Lecture 3.1: G-Protein Coupled Receptors (GPCRs)

Question 1

What is Cell Signalling?

Answer 1

The ability to perceive and respond appropriately to ‘information’ in the environment

Question 2

Principles of Biological Signalling (4 Steps)

Answer 2

1) Reception
2) Transduction
3) Response
4) Amplification

Question 3

What are GPCRs?

Answer 3

Membrane bound receptors that utilise G proteins in the transduction process

Question 4

How many GCPRs does a human have?

Answer 4

800-1000 GPCRs

Question 5

What signals do GCPRs transduce? (4)

Answer 5

• Sensory Information (phototransducers, taste)
• Hormones
• Neurotransmitters
• Growth Factors

Question 6

What processes do GCPRs play mission critical roles in? (7)

Answer 6

• Neurotransmission
• Blood Pressure
• Regulation
• Vascular Function
• Glucose Metabolism
• Immune Response
• Developmental Processes

Question 7

Why are GPCRs important in terms of drugs?

Answer 7

• Have been identified as drug targets
• 1/3 of prescribed drugs act at GPCRs

Question 8

What drugs act at GCPRs? (5)

Answer 8

• Angiotensin receptor blockers
• Bronchodilators
• Anti-histamines for allergy
• H2 receptor antagonists
• GPCRs in oncology as they regulate a number of
  signalling pathways in cancer cells

Question 9

General Structure of GCPRs

Answer 9

• Extracellular N terminus
• Intracellular C terminus
• 7 transmembrane α-helices
• 3 intracellular and 3 extracellular loops

Question 10

Classification of GCPRs: 2 Types

Answer 10

• Class A-F
• GRAFS System

Question 11

Class A of GCPRs

Answer 11

Rhodopsin like (includes neurotransmitters and hormones)

Question 12

Class B of GCPRs

Answer 12

Secretin Receptor Family

Question 13

Class C of GCPRs

Answer 13

Metabotropic Glutamate Receptors

Question 14

Class D of GCPRs

Answer 14

Fungal Mating Pheromone Receptors

Question 15

Class E of GCPRs

Answer 15

cAMP Receptors

Question 16

Class F of GCPRs

Answer 16

Frizzled and Smoothened Receptors

Question 17

GRAFS System

Answer 17

Glutamate
Rhodopsin
Adhesion
Frizzled
Secretin

Question 18

What 3 subunits are G-proteins are made of?

Answer 18

α (alpha)
β (beta)
γ (gamma)

Question 19

How do the α & γ subunits of G-proteins interact with the cell membrane?

Answer 19

They are covalently bound via a range of lipid moieties to the membrane – v.complex!

Question 20

At rest GDP (guanosine diphosphate) binds
to what subunit?

Answer 20

α-subunit

Question 21

When a ligand binds to the receptor GTP
binds to the α-subunit, what does this cause?

Answer 21

Dissociation of the trimer into the α-subunit and a β-γ dimer occurs

Question 22

How many genes code for α?

Answer 22

20

Question 23

How many genes code for β?

Answer 23

6

Question 24

How many genes code for γ?

Answer 24

12

Question 25

How many permutations of G-proteins are possible?

Answer 25

c.1400

Question 26

G-Protein Subtype Gs: Effector? Second Messenger?

Answer 26

Effector: Adenylyl cyclase (+) Second Messenger: cAMP ↑

Question 27

G-Protein Subtype Gi: Effector? Second Messenger? (3 of each)

Answer 27

Effector: * Adenylyl cyclase (-) * K+ channel(+) * VOCC Ca2+ channel(-) Second Messenger: * cAMP↓ * K+↓ * Ca2+ ↓

Question 28

G-Protein Subtype Gq: Effector? Second Messenger?

Answer 28

Effector: Phospholipase C (+) Second Messenger: DAG & IP3 → Ca2+↑

Question 29

G-Protein Subtype Gt: Effector? Second Messenger?

Answer 29

Effector: cGMP phosphodiesterase (+) Second Messenger: cGMP ↓

Question 30

G-Protein Subtype Golf: Effector? Second Messenger?

Answer 30

Effector: Adenylyl cyclase (+) Second Messenger: cAMP ↑

Question 31

What is another name for the G-Protein Subtype Gt? Why is it important?

Answer 31

* Also called transducin * Linked to rhodopsin i.e. it transducers photons to action potentials

Question 32

What is another name for the G-Protein Subtype Golf? Why is it important?

Answer 32

* Also gustducin * Involved in the transduction of taste (bitter, sweet and umami)

Question 33

What are protein kinases?

Answer 33

Enzymes that when activated phosphorylate substrates

Question 34

What does PKA (Protein Kinase A) target? (3)

Answer 34

* Phosphorylates phosphorylase kinase which in turn phosphorylates glycogen phosphorylase leading to glycogen breakdown * Phosphorylation of Cl- channels in small intestine (Cl- enters lumen, Na+ enters lumen = H2O entering n lumen) * Phosphorylation of transcription factors (e.g. CREB), that when phosphorylated are able to initiative transcription)

Question 35

How does adrenaline mobilise glycogen breakdown in muscle cells? (6 Steps)

Answer 35

* Chemical Signal: Adrenaline * Receptor Activation: β-adrenoreceptor * Signal Transduction: Dissociation of G-Protien, Activation of adenylyl cyclase * Message Generation: ↑ cyclic AMP * Enzyme Activation: Activation of PKA, Phosphorylation of glycogen phosphorylase * Biological Response: Glycogen breakdown

Question 36

Why are Phosphodiesterases important?

Answer 36

* They promote the conversion of cAMP → AMP (analogous for cGMP) * Their role in ‘switching off’ signalling pathways offers the potential for targeting them with drugs

Question 37

Theophylline can inhibit type IV Phosphodiesterases, what effect does this have?

Answer 37

Airway relaxation

Question 38

Sildenafil (Viagra) inhibits type ..... Phosphodiesterases, & therefore promotes the vasodilatory effects of cGMP?

Answer 38

V

Question 39

Activation of a muscarinic receptor by acetylcholine

Answer 39

Slide 22

Question 40

What is signal amplification?

Answer 40

A few molecules of an agonist may evoke significant intracellular responses

Question 41

Monomeric G-Proteins

Answer 41

Often called Small GTPases (to distinguish them from GPCRs) and consist of a single unit (~Gα subunit)

Question 42

What do Monomeric G-Proteins bind at rest?

Answer 42

At rest bind GDP, free in cytoplasm

Question 43

How do monomeric G-proteins work? When are they active?

Answer 43

Only active when GTP bound (therefore inactivated by GAPs and activated by GEFs)

Question 44

Ras the monomeric G-protein, why is it important?

Answer 44

* Important role (via PKs) in cytoskeletal reorganisation, cell polarity, cell cycle progression, gene expression * Mutations in Ras genes associated with some 30% of tumours * Three types of Ras in humans considered to be oncogenes * Quite often mutations occur in the GTPase region – G- protein always on (development of GAPs as tumour suppressors?)

Question 45

The process of G-protein insertion and G-Protein degradation is known as...?

Answer 45

G-protein trafficking

Question 46

What are GPCRs regulated by?

Answer 46

G-protein receptor kinases (GRKs)

Question 47

How do GRKs regulate GCPRs?

Answer 47

* Phosphorylation of activated (ligand bound) GPCRs by GRKs allows the binding of β-arrestin proteins * Β-arrestins act as scaffolds for endocytosis leads to transient receptor internalisation