Structure and Function of Activated Rhodopsin

Abstract

AbstractAfter cis/trans isomerization of retinal and early photoproducts, activated rhodopsin (R*) develops signaling states for different proteins in a time‐ordered sequence. Rhodopsin kinase binds all Meta forms, including the early Meta I, while interaction with transducin (Gt) or arrestin requires the deprotonated Schiff base form, Meta II (MII). Gt recognizes a specific conformation, termed MIIb, which arises from an additional, spectrally silent conversion, linked to proton uptake. Collisional coupling with the GDP‐bound Gt holoprotein induces the release of GDP and formation of a stable R*—Gt complex, in which the nucleotide binding site of Gt is empty. The empty site complex, once formed, remains stable, even if the retinal is re‐isomerized to the cis configuration. The cytoplasmic surface of rhodopsin appears to provide the majority of interaction sites for other proteins. Physical analyses and mutagenesis have emphasized the loops connecting helices C/D and E/F of the seven‐helix structure. Any interaction between R* and Gt depends on a conserved charge pair at the interface between the helix C and the adjacent loop CD. Replacements or deletions in loops CD and EF were found to cause more specific functional defects, including slow release of GDP or failure of GTP‐induced complex dissociation. In the dark, signaling states with low activity can be generated by reversible binding of all‐trans‐retinal to opsin. These light‐independent signaling states are different from the Meta forms.