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Transcript
Cell Communication Chapter 7 7.1 Cell Communication: An Overview Cells communicate with one another through • Direct channels of communication • Specific contact between cells • Intercellular chemical messengers Apoptosis Fig. 7-1, p. 140 Intercellular Chemical Messengers Controlling cell • Releases signal molecule that causes response of target cells Target cell processes signal in 3 steps: • Reception, transduction, response Signal transduction • Series of events from reception to response Signal Transduction Fig. 7-2, p. 142 Reception 7.2 Cell Communication Systems with Surface Receptors Peptide hormones and neurotransmitters • Primary extracellular signal molecules recognized by surface receptors in animals Surface receptors • Integral membrane glycoproteins Signaling molecule • Bound by a surface receptor • Triggers response pathways within the cell Surface Receptors Cell communication systems based on surface receptors have 3 components: (1) Extracellular signal molecules (2) Surface receptors that receive signals (3) Internal response pathways triggered when receptors bind a signal Peptide Hormones Peptide hormones • Small proteins Growth factors • Special class of peptide hormones • Affect cell growth, division, differentiation Neurotransmitters Neurotransmitters include • Small peptides • Individual amino acids or their derivatives • Chemical substances Surface Receptors Surface receptors • Integral membrane proteins • Extend entirely through the plasma membrane Binding of a signal molecule • Induces molecular change in the receptor that activates its cytoplasmic end Response of Surface Receptor Cellular Response Pathways (1) Cellular response pathways • Operate by activating protein kinases Protein kinases add phosphate groups • Stimulate or inhibit activities of target proteins, producing cellular response Cellular Response Pathways (2) Protein phosphatases • Reverse response • Remove phosphate groups from target proteins Receptors are removed by endocytosis • When signal transduction is finished Phosphorylation Amplification Each step of a response pathway catalyzed by an enzyme is amplified • Each enzyme activates hundreds or thousands of proteins that enter next step in pathway Amplification • Allows full cellular response when few signal molecules bind to receptors Amplification 7.3 Surface Receptors with Built-In Protein Kinase Activity Receptor Tyrosine Kinases Receptor Tyrosine Kinases Receptor tyrosine kinases bind signal molecule • Protein kinase site becomes active • Adds phosphate groups to tyrosines in the receptor itself, and to target proteins Phosphate groups added to cytoplasmic end of receptor are recognition sites for proteins activated by binding to the receptor Protein Kinase Activity 7.4 G-Protein–Coupled Receptors G proteins: Key molecular switches in secondmessenger pathways Two major G-protein–coupled receptor response pathways involve different second messengers G-Protein-Coupled Receptors G-protein-coupled receptors activate pathways • Binding of the extracellular signal molecule (first messenger) activates a site on the cytoplasmic end of the receptor G-Protein-Coupled Receptors Fig. 7-8, p. 147 G-Protein Activation Activated receptor turns on a G protein, which acts as a molecular switch G protein • Active when bound to GTP • Inactive when bound to GDP Active G Protein Active G protein • Switches on the effector of the pathway (enzyme that generates second messengers) Second messengers • Small internal signal molecules • Activate the protein kinases of the pathway Response Pathways Second Messengers: cAMP 1st of two major pathways triggered by Gprotein-coupled receptors Effector (adenylyl cyclase) generates cAMP as second messenger cAMP activates specific protein kinases cAMP Receptor-Response Pathways Fig. 7-10, p. 148 cAMP Fig. 7-11, p. 148 Second Messengers: IP3 and DAG (1) 2nd major pathway triggered by G-protein– coupled receptors Activated effector (phospholipase C), generates two second messengers, IP3 and DAG Second Messengers: IP3 and DAG (2) IP3 activates transport proteins in the ER • Releasing stored Ca2+ into the cytoplasm Released Ca2+ (alone or with DAG) activates specific protein kinases • Adds phosphate groups to target proteins IP3/DAG Receptor-Response Pathways Pathway Controls cAMP and IP3/DAG pathways are balanced by reactions that eliminate second messengers • Stopped by protein phosphatases that continually remove phosphate groups from target proteins • Stopped by endocytosis of receptors and their bound extracellular signals Mutations Mutated systems can turn on the pathways permanently, contributing to progression of some forms of cancer Gene Regulation: Ras Some pathways in gene regulation link certain receptor tyrosine kinases to a specific G protein (Ras) When the receptor binds a signal molecule, it phosphorylates itself • Adapter proteins then bind, bridging to and activating Ras Activated Ras Activated Ras turns on the MAP kinase cascade Last MAP kinase in cascade phosphorylates target proteins in the nucleus • Activates them to turn on specific genes Many of these genes control cell division Gene Regulation 7.5 Pathways Triggered by Internal Receptors: Steroid Hormone Receptors Steroid hormones have widely different effects • Depend on relatively small chemical differences Response of a cell to steroid hormones • Depends on internal receptors and the genes they activate Steroid Hormone Receptors Steroid hormones penetrate plasma membrane • Bind to receptors within the cell Internal receptors • Regulatory proteins that turn on specific genes when activated by binding a signal molecule • Produce cellular response Two Domains of Steroid Hormone Receptors Steroid hormone receptors • One domain recognizes and binds a specific steroid hormone • One domain interacts with the controlling regions of target genes Gene Activation: Steroid Hormone Receptors Cell Response Cell response to a steroid hormone • Depends on whether it has an internal receptor for the hormone Type of response within the cell • Depends on the genes that are recognized and turned on by an activated receptor 7.6 Integration of Cell Communication Pathways Cross talk Cross-Talk Cell signaling pathways communicate with one another to integrate responses to cellular signals May result in a complex network of interactions between cell communication pathways Cross-Talk Modification of Cell Response Cross-talk often results in • Modifications of cellular responses controlled by the pathways • Fine-tuning effects of combinations of signal molecules binding to receptors of a cell Cell Communication Pathways In Animals Inputs from other cellular response systems also can become involved in the cross-talk network • Cell adhesion molecules • Molecules arriving through gap junctions