Part 3 · Chapter 3.2

G-Protein Signaling

G-protein-coupled receptors are the largest receptor superfamily — 800+ genes, 35% of approved drug targets, 3 Nobel Prizes (Gilman 1994, Lefkowitz & Kobilka 2012). This chapter covers the 7-TM architecture, the GTP–GDP cycle, the four heterotrimeric G-protein families (G_s, G_i, G_q, G_12/13), and the signalling amplification that makes a single photon drive a rod response.

1. 7-TM Architecture

GPCRs share a seven-transmembrane-helix topology with an extracellular N-terminus and intracellular C-terminus. Ligand binding occurs in the helix bundle (small molecules: β-adrenergic, muscarinic) or at the extracellular N-terminus (peptide hormones: glucagon, parathyroid). Class A (rhodopsin-like) is the largest; Class B (secretin), Class C (metabotropic glutamate) are smaller. Rasmussen 2011 crystal structures of active β₂AR-G_s resolved the inward-outward rotation of helix 6 that couples agonist binding to Gα release.

2. The GTPase Cycle

Heterotrimeric G-proteins (Gαβγ) cycle between GDP-bound inactive and GTP-bound active states:

\[ \text{R}^*\ :\ \text{G}\alpha\text{-GDP}\ +\ \text{GTP}\ \longrightarrow\ \text{G}\alpha\text{-GTP}\ +\ \text{GDP}\ +\ \text{G}\beta\gamma \]

Gα-GTP dissociates from βγ and activates effectors (adenylyl cyclase, PLCβ, RhoGEF). Intrinsic GTPase activity hydrolyses GTP to GDP, terminating the signal; RGS proteins accelerate hydrolysis. The cycle duration (seconds) sets the timescale of GPCR signalling.

3. G-Protein Families

G_s: activates adenylyl cyclase → ↑cAMP. β-adrenergic, glucagon, TSH.
G_i/o: inhibits adenylyl cyclase → ↓cAMP. μ-opioid, M2 muscarinic, α₂-adrenergic. Pertussis toxin ADP-ribosylates Gα_i.
G_q: activates PLCβ → IP₃ + DAG. α₁-adrenergic, M1/M3 muscarinic, angiotensin II.
G_12/13: RhoGEF → Rho GTPase → cytoskeletal rearrangement. Thrombin, sphingosine-1-phosphate.

Simulation: Cascade Amplification

Python

script.py40 lines

import numpy as np, matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt

# GPCR activation cascade kinetics - amplification through GTPase cycle
t = np.linspace(0, 30, 500)
agonist_on = t > 2
agonist_off = t > 15

# Rate constants
k_act = 2.0
k_inact = 0.3

R_active = np.zeros_like(t)
for i in range(1, len(t)):
    dt = t[i] - t[i-1]
    stim = 1.0 if (agonist_on[i] and not agonist_off[i]) else 0.0
    R_active[i] = R_active[i-1] + dt * (k_act*stim - k_inact*R_active[i-1])

# Amplification: each R activates many G, each G activates many effectors
G_active = R_active * 20
cAMP = G_active * 50

fig, ax = plt.subplots(figsize=(11, 6), facecolor='#0a0f1e')
ax.set_facecolor('#111e2f'); ax.tick_params(colors='#bae6fd')
for s in ax.spines.values(): s.set_color('#334155')
ax.plot(t, R_active, color='#60a5fa', lw=2.5, label='Active R*  x1')
ax.plot(t, G_active, color='#34d399', lw=2.5, label='Active G-alpha  x20')
ax.plot(t, cAMP/10, color='#fbbf24', lw=2.5, label='cAMP (/10)  x1000')
ax.axvspan(2, 15, alpha=0.1, color='#60a5fa', label='Agonist applied')
ax.set_xlabel('Time (s)', color='#bae6fd')
ax.set_ylabel('Relative activity', color='#bae6fd')
ax.set_title('GPCR cascade amplification ~1000x',
             color='#67e8f9', fontweight='bold')
ax.legend(facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1')
ax.grid(True, color='#334155', alpha=0.3)
plt.tight_layout()
plt.savefig('output.png', dpi=120, bbox_inches='tight', facecolor='#0a0f1e')
print('Single rhodopsin* activates ~800 transducin in visual cascade')
print('Each PDE6 hydrolyses ~1000 cGMP/s -> total gain ~1e6 per photon')

Click Run to execute the Python code

Code will be executed with Python 3 on the server

4. β-Arrestin & Biased Signalling

Following agonist binding, GRKs phosphorylate the GPCR’s intracellular C-terminus, recruiting β-arrestin. β-arrestin uncouples the receptor from G-protein (desensitisation) and initiates a second signalling branch through MAPK, Src, PI3K. Biased agonists can activate G-protein only, β-arrestin only, or both — enabling pathway-selective therapeutics (Lefkowitz 2011).

Key References

• Gilman, A. G. (1987). “G proteins: transducers of receptor-generated signals.” Annu. Rev. Biochem., 56, 615–649.

• Rasmussen, S. G. F. et al. (2011). “Crystal structure of the β₂AR-G_s complex.” Nature, 477, 549–555.

• Wettschureck, N. & Offermanns, S. (2005). “Mammalian G proteins and their cell type specific functions.” Physiol. Rev., 85, 1159–1204.

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