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.