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Signal Transduction

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Biological Sciences
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Signal Transduction in Cell Physiology Ligand reception  chemical binds to a receptor or a stimulus Information transfer  receptor undergoes conformational change that effectively brings the signal across the membrane into the cell. Some ligands pass directly thru membrane and act on cytoplasmic receptors. Amplification and networking  occur when receptors or downstream components act catalytically and interact through common downstream pathways Adaptation and desensitization  processes which attenuate a cell’s response during persistent stimulation Common themes in signal transduction (general mech. used for transfer of info) • Ion channels o Transfer info by changing resting membrane potential o Binding of ligand alters the gating properties of the receptor, aqllowing ions to pass • G-protein linked o G-protein coupled receptors are linked to intracellular G-proteins o Excited receptor interacts with the G-protein and causes it to release GDP o Binds GTP and becomes activated, then activates downstream proteins • Tyrosine kinase linked o Some contain catalytic domains o Others the tyrosine kinase activities are located on separate associated proteins • Receptor enzymes o Carry catalytic activities such as guanylyl cyclases or protein kinases within their cytoplasmic domains o Ligand binding causes an increase in the enzymatic activity • Cytoplasmic receptors o Ligand simply passes directly thru membrane Amplification and Networking • Second messengers  very small amounts create large response  are rapidly destroyed • GTPase switch proteins o Binding of GTP triggers a conformational change that switches the protein from an inactive form to an active form o Have an intrinsic GTPase activity that hydrolyses the bound GTP molecule • Protein kinases and phosphatases o Phosphorylation of serine, threonine, tyrosine, histidine, or aspartate residues on signal transduction proteins o Phosphorylation by protein kinases typically causes a conformational change in the target protein that acts as a reversible switch o Switch is reversed by a specific phosphatase that removes phosphate and restores the amino acid to its unmodified form • Adaptors and scaffolds o Proteins that contain multiple protein-protein interaction domains and this can bind multiple other proteins of the pathway o They recruit other proteins to specific locations and organize groups of proteins within the pathway o Facilitate transmission of the signal along the pathway Classes of cell surface receptors • Growth factor (tyrosine kinase)  growth control o Consist of 2 identical signle membrane span subunits which dimerize upon binding ligand o Dimerizatoin causes autophosphorylation of tyrosine residues on the cytoplasmic domains o Phosphorylated tyrosine residues bind cytosolic proteins containing SH2- domains (Src-homology), which leads to the downstream signaling events o Specific SH2 domains recognize specific phosphotyrosine residues based on a short sequence of amino acids immediately surrounding the tyrosine • Seven helix (G protein coupled)  growth control o Largest class of receptors o Binding of ligand on the outer face of the membrane causes movement of rd th the ends of the helices, primarily the 3 and 6 , on the inner face. o Receptor interacts with a hetrotrimeric G-protein, consisting of α- and βγ- subunits. o α- releases GDP, binds GTP, and then dissociates from the βγ- subunits. o The free subunits can each activate downstream effectors o Many GPCRs undergo agonist-induced phosphorylation, which plays a role in desensitization and internalization of these receptors • Cytokine (tyrosine kinase)  growth control o Consist of two homologous but non-identical single membrane span subunit o Do not contain enzymatic activities in their cytoplasmic domains o They recruit and activate cytosolic tyrosine kinases (JAKs) o JAKs phosphorylate TFs called STATs, providi
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