MICR 304 Immunology & Serology
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Transcript of MICR 304 Immunology & Serology
MICR 304 Immunology &
Serology
MICR 304 Immunology &
Serology
Lecture 7A Signal Transduction
Chapter 6.1 –6.9, 6.12-6.16
Lecture 7A Signal Transduction
Chapter 6.1 –6.9, 6.12-6.16
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
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
…CTWAEPYCH….
P
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
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
Receptor Clustering
Membrane is rich in cholesterol and saturated phospholipids
• Kinase activation upon receptor clustering• Occurs in special membrane region called lipid rafts
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
Propagation of Signal through Scaffolds
• Larger unphosphorylated proteins• Tyrosine phosphorylation on multiple sites• Recruit many different proteins
Scaffolds become tyrosine phosphorylated.
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
PYXXZ
Example for an Adaptor Protein
Adaptor proteins bind to phosphorylated proteins.
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
Means of Signaling Pathway Amplification
Activation ofkinase cascade
Increase of intracellular Ca2+
Activation of downstream signaling molecules such as calmodulin
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
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– PLC, GEF
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)
• IP3 increases 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
Calcium Release Activated Calcium channels
Antigen Receptor
PLC
PIP2
DAG IP3
Protein Kinase CSmall G proteinMAP Kinases
Intracellular Ca2+Calcineurin
AP-1NFkB NFAT
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)
Reversible Action of Small G-Proteins
Inactive(Off)
Active(On)
Inactive(Off)
Activation of MAP kinases
MAP Kinase Activation Pathway
MAP Kinase
Activation of Transcription Factors
Turning Off Signaling• Dephsophorylation• Ubiquitination
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.