GPCR Signaling Notes

GPCR Signaling – Comprehensive Notes

• GPCR structure and initiation of signaling

  • Many G protein-coupled receptors have a large extracellular ligand-binding domain.
  • When an appropriate protein ligand binds to this domain, the receptor undergoes a conformational change that is transmitted to its cytosolic regions, which now activate a trimeric G protein (G protein).
  • The G protein is composed of three subunits: alpha (Gα), beta (Gβ), and gamma (Gγ).
  • Both the Gα and Gγ subunits have covalently attached lipid tails that help anchor the G protein in the plasma membrane.
  • In the absence of a signal, the Gα subunit binds GDP and the G protein is inactive.

• Receptor–G protein association states

  • In some cases, the inactive G protein is pre-associated with the inactive receptor; in other cases, the G protein binds only after receptor activation.
  • In either scenario, an activated receptor induces a conformational change in the Gα subunit causing GDP to dissociate.
  • GDP is released and GTP, which is abundant in the cytosol, binds in place of GDP.

• G protein activation and dissociation patterns

  • GTP binding causes a further conformational change in the G protein, activating both the Gα subunit and the Gβγ dimer.
  • Depending on the specific G protein, the activated Gα subunit may dissociate from the activated Gβγ complex, or the two activated components may stay together.

• Downstream signaling by activated G proteins

  • Both activated components (Gα-GTP and/or Gβγ) can regulate the activity of target proteins in the plasma membrane.
  • The activated target proteins relay the signal to downstream components in the signaling cascade.

• Termination of G protein signaling

  • The Gα subunit hydrolyzes its bound GTP to GDP, inactivating the subunit.
  • This GTPase activity is often accelerated by a regulator of G protein signaling (RGS).
  • The now GDP-bound Gα subunit re-associates with the Gβγ dimer to reform an inactive heterotrimeric G protein, turning off downstream events.
  • Mathematical note (conceptual): the activation cycle can be viewed as
    {G_ ext{α}- ext{GTP}}
    ightarrow {G_ ext{α}- ext{GDP}} + P_i,
    with RGS accelerating the rate of this hydrolysis.

• Persistent receptor stimulation and desensitization

  • As long as the signaling receptor remains stimulated, it can continue to activate G proteins.
  • Upon prolonged stimulation, receptors themselves become desensitized and inactivate even if the activating ligands remain bound.

• Desensitization via receptor phosphorylation and arrestin

  • A receptor kinase phosphorylates the cytosolic portions of the activated receptor.
  • Once phosphorylated, the receptor binds with high affinity to an arrestin protein.
  • Arrestin inactivates the receptor by preventing its interaction with G proteins.
  • Arrestins also function as adapter proteins, recruiting phosphorylated receptors to clathrin-coated pits for endocytosis.
  • Endocytosed receptors can be degraded in lysosomes or can activate new signaling pathways from endosomes.

• Receptor endocytosis and fate

  • Clathrin-coated pit-mediated endocytosis follows arrestin recruitment.
  • Post-endocytosis, receptors may be degraded in lysosomes or may participate in alternate signaling pathways.

• Receptor kinases and their role in desensitization

  • Receptor kinases (often GRKs) phosphorylate cytosolic regions of activated receptors.
  • Phosphorylation promotes arrestin binding and receptor desensitization.

• Clarification: N-terminus (NH) of the receptor

  • Question: What does the NH refer to on the receptor with a C-terminal tail and NH on the other side?
  • Answer: NH refers to the N-terminus of the protein, which is the amino (–NH2) terminus.
  • In membrane proteins like many receptors, the N-terminus is typically extracellular, while the C-terminus is cytoplasmic.
  • Polypeptides are synthesized from the N-terminus to the C-terminus, so the N-terminus is the starting end of the chain.
  • Note: The in-class discussion here emphasizes orientation: N-terminus (NH2) tends to be outside the cell; C-terminus (carboxyl end) tends to be inside (cytoplasmic tail).

• Connections to foundational concepts and relevance

  • Receptor–ligand binding triggers conformational changes that propagate signals to intracellular proteins (allostery).
  • The G protein cycle (GDP/GTP exchange and hydrolysis) acts as a molecular switch mechanism for signaling duration.
  • The separation or coordination of Gα and Gβγ regulates distinct downstream effectors.
  • Desensitization and receptor internalization are critical for preventing overstimulation and for receptor reuse.
  • Arrestin-mediated signaling provides alternative pathways independent of G proteins, contributing to signaling diversity.
  • GRKs and arrestin interactions illustrate a general principle of signal termination and pathway switching.

• Practical and real-world implications

  • GPCRs are common drug targets; understanding desensitization and internalization informs therapeutic strategies and drug design.
  • Biased agonism and receptor trafficking can influence drug efficacy and side effects by altering signaling routes (G protein vs arrestin pathways).

• Quick summary of the signaling flow (conceptual cycle)

  • Ligand binding to extracellular domain → receptor activation → conformational change in G protein → GDP dissociation from Gα → GTP binding → activation of Gα and/or Gβγ → regulation of plasma membrane targets → downstream cascade → GTP hydrolysis via intrinsic GTPase activity (accelerated by RGS) → reformation of inactive Gαβγ → receptor desensitization via phosphorylation by receptor kinase → arrestin binding → receptor internalization via clathrin-mediated endocytosis → potential lysosomal degradation or alternative signaling from endosomes.