Medical Biochemistry Membranes: Membrane receptors; G-proteins Lecture 73 Membranes: Membrane receptors; G-proteins Lecture 73

Hormone Receptors All receptors have at least two functional domains –Recognition domain binds hormone –Second regions generates signal to some intracellular function

Two general groups of hormones Signal transduction occurs in two general ways –Polypeptide hormones, catecholamines bind receptors in plasma membrane, generates signal that regulates intracellular function (often changing enzyme activity) –Steroid, thyroid hormones bind intracellular receptors, complex provides the signal

Group I Hormones Have intracellular receptors Affect gene expression –activates or inactivates specific gene expression At least two control sites –PE - controls transcription initiation –HRE - modulate initiation rate (functions like enhancer)

Group II Hormones Largest group of hormones Have membrane receptors Use intracellular messengers –cAMP –cGMP –calcium or phoshatidylinositols –protein kinase cascade

cAMP as Second Messenger Intracellular [cAMP] increased or decreased by various hormones Effect varies by tissue cAMP derived from ATP by adenylyl cyclase can terminate signal by cAMP hydrolysis by phosphodiesterase –inhibited by caffeine (mimics or prolongs action of hormones)

Adenylyl cyclase system Receptors that couple to effectors through G protein typically have 7 membrane-spanning domains Adenylyl cyclase (AC) regulated by G s (stimulatory) and G i (inhibitory) complexes

Bacterial toxins irreversibly activate adenylyl cyclase Cholera toxin inactivates G  s GTPase activity, activates AC Pertussis toxin prevents G  i from being activated, activates AC

Superfamily of GTPases Classified into four subfamilies Some  i stimulate K + channels, inhibit Ca 2+ channels, some  s have opposite effects G q family members activate phospholipase C GsGs GiGi GqGq G 12

cAMP-dependent protein kinase cAMP binds to inactive, heterotetrameric protein kinase cAMP-regulatory subunits dissociate from catalytic subunits that can phosphorylate and activate protein substrates (e.g., cAMP-response element binding protein - CREB) gives rise to diverse biological responses (e.g., induction of glycogen breakdown in muscle cells by epinephrine) Animation: Extracellular_signaling.mov hormonal control of phosphoprotein phosphatases (dephosphorylation)

cGMP as Second Messenger Guanylyl cyclase forms cGMP from GTP Atriopeptins (e.g., atrial natriuretic factor - ANF) in cardiac atrial tissues cause natriuresis, diuresis, vasodilation, and inhibition of aldosterone secretion Nitric oxide (NO) binds soluble guanylyl cyclase, increase cGMP, activates cGMP-dependent protein kinase, phosphorylates smooth muscle proteins  vasodilation –inhibitors of cGMP phosphodiesterase enhance and prolong responses (Viagra)

Hormones act through calcium Ionized calcium regulates muscle contraction, stimulus-secretion coupling, blood clotting cascade, enzyme activity, and membrane excitability Three ways of changing cytosolic [Ca 2+ ] –hormones that enhance membrane permeability to Ca 2+ (e.g., acetylcholine) using Na + -Ca 2+ exchange –Ca 2+ -2H + ATPase-dependent pump that extrudes Ca 2+ in exchange for H + –Ca 2+ can be mobilized from mitochondrial and ER pools

Calmodulin Calcium-dependent regulatory protein –Four Ca 2+ binding sites, binding leads to conformational change –Ca 2+ -calmodulin can activate or inactivate enzymes (analogous to binding of cAMP to protein kinase)

Phosphotidylinositol metabolism Binding of hormones (e.g., acetylcholine, antidiuretic hormone) to receptors coupled to G q leads to activation of phospholipase C (PLC) Catalyzes hydrolysis of PIP 2  IP 3 + diacylglycerol (DAG) IP 3 binds intracellular receptor, releases Ca 2+ from sarcoplasmic reticulum and mitochondria

Phosphotidylinositol metabolism DAG (plus free calcium) activates protein kinase C (PKC) both activated PKC and Ca 2+ -calmodulin dependent protein kinase can phosphorylate and activate specific substrates

Receptor tyrosine kinase (RTK) cascade Several receptors involved in growth control and differentiation have intrinsic tyrosine kinase activity (e.g., insulin, EGF) Binding ligand to receptor leads to receptor phosphorylation and activation of a cascade of protein kinases phosphorylation of transcription factors activates (inactivates) gene transcription

Non-receptor tyrosine kinase Hormone-receptor interaction (e.g., growth hormone, cytokines) activates cytoplasmic tyrosine kinase (e.g., JAK1) Tyrosine kinases phosphorylate proteins that dock with other proteins via SH2 domains (bind to phosphotyrosines) STAT binds phosphorylated receptor, becomes phosphorylated, dimerizes, translocates to nucleus, binds specific DNA elements, regulates transcription

Signaling crosstalk and convergence The same cellular responses (e.g., glycogen breakdown) may be induced by multiple signaling pathways Many RTKs and GCPRs activate multiple signaling pathways, and different second messengers sometimes mediate the same cellular response Interaction of different signaling pathways permits fine-tuning of cellular activities