Cellular Signal Transduction

1. Cellular Signal Transduction

2. Organisms can function normally only if: • All the cells of their different organs communicating effectively with their surroundings. • Once a cell picks up a hormonal or sensory signal, it must transmit this information from the surface to the interior parts of the cell— Example, to the nucleus. • This occurs via signal transduction pathways that are: very specific, both in their activation and in their downstream actions. • Thus, the various organs in the body respond in an appropriate manner and only to relevant signals.

3. Signal Transduction The transmission of molecular signals from a cell's exterior to its interior. Molecular signals transmitted between cells by: Hormones Other chemical factors 3. Sensory signals received from the environment, in form of: Light, Taste, Sound, Smell, and Touch.

6. Upon Environment changes: Monad: responds directly Multicellular organisms: Respond with signal through a system of intercellular or intracellular communication，and consequently regulate functions of organisms.

7. Signal transduction

9. 1. Signaling Molecules

10. Signaling molecules • Released by signal-producing cells, • Transfer biological signals to their target cells to initiate specific cellular responses.

11. Two types.. • Extracellular molecules • Intracellular molecules

12. 1. Extracellular molecules protein & peptides: Hormone, cytokine AA & its derivatives: Gly, Glu, adrenaline, thyroxine Steroid: Sex Hormone, glucocorticosteroid Fatty acid derivatives: prostaglandin

13. (1) Paracrine signaling (local chemical mediators) • Secreted by common cells. • Reach neighboring target cells by passive diffusion. • Time of action is short. • Such as GF

14. (2) Endocrine signal • Secreted by endocrine cells. • Reach target cells by blood circulation. • Time of action is long. • Such as insulin, thyroxine, adrenalin

15. (3) Synaptic signal (neurotransmitters) • Secreted by neuronal cells. • Reach another neuron by synaptic gap. • Time of action is short. • Such as Acetylcholine (Ach), noradrenaline

16. (4) Gaseous signal • Simple structure, half life is short and active in chemistry . • Such as NO, CO.

17. GAS MOLECULE

18. (5) Autocrine signal • Act back to their own cells. • Such as GF, cytokine, interferon, interleukin.

19. 2.Intracellular molecule • Ca2+ ions • DG, ceramide lipid derivatives • IP3 carbohydrate derivatives • cAMPcGMP nucleotides • Ras, JAK, Raf proteins

20. Second messenger: Small molecules synthesized in cells in response to an external signal, responsible for intracellular signal transduction. Such as Ca2+, DG, Cer, IP3,cAMP, cGMP

21. Third messengers: The molecules which transmit message from outside to inside of nucleous or from inside to outside of nucleous, also called DNA binding protein.

22. Proteins and peptides: Hormones, cytokines Effect by membrane receptors Amino acid derivatives: Catecholamines Fatty acid derivatives: Extracellular molecules Prostaglandins Effect by intracellular receptors Signal molecules Steroid hormones, Thyroxine, VD3 Intracellular molecules cAMP, cGMP, IP3, DG, Ca2+

23. 2. Receptors

24. Receptors Specific membrane proteins, able to recognize and bind to corresponding ligand molecules, become activated, and transduce signal to next signaling molecules. Glycoprotein or Lipoprotein

25. ligand A small molecule that binds specifically to a larger one; Example: A hormone is the ligand for its specific protein receptor.

26. Membrane receptors membrane Glycoprotein • Intracellular receptors Cytosolor nuclei DNA binding protein

27. 1. membrane receptors (1) Ligand-gate ion channels type (cyclic receptor) ligand→receptor→ion channel open or close

31. Properties of binding of H and R • highly specificity • highly affinity • saturation • reversible binding • special function model

32. Control of receptor activity • Phosphorylation or dephosphorylation of R • Phospholipid of membrane • Enzyme catalyzed hydrolysis • G protein regulation

33. Function of receptor (1) Recognize the special ligand (2) Binding to special ligand (3) Signal transduction biological effect

34. Pathway of Signal Transduction

35. Many enzymes are regulated by covalent attachment of phosphate, in ester linkage, to the side-chain hydroxyl group of a particular amino acid residue (serine, threonine, or tyrosine).

36. A protein kinasetransfers the terminal phosphate of ATP to a hydroxyl group on a protein. • A protein phosphatase catalyzes removal of the Pi by hydrolysis.

37. Phosphorylation may directly alter activity of an enzyme, e.g., by promoting a conformational change. Alternatively, altered activity may result from binding another protein that specifically recognizes a phosphorylated domain. • E.g., 14-3-3 proteins bind to domains that include phosphorylated Ser or Thr in the sequence RXXX[pS/pT]XP, where X can be different amino acids. • Binding to 14-3-3 is a mechanism by which some proteins (e.g., transcription factors) may be retained in the cytosol, & prevented from entering the nucleus.

38. Protein kinases and phosphatases are themselves regulated by complex signal cascades. For example: • Some protein kinases are activated by Ca++-calmodulin. • Protein Kinase A is activated by cyclic-AMP (cAMP).

39. Adenylate Cyclase (Adenylyl Cyclase) catalyzes:  ATPàcAMP + PPi Binding of certain hormones (e.g., epinephrine) to the outer surface of a cell activates Adenylate Cyclase to form cAMP within the cell. Cyclic AMP is thus considered to be a second messenger.

40. Phosphodiesteraseenzymes catalyze: cAMP + H2OAMP The phosphodiesterase that cleaves cAMP is activated by phosphorylation catalyzed by Protein Kinase A. Thus cAMP stimulates its own degradation, leading to rapid turnoff of a cAMP signal.

41. Protein Kinase A (cAMP-Dependent Protein Kinase) transfers Pi from ATP to OH of a Ser or Thr in a particular 5-amino acid sequence. Protein Kinase A in the resting stateis a complex of: • 2 catalytic subunits(C) • 2 regulatory subunits(R). R2C2

42. R2C2 Each regulatory subunit (R) of Protein Kinase Acontains a pseudosubstrate sequence, like the substrate domain of a target protein but with Ala substituting for the Ser/Thr. The pseudosubstrate domain of (R), which lacks a hydroxyl that can be phosphorylated, binds to the active site of (C), blocking its activity.

43. R2C2 + 4 cAMPR2cAMP4 + 2C When each (R) binds 2 cAMP, a conformational change causes (R) to release (C). The catalytic subunits can then catalyze phosphorylation of Ser or Thr on target proteins. PKIs, Protein Kinase Inhibitors, modulate activity of the catalytic subunits (C).

44. Pathway of G protein linked receptor H R G protein Es secondary messeger Protein kinase Phophorylation of Es or functional protein Biological effect

45. G Protein Signal Cascade Mostsignal molecules targeted to a cell bind at the cell surface to receptors embedded in the plasma membrane. Only signal molecules able to cross the plasma membrane (e.g., steroid hormones) interact with intracellular receptors. A large family of cell surface receptors have a common structural motif, 7 transmembranea-helices. Rhodopsin was the first of these to have its 7-helix structure confirmed by X-ray crystallography.

46. Rhodopsin is unique. It senses light, via a bound chromophore, retinal. • Most7-helix receptors have domains facing the extracellular side of the plasma membrane that recognize & bind signal molecules (ligands). E.g., the b-adrenergic receptor is activated by epinephrine & norepinephrine. Crystallization of this receptor was aided by genetically engineering insertion of the soluble enzyme lysozyme into a cytosolic loop between transmembranea-helices.

47. The signal is usually passed from a 7-helix receptor to an intracellular G-protein. • Seven-helix receptors are thus called GPCR, or G-Protein-Coupled Receptors. • Approx. 800 different GPCRs are encoded in the human genome.

48. G-protein-Coupled Receptors may dimerize or form oligomeric complexes within the membrane. Ligand binding may promote oligomerization, which may in turn affect activity of the receptor. Various GPCR-interacting proteins (GIPs) modulate receptor function. Effects of GIPs may include: • altered ligand affinity • receptor dimerization or oligomerization • control of receptor localization, including transfer to or removal from the plasma membrane • promoting close association with other signal proteins

49. G-proteins are heterotrimeric, with 3 subunits a, b, g. • A G-protein that activates cyclic-AMP formation within a cell is called a stimulatory G-protein, designated Gs with alpha subunit Gsa. • Gs is activated, e.g., by receptors for the hormones epinephrine and glucagon. The b-adrenergic receptor is the GPCR for epinephrine.

50. The asubunit of a G-protein (Ga) binds GTP, & can hydrolyze it to GDP + Pi. a & g subunits have covalently attached lipid anchors that bind a G-protein to the plasma membrane cytosolic surface. Adenylate Cyclase (AC) is a transmembrane protein, with cytosolic domains forming the catalytic site.