Solution Structure of the Rod Arrestin Tetramer Explains the Mode of Its Formation and Its Inability to Bind Rhodopsin

Abstract

 

Purpose:

 

We have previously demonstrated that the shape of the solution tetramer of rod arrestin is dramatically different from that in the crystal. Here we elucidate the structure of the physiologically relevant tetramer that rod arrestin forms in solution.

 

Methods:

 

We employed the ROSETTA docking software to generate molecular models of the solution tetramer based on the monomeric arrestin crystal structure. The resulting models were filtered using the ROSETTA energy function, experimental inter-subunit distances measured with DEER spectroscopy, and inter-subunit contact sites identified by mutagenesis and site-directed spin labeling.

 

Results:

 

Our analysis resulted in a single best model for experimental evaluation. The validity of the model is strongly supported by model-guided cross-linking of four different residues and targeted mutagenesis that yields arrestin variants deficient in self-association.

 

Conclusions:

 

We have previously demonstrated that only monomeric arrestin can bind receptor and that the arrestin does not form oligomers greater than tetramer. A "closed" diamond-shaped structure (shown) of the solution tetramer explains why it forms via dimers in a cooperative fashion, and why arrestin oligomerization stops at tetramer. Previously identified rhodopsin-binding elements in arrestin are either directly involved in self-association or shielded by "sister" subunits, providing a structural basis for the inability of tetrameric arrestin to bind rhodopsin. Our model paves the way for the construction of self-association deficient arrestin mutants for experimental studies of the physiological role of rod arrestin self-association.  

 

Keywords: photoreceptors • protein structure/function • signal transduction