Conformational Differences among Metarhodopsin I, Metarhodopsin II, and Opsin Probed by Wide-Angle X-ray Scattering

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Biophysics

Background:

• Vertebrate rhodopsin is a G protein-coupled receptor crucial for vision.
• Upon light absorption, rhodopsin undergoes conformational changes, activating downstream signaling.
• Metarhodopsin II (Meta-II) is the primary signaling-competent intermediate, characterized by transmembrane helix rearrangement.

Purpose of the Study:

• To directly monitor light-induced helical rearrangement of membrane-embedded rhodopsin.
• To characterize the structural differences between metarhodopsin I, metarhodopsin II, and acid-induced active opsin (Opsin*).
• To elucidate the structural basis for the distinct signaling states of rhodopsin photoproducts.

Main Methods:

• Wide-angle X-ray scattering (WAXS) was employed to monitor structural changes in rhodopsin.
• Nanodiscs were used to embed rhodopsin in a native-like membrane environment.
• Structural models were generated based on crystal structures of dark-state and Meta-II rhodopsin.

Main Results:

• Metarhodopsin II formation exhibited a distinct WAXS signature with a peak at 0.2 Å⁻¹ and a valley at 0.6 Å⁻¹.
• Metarhodopsin I and opsin lacked this characteristic WAXS profile.
• Opsin* showed the 0.2 Å⁻¹ peak but not the 0.6 Å⁻¹ valley, indicating partial helix rearrangement.
• Structural analysis suggested outward movement of helix VI in Opsin*, but restoration of helices III and V, unlike in Meta-II.

Conclusions:

• Metarhodopsin II possesses a unique active structure with specific transmembrane helix rearrangements.
• Opsin* exhibits a distinct, less stable active conformation compared to Meta-II.
• The disruption and restoration of the cytoplasmic ionic lock play a critical role in stabilizing the G-protein-activating structure of rhodopsin.