Structural evidence for visual arrestin priming via complexation of phosphoinositols

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• Visual arrestin (Arr1) is crucial for terminating rhodopsin signaling in photoreceptors.
• Arr1 translocates to the outer segment upon light stimulation to block rhodopsin-transducin interaction.
• The precise molecular mechanism of inositol phosphate (InsP) regulation of Arr1 activity is not well understood.

Purpose of the Study:

• To elucidate the molecular interaction between inositol phosphates (InsPs) and visual arrestin (Arr1).
• To determine the structural basis for InsP-mediated regulation of Arr1 activity and translocation.

Main Methods:

• X-ray crystallography was used to determine the structure of bovine Arr1 in a ligand-free state.
• A near-complete model of the Arr1 C-tail was resolved.
• Structural analysis focused on the interaction sites of InsPs and the Arr1 C-tail.

Main Results:

• The structure of ligand-free bovine Arr1 revealed a near-complete C-tail, critical for regulating activity.
• Inositol phosphates (InsPs) bind to the N-domain basic patch of Arr1.
• InsP binding displaces the Arr1 C-tail, suggesting a priming mechanism for rhodopsin interaction and translocation.
• The polar core remained intact upon InsP binding, indicating C-tail displacement alone does not fully activate Arr1.

Conclusions:

• Inositol phosphates (InsPs) directly interact with visual arrestin (Arr1) at the N-domain basic patch.
• InsP binding to Arr1 displaces the C-tail, priming the protein for light-induced signaling termination and translocation.
• These findings provide a detailed molecular understanding of how endogenous InsPs regulate Arr1 activity in photoreceptors.