Transducin: The G-Protein Mediator of Vertebrate Phototransduction

Molecular Mechanism of Visual Signal Transduction

Transducin (G_t) is a heterotrimeric G protein (α, β, γ subunits) that couples photon absorption by rhodopsin to the phosphodiesterase (PDE6) cascade in rod and cone photoreceptors.

Phototransduction Cascade (Rod Pathway)

1. Photon Absorption: 11-cis-retinal in rhodopsin isomerizes to all-trans → conformational change to metarhodopsin II (R*).
2. R* Activates Transducin: Catalyzes GDP→GTP exchange on G_αt (rate: ~100–1000 transducins/R*).
3. Subunit Dissociation: G_αt-GTP separates from G_βγ.
4. PDE6 Activation: G_αt-GTP binds inhibitory γ-subunits of PDE6 → relieves inhibition of catalytic α/β subunits.
5. cGMP Hydrolysis: Activated PDE6 rapidly hydrolyzes cGMP → [cGMP] falls.
6. Channel Closure: cGMP-gated cation channels (CNGA1/CNGB1) close → Na⁺/Ca²⁺ influx stops.
7. Hyperpolarization: Photoreceptor membrane potential moves from ~–40 mV to –70 mV → reduced glutamate release.
8. Signal Amplification: Single R* activates hundreds of transducins → each PDE hydrolyzes ~1000 cGMP/s → massive amplification.

Signal Termination (Recovery)

1. R* Quenching: Rhodopsin kinase (GRK1) phosphorylates R* → arrestin binding.
2. G_αt GTPase: Intrinsic GTPase activity (accelerated by RGS9) hydrolyzes GTP → GDP.
3. Reassociation: G_αt-GDP + G_βγ → inactive heterotrimer.
4. PDE6 Inactivation: γ-subunits rebind catalytic subunits.
5. cGMP Restoration: Guanylate cyclase (GCAP1/2 regulated) synthesizes cGMP → channels reopen.

Light Adaptation

• Background light → steady cGMP reduction → channels partially closed → response compressed (Weber-Fechner law).
• Calcium feedback via GCAPs and recoverin modulates gain.

Clinical Relevance

• Mutations in GNAT1 (G_αt), PDE6A/B, CNGA1 cause congenital stationary night blindness or retinitis pigmentosa.
• Transducin translocation between outer/inner segments in light/dark adaptation.