Mapping light-dependent structural changes in the cytoplasmic loop connecting helices C and D in rhodopsin: a...

Area of Science:

• Biochemistry
• Structural Biology
• Membrane Proteins

Background:

• Rhodopsin, a G protein-coupled receptor, is crucial for vision.
• Understanding its structure and dynamics is key to deciphering visual transduction.
• The C-D interhelical loop and C helix are important for transducin interaction.

Purpose of the Study:

• To investigate the structural and dynamic changes in bovine rhodopsin's C helix and C-D loop upon photoexcitation.
• To map the accessibility and mobility of residues within this region.

Main Methods:

• Site-directed mutagenesis to introduce cysteine residues.
• Modification with a sulfhydryl-specific nitroxide reagent.
• Electron paramagnetic resonance (EPR) spectroscopy in dark and photoexcited states.
• Analysis of nitroxide accessibility and mobility.

Main Results:

• Rhodopsin's polypeptide chain crosses an aqueous/hydrophobic boundary near V138 and H152.
• The C helix is largely embedded in protein, not lipid, near the cytoplasmic surface.
• Photoexcitation induces EPR spectral changes in the C helix and C-D loop, indicating movement.

Conclusions:

• Photoexcitation of rhodopsin likely involves a rigid body movement of the C helix relative to other helices.
• These movements are significant for the receptor's function in visual signaling.