Tracking G-protein-coupled receptor activation using genetically encoded infrared probes

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• Rhodopsin, a G-protein-coupled receptor (GPCR), mediates dim-light vision.
• Light-induced retinal isomerization triggers rhodopsin activation via transmembrane helix rearrangements.
• The exact timing and mechanism of these helix movements remain incompletely understood.

Purpose of the Study:

• To elucidate the temporal sequence of conformational changes during rhodopsin activation.
• To investigate the early molecular events preceding G-protein binding and receptor activation.

Main Methods:

• Site-directed mutagenesis incorporating p-azido-l-phenylalanine into rhodopsin.
• Infrared spectroscopy to monitor azido probe vibrational signatures.
• Tracking conformational changes throughout the rhodopsin activation pathway.

Main Results:

• Significant alterations in probe electrostatic environments were detected in the inactive Meta I state.
• Early changes indicate H6 rotation and H5 movement prior to the active Meta II state.
• A pronounced outward tilt of H6 defines the active Meta II state, opening the cytoplasmic surface.

Conclusions:

• Conformational changes occur early in rhodopsin activation, preceding major helix rearrangements.
• These findings provide a framework for interpreting GPCR crystal structures.
• The study sheds light on conformational substates during the activation of other GPCRs.