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Light-dependent transducin activation by an ultraviolet-absorbing rhodopsin mutant
1Howard Hughes Medical Institute, Rockefeller University, New York, New York 10021.
Biochemistry
|September 7, 1993
Summary
Visual pigments can be activated by UV light, challenging the necessity of a protonated Schiff base for efficient photoactivation. This study reveals alternative pathways for rhodopsin activation.
Area of Science:
- Biochemistry
- Photochemistry
- Molecular Biology
Background:
- Rhodopsin photoactivation is crucial for vision.
- A mutant rhodopsin (E113Q) exhibits pH-dependent spectral forms absorbing at 380 nm (unprotonated Schiff base) and 490 nm (protonated Schiff base).
Purpose of the Study:
- To investigate the role of the Schiff base proton in rhodopsin photoactivation.
- To determine if UV-absorbing forms of rhodopsin can initiate the visual signaling cascade.
Main Methods:
- Measuring transducin activation action spectra of wild-type rhodopsin and the E113Q mutant.
- Comparing UV-visible absorption spectra with action spectra.
Main Results:
- Transducin activation action spectra closely matched the absorption spectra of both rhodopsin and E113Q mutant.
- The UV-absorbing (380 nm) form of E113Q was directly activated by UV light to catalyze transducin nucleotide exchange.
- The quantum efficiency of photoactivation was similar for both UV- and visible-absorbing forms of E113Q.
Conclusions:
- A protonated Schiff base is not essential for efficient photoactivation of visual pigments.
- The protonated Schiff base likely stabilizes the ground state and enables visible light absorption (>420 nm).
- Rhodopsin's active state can be formed through multiple pathways, including those involving UV-activated forms.