Signal transduction pathways Ligand binds to specific receptor Pathways relay information from plasma membrane to inter

1. Signal transduction pathways Ligand binds to specific receptor Pathways relay information from plasma membrane to interior of cell Changes are initiated along the way; effect on cell behavior and/or gene expression protein phosphorylation second messengers

2. General description of pathways Interactions with specific receptors Triggering events

3. Ion channels Tyrosine kinase pathways G protein-linked receptors Intracellular receptors

4. Ion channels Salty taste Sour taste (acids) Calcium channels Often provides electrical stimulation of a neuron

5. Pathways pass on information through binding events bindingchange in protein conformation  activation Phosphorylation is often the triggering event Kinases: enzymes that catalyze this

6. Tyrosine kinases phosphorylate themselves Most kinases phosphorylate other proteins, on serine or threonine (OH groups)

8. Proteins are activated by kinase activity Inactivated by protein phosphatases Many different kinases, each with a different substrate specificity If kinases are defective in some way they may not be regulated properly

9. Example: src A normal (cellular) form (c-src)and an oncogenic form (v-src) of the genes are known c-Src contains a tyrosine residue that, when phosphorylated, INACTIVATES the protein v-Src is missing this residue, so it is stuck in the “on” position Cell growth is uncontrolled, leads to cancer (oncogene)

10. Protein kinase defects are associated with several types of cancer Kinase inhibitors may control growth of these cells Example: Gleevec inhibits Bcr-Abl kinase (aberrant kinase that causes a form of leukemia) Since inhibitor is specific for that kinase, it can target cancer cells with few side effects

11. Second messengers small, water-soluble molecules that diffuse rapidly through cells cyclic AMP calcium are especially common; these can interact with many different molecules

12. Where is adenylyl cyclase?

13. Note G protein

14. cAMP is quickly inactivated by phospho- diesterase Primary target of cAMP is protein kinase A (PKA) PKA has 4 subunits: 2 regulatory, 2 catalytic cAMP binds to regulatory subunits catalytic subunits now detach and are activated

15. Some effects of cAMP (cell-specific) In skeletal muscle and liverglycogenolysis Cardiac muscle- strengthens muscle contraction Smooth muscle- inhibits contraction Intestinal epithelium- movement of salt and water into gut

16. Why different effects? Some G proteins stimulate signal transduction (Gs; s stands for stimulatory) Some inhibit signal transduction (Gi) There are many different kinds of G proteins some have 3 subunits, some only one Distributed differently on different cells

17. What sorts of binding events affect cAMP levels? Epinephrine- glycogen metabolism break down glycogen; inhibit glycogen formation Serotonin- enhances synaptic transmission (via PKA activation) Gene expression (PKA-activated protein actually binds to DNA and promotes transcription)

18. Some compounds can affect this pathway without ligand/G-protein interaction Caffeine inhibits phosphodiesterase (to what effect?) Theophylline- same effect; smooth muscle relaxant

19. Disruption of G protein signaling and disease A tale of two toxins Cholera toxin modifies Gs so that it cannot be inactivated Pertussis toxin modifies Gi so it cannot inhibit adenylyl cyclase Same effect, two different mechanisms, different target cells

20. One response to cAMP-mediated signaling

21. Calcium ions and their associates Neurotransmitters Growth factors Some hormones Increase in cytoplasmic concentration of calcium contributes to: Muscle cell contraction Neurotransmitter release Antibody production Insulin secretion Among others

22. Both G-protein and tyrosine kinase pathways can utilize calcium as a second messenger Calcium concentrations in cytoplasm are normally very low Where does the calcium come from? from the extracellular environment from the ER (calcium is exported by calcium pumps within cell, to keep internal calcium levels low)

24. How is calcium released from internal stores? Phospholipase C is activated (different forms are activated by receptor tyrosine kinases than by G protein-linked receptors) It cleaves a membrane phospholipid (PIP2) to form diacylglycerol (DAG) and inositol trisphosphate (IP3)

26. Both DAG and IP3 act as second messengers in many types of cells Discovered in insect salivary glands (early 1980s) Other functions: platelet activation muscle contraction antibody secretion Among others What do these second messengers do?

27. DAG activates protein kinase C (PKC) phosphorylates many target proteins, at serines or threonines PKC activation (you guessed it) does many things in cells, including: Cell growth Activation of ion channels Protein secretion Through its ability to phosphorylate kinases associated with growth

28. What about IP3? Ca++ levels are now high enough to bind calmodulin

29. Many cellular proteins have binding sites for calcium-calmodulin complex Actual identities of these proteins varies within cells Tend to be kinases and phosphatases When calcium levels drop, calmodulin releases calcium

30. Outcomes of cell signaling Enzyme activation Amplification Transcription and translation of specific gene(s)

31. Enzyme activation; amplification

32. (why is there gene expression in response to this ligand?

33. How can cells respond differently to same stimulus Own set of receptors (with characteristic tyrosine kinases, G proteins, etc.) Own set of relay and functional proteins