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Phosphorylation stabilizes the active conformation of rhodopsin
S K Gibson1, J H Parkes, P A Liebman
1Department of Biochemistry and Biophysics, University of Pennsylvania Medical Center, Philadelphia 19104-6059, USA.
Biochemistry
|August 26, 1998
Summary
Rhodopsin phosphorylation surprisingly enhances the formation of metarhodopsin II (MII), the G protein-activating conformation. This finding challenges previous assumptions about G protein-coupled receptor (GPCR) deactivation mechanisms.
Area of Science:
- Biochemistry
- Molecular Biology
- Structural Biology
Background:
- G protein-coupled receptors (GPCRs) are crucial cell signaling proteins.
- Receptor phosphorylation is typically associated with GPCR deactivation.
- Rhodopsin, a well-studied GPCR, exists in equilibrium between metarhodopsin I (MI) and metarhodopsin II (MII) upon activation.
Purpose of the Study:
- To investigate the effect of rhodopsin phosphorylation on the MI-MII conformational equilibrium.
- To determine if phosphorylation promotes or inhibits the formation of the G protein-activating MII conformation.
Main Methods:
- Experimental phosphorylation of rhodopsin to varying degrees.
- Analysis of the MI-MII equilibrium using kinetic measurements.
- Quantification of the equilibrium constant and rate constants (k1 and k-1).
Main Results:
- Rhodopsin phosphorylation was found to increase, not decrease, the formation of MII.
- The MI-MII equilibrium constant shifted towards MII with increasing phosphorylation stoichiometry.
- Phosphorylation increased the rate of MII formation (k1) and decreased its rate of decay (k-1).
Conclusions:
- Contrary to expectations, rhodopsin phosphorylation stabilizes the active MII conformation.
- Cytoplasmic phosphorylation can directly modulate intramolecular conformational equilibria in membrane proteins.
- This mechanism may be relevant for the functional regulation of other GPCRs and membrane proteins.