Last lecture: reversible phosphorylation regulation of transcription This lecture: signal transduction

Fridays MC H313 Biological Sciences Seminar Series This Friday (19th): The smallest hormone: birth, life and many deaths of nitric oxide

Intracellular signal transduction: receptors or sensors second messengers kinases, phosphatases, transcription factors downstream targets (proteins)

Intracellular Signal Transduction Hormone receptors within the cell

Intracellular receptors: - cytosolic - nuclear

T4 T3

Intracellular Signal Transduction Hormone receptors within the cell Hormone receptors on cell membrane (facing external environment)

2. Membrane receptors: - laterally mobile - interact with other membrane proteins - catalytic

Intracellular Signal Transduction Hormone receptors within the cell Hormone receptors on cell membrane (facing external environment) Signaling of entirely intracellular events

3. Detection & signaling of entirely intracellular events

ATP synthetic processes ATP ADP AMP ↑work → ↑[AMP] AMP kinase ATP synthetic processes

Intracellular Signal Transduction Hormone receptors within the cell Hormone receptors on cell membrane (facing external environment) Signaling of entirely intracellular events

Roles of signal transduction?

Roles of signal transduction (1) Permit adaptation (2) Regulate response (3) Coordinate related events

Features of signal transduction 2. Amplification 3. Diversification 4. Transience

Features of signal transduction 1. Transduction - conversion of one form to another 2. Amplification 3. Diversification 4. Transience

Features of signal transduction 1. Transduction - conversion of one form to another 2. Amplification - one receptor/binding event can affect big changes 3. Diversification 4. Transience

Features of signal transduction 1. Transduction - conversion of one form to another 2. Amplification - one receptor/binding event can affect big changes 3. Diversification - may affect related but different targets 4. Transience -

Features of signal transduction 1. Transduction - conversion of one form to another 2. Amplification - one receptor/binding event can affect big changes 3. Diversification - may affect related but different targets 4. Transience - can be turned off

An example of signal transduction: Receptors & G-proteins

G-protein activated by binding GTP

1. Receptors that activate G-proteins Largest family of receptors on cell surface (>1000 members) Egs. epinephrine (adrenergic) receptor Heterotrimers (one each of α, β, γ subunits) Interact with a variety of effector molecules Depend on lateral mobility of proteins in membrane

Eg. G-protein acting via adenylyl cyclase (AC)

In the cytosol:

Mobilization of glucose subunits from glycogen: Hormone + receptor → G-protein → adenylyl cyclase → cAMP↑ → PKA → phosphoprotein kinase → glycogen phosphorylase

G-proteins associated with phospholipase C and protein kinase C

G-proteins can: Be stimulatory or inhibitory (eg. differential response to epinephrine) Have different actions; depends on: - particular combination of subunits (16 possible α isoforms; 5β; 11γ ) - presence of effectors nearby (egs. AC, phosphodiesterase, phospholipase) Affect changes in cAMP, Ca2+, diacylglycerol

Receptors that are enzymes Tyrosine kinases Guanylyl cyclases Catalyze cleavage (proteolysis) reactions (these may release an active transcription factor)

Example 1: the insulin receptor Heterotetramer (2α and 2β subunits) β subunit is tyrosine kinase (autophosphorylates tyrosines on receptor, then on other proteins) Active IR stimulates PI 3-kinase activity GLUT4 protein recruited to membrane from intracellular vesicles

Example 2: Nitric oxide receptor Has guanylyl cyclase activity: GTP → cGMP

Protein regulation and signal transduction - summary and overview Many strategies for altering specific and cellular activities Fast vs slow responses Specific activity vs amount of a protein Protein kinases/phosphatases Many transcription factors and HREs Receptors: membrane, cytosol, nucleus Receptor – effector mechanisms Second messengers: signal amplification and/or diversification (egs. Ca++, cAMP, cGMP)

Read chapter 3 for Thursday