.g. insulin receptor). 3. G protein coupled receptors (e.g. muscarinic acetylcholine receptors). Notes: ֍Each receptor subtype is specific for one (or a very limited number of chemicals (‘LIGAND’). ֍Ligand binding activated the receptor, which in turn directly or indirectly brings about a change in cellular activity. 2 nd Objectives To understand how activated G protein coupled receptors initiate a linear sequence of signaling events involving G proteins and enzyme or ion channel effector proteins.  G protein coupled receptors (GPCRs) definition and properties GPCRs are a family of receptors that act by altering the activity of effectors (e.g. enzymes, ions and channels). They achieve this via the activation of one or more types of (guanine nucleotide binding proteins; G-Proteins) thereby changing cellular activity. (G protein coupled receptors) alter the activities of effectors, which may be second messenger generating enzymes (e.g. adenylyl cyclase) or ion channels, via activation of one or more types of (guanine nucleotide binding proteins; G proteins). G protein-coupled receptors features There are >700 GPCRs identified in the human genome.All GPCRs share a common basic structure:  How do GPCRs cause a change in cellular activity? 1. Agonist binds receptor. 2. Protein-protein interaction releases GDP, binds GTP. 3. GTP released and interact with effectors. 4. GTP hydrolysed to GDP. 5. GDP reform heterotrimer.  An activated GPCRs must interact with another protein called a guanine nucleotide binding protein (G protein). G proteins are made up of three subunits they are (heterotrimeric): α (alpha), β (beta), and ɣ (gamma). The functional units of the G protein are the α subunit and βɣ subunit. The GPCRs and G protein interaction activates the G protein by causing GTP to exchange for GDP on the G protein α subunit.  Th α-βɣ complex e immediately dissociates each can then interact int α-GTP with effector subunits + proteins o free (second βɣ messenger subunits generating and enzymes, or ion channels). The α- and/ βɣ interaction effectors GTP or with the α GTPa activity lasts hydrolyses GTP back tose until subunit GDP. The α-GDP and βɣ subunits then reform an inactive heterotrimeric complex.  G protein diversity The human genome encodes 20 G α (alpha), 5 G β (beta) and 12 + G ɣ (gamma) proteins. Therefore, there are > 1000 possible G α-βɣ protein combinations. But there is a specificity i,e activated GPCRs preferentially interact with specific types of G protein. In this way, an extracellular signal, working via a specific GPCR, will activate a single, or small sub- population of G proteins and effectors in the cell to bring about a specific cellular response.  G protein GTP-for-GDP exchange and GTP hydrolysis on/off switch and timer G protein GTP-for-GDP exchange and GTP hydrolysis on/off switch and timer G proteins can be thought of as on/off switches and timers. The on switch is receptor-facilitated GDP/GTP exchange and the timer/off switch is governed by the length of time taken for GTP hydrolysis. What are the Cellular Targets for Activated G- Proteins? Stimulatory Gs (αs-GTP)stimulates adenylyl cyclase to produce cAMP.  Inhibitory Gi pathways, which reduce cAMP levels by inhibiting adenylyl cyclase. Gq/11 proteins preferentially interact with the membrane bound enzyme phospholipase C to generate the messengers inositol and diacyglycerol. Transducing Gt that is activated by light- sensing proteinrhodopsin turn which in hydroly cyclic GMP to activates ses 5’-GMP. Diseases associated with signal transduction Mutation to G protein coupled receptors (GPCRs) Genetic changes to GPCRs result in loss-of-function or gain-of-function mutation: 1. Retinitis pigmentosa can be caused by a loss of function mutation to rhodopsin. 2. Nephrogenic diabetes insipidus can be caused by a loss of function mutation to the V2 vasopressin receptor 3. Familial male precocious puberty can be caused by a gain of function mutation to the luteinizing hormone (LH) receptor.  Experimental Manipulation of the G-Protein Cycle Cholera Toxin (CTx) and Pertussis Toxin (PTx) contain an enzyme ADP-Ribosyl transferase that specifically modifies G Proteins. Cholera Toxin (CTx): CTx eliminates the GTPase activity of Gsα. This leads Gsα to become irreversibly activated. Pertussis Toxin (PTx): PTx interferes with the GDP/GTP exchange on Giα. This leads Giα to become irreversibly inactivated.  The enzyme is an ADP-ribosyl transferase that covalently modifies (ADP-ribosylates) specific G proteins Thank you