1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 11: Cell Communication - The Cellular Internet Cell-to-cell communication is essential for multicellular organisms Biologists have discovered some universal mechanisms of cellular regulation External signals are converted into responses within the cell A signal-transduction pathway is a series of steps by which a signal on a cell’s surface is converted into a specific cellular response Pathway similarities suggest that ancestral signaling molecules evolved in prokaryotes and have since been adopted by eukaryotes

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings What will we cover? Local (paracrine and synaptic) signaling vs. long distance (endocrine) signaling The three main steps of signaling inside of a cell – Reception: What types of receptors are there? – Transduction: Different types of signal propagation (kinase cascades, 2 nd messengers) – Response: The final output of the signal Regulation of signaling Termination of signaling

3. Exchange of mating factors Mating Receptor a   factor a  a factor Yeast cell, mating type a Yeast cell, mating type  a/  New a/  cell Even unicellular yeast signal!!! Cell signaling between Yeast cells regulates their mating They always mate to cells of the different mating type (a vs.  )

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Local and Long-Distance Signaling Cells in a multicellular organisms communicate by chemical messengers Local signaling: – Cells are directly connected: Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells (gap junctions in animals, plasmodesmata in plants) – Animal cells may communicate by direct contact in cell to cell recognition – Paracrine signaling – local regulators (signaling molecules/ligands) are released into extracellular fluid and nearby cells bind the released ligand – Synaptic signaling – signaling between neurons in the nervous system Long distance signaling – endocrine signaling – hormones in blood

5. Plasma membranes Gap junctions between animal cells Cell junctions Cell-cell recognition Plasmodesmata between plant cells Local Signaling

6. Paracrine signaling Local regulator diffuses through extracellular fluid Secretory vesicle Secreting cell Target cell Local signaling Electrical signal along nerve cell triggers release of neurotransmitter Neurotransmitter diffuses across synapse Endocrine cell Blood vessel Long-distance signaling Hormone travels in bloodstream to target cells Synaptic signaling Target cell is stimulated Hormonal signaling Target cell In many other cases, animal cells communicate using local regulators, messenger molecules that travel only short distances (local signaling) In long-distance signaling, plants and animals use chemicals called hormones Local Signaling vs. Long distance signaling

7. EXTRACELLULAR FLUID Reception Plasma membrane Transduction CYTOPLASM Receptor Signal molecule Relay molecules in a signal transduction pathway Response Activation of cellular response The Three Stages of Cell Signaling: A Preview Earl W. Sutherland discovered how the hormone epinephrine acts on cells Sutherland suggested that cells receiving signals went through three processes: Reception, Transduction, Response

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Reception (1 st stage): A signal molecule (ligand) binds to a receptor, causing it to change shape The binding between a signal molecule (ligand) and receptor protein is highly specific A conformational change in a receptor is often the initial transduction of the signal Most signal receptors are plasma membrane proteins – they span the plasma membrane Some receptor proteins are intracellular, (within the cell) found in the cytosol or nucleus of target cells Small or hydrophobic chemical messengers can readily cross the membrane and activate these intracellular receptors Examples of hydrophobic messengers are the steroid and thyroid hormones of animals An activated hormone-receptor complex can act as a transcription factor, turning on specific genes

9. EXTRACELLULAR FLUID Plasma membrane The steroid hormone testosterone passes through the plasma membrane. Testosterone binds to a receptor protein in the cytoplasm, activating it. The hormone- receptor complex enters the nucleus and binds to specific genes. The bound protein stimulates the transcription of the gene into mRNA. The mRNA is translated into a specific protein. CYTOPLASM NUCLEUS DNA Hormone (testosterone) Receptor protein Hormone- receptor complex mRNA New protein The steps in Signaling using an intracellular receptor THIS IS WHAT HORMONES DO = ENDOCRINE/LONG DISTANCE SIGNALING!

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Receptors in the Plasma Membrane Most water-soluble ligands bind to specific sites on receptor proteins in the plasma membrane – What kinds of characteristics would you expect trans-membrane receptor proteins to have? These water-soluble ligands are polar and hydrophelic, so they do not easily cross the plasma membrane There are three main types of membrane receptors: – G-protein-linked receptors – Receptor tyrosine kinases – Ion channel receptors

11. Segment that interacts with G proteins Signal-binding site G-protein-linked receptors A G-protein-linked receptor is a plasma membrane receptor that works with the help of a G protein The G-protein acts as an on/off switch: If GDP is bound to the G protein, the G protein is inactive; when the protein is bound to GTP, it is active

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings G-protein linked receptor signaling G protein-coupled receptor Plasma membrane Enzyme G protein (inactive) GDP CYTOPLASM Activated enzyme GTP Cellular response GDP P i Activated receptor GDP GTP Signaling molecule Inactive enzyme 1 2 3 4

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Receptor tyrosine kinases are membrane receptors that attach phosphates to tyrosine amino acids in proteins A receptor tyrosine kinase can trigger multiple signal transduction pathways at once Upon ligand binding, the receptors dimerize The kinase domain (the protein domain that phosphorylates other proteins) is activated and the receptor proteins cross-phosphorylate each other The activated receptor then relays its signal to other proteins in the signaling pathway Receptor tyrosine kinases

14. Signal molecule  Helix in the membrane Signal-binding site Tyr Tyrosines Receptor tyrosine kinase proteins (inactive monomers) CYTOPLASM Tyr Activated tyrosine- kinase regions (unphosphorylated dimer) Signal molecule Dimer Fully activated receptor tyrosine-kinase (phosphorylated dimer) Tyr P P P P P P ATP 6 ADP Tyr P P P P P P Inactive relay proteins Cellular response 2 Cellular response 1 Activated relay proteins 6 Receptor tyrosine kinases

15. Signal molecule (ligand) Gate closed Ions Ligand-gated ion channel receptor Plasma membrane Gate closed Gate open Cellular response An ion channel receptor acts as a gate when the receptor changes shape When a signal molecule binds as a ligand to the receptor, the receptor changes shape and the ‘ gate ’ allows specific ions, such as Na + or Ca 2+, through a channel in the receptor These receptors are common in neurons where they must propogate a signal down the length of an axon (synaptic signaling) Ion channel receptors

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Signal Transduction (2 nd stage): Cascades of molecular interactions relay signals from receptors to target molecules in the cell Signal transduction usually involves multiple steps Multistep pathways can amplify a signal: A few molecules can produce a large cellular response Multistep pathways provide more opportunities for coordination and regulation The molecules that relay a signal from receptor to response are mostly proteins Like falling dominoes, the receptor activates another protein, which activates another, and so on, until the protein producing the response is activated At each step, the signal is transduced into a different form, usually a conformational change in a protein

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Protein Phosphorylation and Dephosphorylation In many pathways, the signal is transmitted by a cascade of protein phosphorylations These phosphorylations are carried out by proteins known as kinases Often, one kinase is activated by being phosphorylated, which in turn then phosphorylates other kinases, activating them Phosphatase enzymes remove the phosphates, thus inactivating the proteins This phosphorylation and dephosphorylation system acts as a molecular switch, turning activities on and off

18. Signal molecule Activated relay molecule Receptor Inactive protein kinase 1 Active protein kinase 1 Inactive protein kinase 2 Active protein kinase 2 Inactive protein kinase 3 Active protein kinase 3 ADP Inactive protein Active protein Cellular response Phosphorylation cascade ATP PP P i ADP ATP PP P i ADP ATP PP P i P P P A typical signaling phosphorylation cascade

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Small Molecules and Ions as Second Messengers Second messengers are small, nonprotein, water-soluble molecules or ions The extracellular signal molecule that binds to the membrane is a pathway’s “first messenger” Second messengers can readily spread throughout cells by diffusion Second messengers participate in pathways initiated by G-protein- linked receptors and receptor tyrosine kinases Cyclic AMP (cAMP) is one of the most widely used second messengers Adenylyl cyclase, an enzyme in the plasma membrane, converts ATP to cAMP in response to an extracellular signal

20. ATPCyclic AMPAMP Adenylyl cyclase Pyrophosphate PP i Phosphodiesterase H2OH2O Adenylyl cyclase, an enzyme in the plasma membrane, converts ATP to cAMP in response to an extracellular signal

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Many signal molecules trigger formation of cAMP Other components of cAMP pathways are G proteins, G-protein-linked receptors, and protein kinases cAMP usually activates protein kinase A, which phosphorylates various other proteins Further regulation of cell metabolism is provided by G-protein systems that inhibit adenylyl cyclase cAMP as a second messenger

22. cAMP ATP Second messenger First messenger (signal molecule such as epinephrine) G-protein-linked receptor G protein Adenylyl cyclase Protein kinase A Cellular responses GTP cAMP as a second messenger

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Calcium ions and Inositol Triphosphate (IP 3 ) Calcium ions (Ca 2+ ) act as a second messenger in many pathways Calcium is an important second messenger because cells can regulate its concentration A signal relayed by a signal transduction pathway may trigger an increase in calcium in the cytosol Pathways leading to the release of calcium ALSO involve inositol triphosphate (IP 3 ) and diacylglycerol (DAG) as second messengers

24. ATP EXTRACELLULAR FLUID ATP Mitochondrion Ca 2+ pump Plasma membrane CYTOSOL Endoplasmic reticulum (ER) Ca 2+ pump Ca 2+ pump High [Ca 2+ ] Key Nucleus Low [Ca 2+ ] Calcium is stored In regions of the cell such as the mitochondria and ER It is released when signaled

25. CYTOSOL Ca 2+ Endoplasmic reticulum (ER) IP 3 -gated calcium channel IP 3 (second messenger) DAG PIP 2 G-protein-linked receptor Phospholipase C G protein Signal molecule (first messenger) EXTRACELLULAR FLUID GTP Ca 2+ (second messenger) Various proteins activated Cellular re- sponses A signal from the exterior of the cell triggers Calcium release

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Response (3 rd stage): Cell signaling leads to regulation of cytoplasmic activities or transcription The cell’s response to an extracellular signal is sometimes called the “output response” Ultimately, a signal transduction pathway leads to regulation of one or more cellular activities The response may occur in the cytoplasm or may involve action in the nucleus Many pathways in some way regulate the activity of enzymes – Enzymes can be activated or deactivated by being phosphorylated or dephosphorylated in response to the signal – Also, an enzyme may be signaled to be expressed when it is not already present in a cell

27. Binding of epinephrine to G-protein-linked receptor (1 molecule) Reception Transduction Inactive G protein Active G protein (10 2 molecules) Inactive adenylyl cyclase Active adenylyl cyclase (10 2 ) ATP Cyclic AMP (10 4 ) Inactive protein kinase A Inactive phosphorylase kinase Active protein kinase A (10 4 ) Active phosphorylase kinase (10 5 ) Active glycogen phosphorylase (10 6 ) Inactive glycogen phosphorylase Glycogen Response Glucose-1-phosphate (10 8 molecules) A representative signaling pathway

28. Reception Growth factor Receptor Phosphorylation cascade Transduction CYTOPLASM Inactive transcription factor Active transcription factor P Response Gene mRNA DNA NUCLEUS Many other signaling pathways regulate the synthesis of enzymes or other proteins, usually by turning genes on or off in the nucleus The final activated molecule may function as a transcription factor Cell signaling pathways often control gene expression

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fine-Tuning of the response and signal amplification Multistep pathways have two important benefits: – Amplifying the signal (and thus the response) At each step, the number of activated products is much greater than in the preceding step – Contributing to the specificity of the response Different kinds of cells have different collections of proteins These differences in proteins give each kind of cell specificity in detecting and responding to signals The response of a cell to a signal depends on the cell’s particular collection of proteins Pathway branching and “cross-talk” between multiple signaling pathways further help the cell coordinate incoming signals

30. Signal molecule Receptor Relay molecules Response 1 Response 2Response 3 Cell B. Pathway branches, leading to two responses Cell A. Pathway leads to a single response Cell C. Cross-talk occurs between two pathways Response 4 Response 5 Activation or inhibition Cell D. Different receptor leads to a different response Different signals can lead to different responses in different cell types

31. Signal molecule Plasma membrane Receptor Scaffolding protein Three different protein kinases Signaling Efficiency: Scaffolding Proteins and Signaling Complexes Scaffolding proteins are large relay proteins to which other relay proteins are attached Scaffolding proteins can increase the signal transduction efficiency

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Termination of the Signal Inactivation mechanisms are an essential aspect of cell signaling When signal molecules leave the receptor, the receptor reverts to its inactive state Other signals often lead to de-phosphorylation, and hence inactivation of the proteins along a signaling pathway

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Put these events in the most likely order for signaling from one cell to another a. The receptor tyrosine kinase proteins cross- phosphorylate one another b. An enzyme is activated by phosphorylation to start glycolysis c. The ligand binds to a receptor tyrosine kinase d. An inactive relay protein binds to the receptor tyrosine kinase e. A signaling molecule/ligand is released from one cell f. The receptor tyrosine kinase forms a dimer g. The relay protein becomes activated h. A kinase, or phosphorylation cascade is initiated