MICR 304 Immunology & Serology

1. MICR 304 Immunology & Serology Lecture 7A Signal Transduction Chapter 6.1 –6.9, 6.12-6.16

2. Overview of Today’s Lecture • Principles of signal transduction • Structure of signal transducing receptors • Membrane properties in regions with signal transduction • Major signal transducing pathways

3. Signal Transduction • Transmembrane receptors convert extracellular signals to intracellular biochemical events • Surface receptors with signaling functions are either transmembrane proteins or associate with transmembrane proteins • Ligand binding and receptor clustering trigger enzymatic activity • Protein kinases are most commonly activated P …CTWAEPYCH….

4. Protein Phosphorylation • Addition of phosphate group to an amino acid by kinases • Tyrosine in early activation events • Serine and threonine in later events • Kinase can be part of receptor or can be associated with receptor • Enzyme activation • Creation of binding sites for other proteins (adaptor proteins) • Quick • Reversible through phosphatases

5. Kinase Activity of Signal Transducing Receptors Kinase domain is intrinsic part of receptor e.g., many growth factors Kinase is non-covalently associated with receptor e.g., antigen receptor, many cytokine receptors

6. Receptor Clustering • Kinase activation upon receptor clustering • Occurs in special membrane region called lipid rafts Membrane is rich in cholesterol and saturated phospholipids

7. Propagation of Signals by Assembly of Signaling Complexes • Creates large multiprotein signaling complexes • Involves protein interaction domains • Scaffold and adaptor proteins are used • Do not have enzymatic activity • Recruit other proteins to a signaling complex

8. Propagation of Signal through Scaffolds • Larger unphosphorylated proteins • Tyrosine phosphorylation on multiple sites • Recruit many different proteins Scaffolds become tyrosine phosphorylated.

9. Propagation of Signal throughAdaptor Proteins • Smaller proteins • Link two signaling proteins together • Bind to phosphorylated tyrosin • 2 or 3 functional domains • SH2 domain is one of most important domains • Src homology 2 domain • Binds to phosphotyrosine in a sequence specific fashion • SH3 domain binds to proline-rich motifs P YXXZ

10. Example for an Adaptor Protein Adaptor proteins bind to phosphorylated proteins.

11. Signaling Pathways Amplify the Initial Signal • Activation of enzymes • Generation of small biochemical mediators known as second messengers • One activated enzyme can produce hundreds of second messengers

12. Means of Signaling Pathway Amplification Increase of intracellular Ca2+ Activation of downstream signaling molecules such as calmodulin Activation of kinase cascade

13. Selected Common Signal Transduction Pathways • Common in many cell types (not only lymphocytes) • Triggered by various stimuli through specific receptors • Examples • Src (sarc) tyrosine kinase family • Phospholipase C/protein kinase C amplification • Small G-proteins Activation of transcription factors

14. The src - Family of Tyrosine Kinases • Receptor associated tyrosine kinases • Initial activation event • Add phosphate group to tyrosine residues • Src from “sarcoma” • Oncogene • Viral gene that induces tumors • Host has related genes involved in cell growth • Mutated genes in tumors • Activate other signal transducing molecules • PLCg, GEF

15. Amplification of Signal via PLC/PKC • Phospholipase C becomes phosphorylated • Catalyzes the break down of the membrane lipid phosphatidylinositol biphosphate (PIP2) to inositol triphosphate (IP3, second messenger) and diacylglocerol (DAG, membrane lipid) • IP3increases intracellular calcium • DAG and Ca2+activate protein kinase C (serine/threonine kinase) • Activation of three different transcription factors • NFkB (nuclear factor kappa B) • NFAT (nuclear factor of activated T cells) • AP-1

16. Calcium Release Activated Calcium channels

17. Small G-Protein Cascade • Small GTP binding proteins • E.g. : Ras • Oncogene in tumor inducing viruses • Related genes in all eukaryotic cells • Mutant forms found in many tumors • Are activated by GEF (Guanine-nucleotide Exchange Factor) • Key components of several signaling pathways • MAP kinase pathway (Mitogen Activated Protein kinases)

18. Reversible Action of Small G-Proteins Inactive (Off) Active (On) Inactive (Off) Activation of MAP kinases

19. MAP Kinase Activation Pathway MAP Kinase Activation of Transcription Factors

20. Turning Off Signaling • Dephsophorylation • Ubiquitination

21. Today’s Take Home Message • Signal transducing events commonly involve phosphorylation cascades. • Tyrosine kinases are important in early steps of cell activation, serine and threonine kinases are important in later steps of activation. • Src kinase, small G proteins and MAP Kinases, and phospholipase C/Protein kinase C are common signal tranducing pathways. • Amplification of the signals occurs through increase of intracellular calcium concentration.