BCOR 103 1st Edition Lecture 16 Outline of Last Lecture I Cell Signaling and Cellular Responses II Protein Kinase Phosphatase III Signaling Molecules Outline of Current Lecture I II III Receptors Receptor Tyrosine Kinases Transmitters Second Messengers Current Lecture Cell Surface Receptors Cell surface receptors o Molecule binding to receptor initiates signal transmission o Most reside on cell surface of target cell exception steroid hormone receptors o Transmit signal by regulating activity of intracellular proteins o Common receptor domains Extracellular molecule binding domain Membrane spanning domain Cytosolic catalytic domain o Receptor itself can respond to signal Classes of Cell Surface Receptors 1 2 3 4 G protein coupled receptors most abundant type Receptor tyrosine kinases only kind with enzymatic activity Cytokine receptors usually coupled to non receptor tyrosine kinases Enzyme linked receptors a Tyrosine phosphatases b Serine threonine kinases c Guanlyly cyclase G Protein coupled receptors GPCRs These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Structure serpentine receptors o Extracellular domain ligand specificity o 7 membrane spanning helices o Cytosolic domain functional domain Functions 1 000 in mammalian cells o Coupled to G protein o Act as a molecular switch GDP bound inactive or off state GTP bound active or on state o Bind variety of molecules Neurotransmitters neuropeptides peptide hormones o Involved in many signaling pathways Smell sight taste metabolism cell survival proliferation ion channels GPCRs bind to heterotrimeric G Proteins A heterotrimeric G protein has three different polypeptides and Regulation of Heterotrimeric GPCRs Mammalian cells have 20 subunits 5 subunits and 12 subunits GTP and GDP regulate on off state of GPCRs complexes to receive hormone signal The GTP GDP regulation is an exchange reaction NOT phosphorylation dephosphorylation Receptor Tyrosine Kinases RTKs 59 RTKs found in the human genome o Generally classified by extracellular domains which determine specificity of ligand receptor interactions and dimerization properties Activation of RTKs Activation requires auto phosphorylation of cytosolic kinase domain Changes in RTKs after Activation Auto phosphorylation within kinase domain increases protein kinase activity Auto phosphorylation outside kinase domain creates binding sites for downstream signaling proteins effectors o Effector binds though SH2 domains Src Homology 2 PTB domains phosphotyrosine Responses to RTK Activation Activate or inhibit enzymatic activity through tyrosine phosphorylation Stimulate enzymatic activity via conformational changes Bridge interactions between receptor and enzymatic targets adaptors Localize proteins to plasma membrane and or promote complex formation at plasma membrane docking proteins Inactivating Receptor Tyrosine Kinases General Mechanisms 1 2 3 4 5 6 Phosphatases PTP regulated by localization Ubiquitination ubiquitin ligase Cbl Negative feedback PKC induced inhibitory phosphorylation Endocytosis Antagonistic ligands Hetero oligomerization with kinase domain deficient receptor mutants Epidermal Growth Factor Receptor Multiple complex signaling pathways are downstream of all receptor tyrosine kinases Cytokine Receptors no kinase activity separate polypeptide comes in Cytokine receptors o No biological activity o Bind to intracellular tyrosine kinases Ex Src kinase family immune cell signaling o Stimulates growth differentiation and immune responses Src Kinases Non receptor Tyrosine Kinases Identified in Rous sarcoma virus Roles in differentiation cell adhesion motility immune cell survival Phosphorylation of Y527 keeps Src in auto inhibited inactive conformation SH3 or SH12 binding or dephosphorylation of Y527 activates Src Constitutive Src Kinase identified as First Oncogene Receptors Linked to other Enzymatic Activities Tyrosine phosphatases o Remove phosphate groups from phosphotyrosines o Ligand mediated dimerization inhibits phosphatase o Human cells have 21 protein tyrosine phosphatases Serine threonine Kinases o Phosphorylate serine and threonine residues of substrate o Exhibit open closed auto regulation of kinase activity Cytoplasmic proteins with additional biochemical activity Transmitters and Second Messengers Transmitters o Intracellular molecules that are linked to receptors o Propagates and amplifies signal to intracellular targets o Common transmitters are Second messengers small molecules produced in response to initial signal Small G proteins guanine nucleotide binding proteins that modulate protein kinases cascades Protein kinases phosphorylate enzymatic target or part of signaling cascade Ultimate targets o Final target of signal transduction pathway Transcription factors Nuclear targets Cytoplasmic targets Second Messengers cAMP Pathway Second messengers small molecules that initiate intracellular signaling o Examples cAMP Ca 2 DAG etc cAMP is synthesized from ATP by adenylyl cyclase and degraded to AMP by cAMP phosphodiesterase cAMP acts downstream of hormone epinephrine to signal breakdown of glycogen to glucose in anticipation of muscle activity Regulation of Protein Kinase A PKA by cAMP epinephrine triggers cAMP synthesis via G protein coupled receptors activation of adenylyl cyclase cAMP binds to the two regulatory subunits of PKA PKA regulatory subunits then phosphorylate serine residues on target proteins PKA Directly Affects Enzymatic Activity of Proteins PKA phosphorylation inhibits glycogen synthase PKA phosphorylation activates phosphorylase kinase Amplification of Signal via cAMP Second Messengers Epinephrine response release of glucose in anticipation of muscle usage Activates 100 million intracellular molecules from one signaling molecule