Phototransduction in rod photoreceptors involves closure of cGMP-gated channels upon light exposure. Which G-protein mediates the signal from activated rhodopsin to phosphodiesterase 6 (PDE6)?

• A Transducin (Gαt): activated by metarhodopsin II → Gαt-GTP dissociates and activates PDE6, hydrolyzing cGMP → channel closure → hyperpolarization ✓
• B Gs: activated by rhodopsin → adenylyl cyclase → cAMP opens Na⁺ channels causing depolarization in light
• C Gq: activated rhodopsin → PLC → IP3 → intracellular Ca²⁺ release closes cGMP channels
• D Gi: inhibits adenylyl cyclase reducing cAMP, and cAMP (not cGMP) gates the light-sensitive channels

Explanation

In rod phototransduction, a photon isomerizes 11-cis-retinal to all-trans-retinal within rhodopsin, forming metarhodopsin II (R*). R* acts as a GPCR activating transducin (Gαt-GDP → Gαt-GTP), a member of the Gi/o family. Gαt-GTP activates phosphodiesterase 6 (PDE6), which hydrolyzes cGMP to 5'-GMP. In dark, high cGMP keeps cyclic nucleotide-gated (CNG) Na⁺/Ca²⁺ channels open (dark current, ~40 mV depolarized). Light reduces cGMP → CNG channel closure → hyperpolarization → reduced glutamate release from the synaptic terminal. Recoverin (Ca²⁺-dependent) and GRK1/arrestin terminate rhodopsin signaling (adaptation).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.