LG - Advanced Studies in GPCRs II

Question 1

Q: How are Gα proteins classified and what is their role? (3)

Answer 1

• 21 human Gα subunits grouped into 4 families: Gαs, Gαi, Gαq, Gα12/13
• Each activates distinct signalling pathways
• Used in deorphanization assays to match GPCRs with ligands

Question 2

Q: What are the functions of the Gαs subunit? (2)

Answer 2

• Stimulates adenylyl cyclase → increases cAMP
• Activates PKA, enhancing metabolism and growth-related processes

Question 3

Q: What are the functions of the Gαi subunit? (3)

Answer 3

• Inhibits adenylyl cyclase → decreases cAMP
• Opposes Gαs signalling
• Inhibited by pertussis toxin (PTX), which causes cAMP accumulation

Question 4

Q: What are the functions of the Gαq subunit? (3)

Answer 4

• Activates phospholipase C-β (PLCβ)
• Produces IP3 and DAG → IP3 releases Ca²⁺, DAG activates PKC
• Mediates Ca²⁺ signalling and PKC activation

Question 4

Q: What are the functions of the Gα12/13 subunits? (2)

Answer 4

• Activate RhoGEFs → stimulate RhoA
• Regulates cytoskeleton dynamics and cell motility

Question 5

Q: What are the roles of Gβ proteins in signalling? (3)

Answer 5

A:

• 5 isoforms (Gβ1–Gβ5), tissue-specific expression
• Must pair with Gγ to function (stable dimer)
• Dysregulation linked to cancer, neurodegeneration, immune dysfunction

Question 5

Q: What are the functions of Gγ proteins in GPCR signalling? (3)

Answer 5

• Required for Gβγ dimer formation
• Effector modulation: regulates GIRK, PLCβ, and PI3K
• Prenylation anchors Gγ to the membrane; isoforms (Gγ1–Gγ13) impact signalling specificity

Question 6

Q: What are GRKs (G-protein-coupled receptor kinases) and their types? (3)

Answer 6

• Phosphorylate activated GPCRs to promote desensitization
• Also modulate non-GPCR signalling via non-phosphorylation mechanisms
• Subtypes:
  • GRK1 & GRK7: visual receptors (e.g. rhodopsin)
  • GRK2 & GRK3: activated only by active GPCRs
  • GRK4, GRK5 & GRK6: can phosphorylate both active and inactive GPCRs

Question 7

Q: What are non-canonical functions of GRKs? (2)

Answer 7

• Phosphorylate non-GPCR substrates
• Implicated in diseases like cardiac hypertrophy, neurodegeneration, and cancer

Question 8

Q: What are the roles of arrestins in GPCR signalling? (3)

Answer 8

• Bind active, phosphorylated GPCRs → block G-protein interaction
• Two types:
  • Visual arrestins (Arrestin-1, -4)
  • Non-visual arrestins (Arrestin-2, -3)
• Originally known for desensitisation, now known to mediate alternative pathways

Question 9

Q: What are the GPCR-independent signalling roles of arrestins? (3)

Answer 9

• Mediate GPCR internalisation via clathrin/AP2
• Scaffold components of MAP kinase cascades
• Participate in β-arrestin-dependent signalling pathways

Question 10

Q: What are three key arrestin-mediated signalling cascades? (3)

Answer 10

• ERK1/2 (via Src): Luttrell et al., 1999
• JNK3 (ASK1-MKK4/7-JNK3): McDonald et al., 2000
• c-Raf1–MEK1–ERK1/2: Luttrell et al., 2001

Question 11

Q: What are the key takeaways of GPCR signalling from this lecture? (4)

Answer 11

Gα subunits control major signalling axes:

• Gαs → cAMP↑
• Gαi → cAMP↓
• Gαq → PLCβ → IP3/DAG
• Gα12/13 → RhoA → cytoskeleton

GRKs regulate receptor desensitisation and alternative signalling

Arrestins act as terminators and scaffolds

GPCR signalling is modular and versatile, crucial for cell-specific responses