Cytotoxicity of Monomeric Arrestin1 Explains Its Robust Self-Association

Abstract

Purpose: : To determine the biological role of arrestin1 self-association.

Methods: : Transgenic mice expressing wild type (WT) and oligomerization-deficient arrestin1 mutant at different levels were used to determine the functionality and health of rod photoreceptors

Results: : Arrestin1 ensures timely signal shutoff by binding light-activated phosphorhodopsin. Engineered arrestin1 mutant with increased affinity for Rh* partially compensates for lack of rhodopsin phosphorylation, improving the functional performance of rhodopsin kinase knockout rods. However, self-association of this mutant is greatly impaired. We found that high expression of "enhanced" arrestin1 results in progressive light-independent rod degeneration via apoptosis involving the activation of caspases-9 and -3. In contrast, similar high expression of WT arrestin1 does not affect rod survival. Increasing co-expression of WT arrestin1 dose-dependently alleviates the damage induced by enhanced mutant. Due to "closed" shape of arrestin1 tetramer, where each monomer engages two "sister" subunits via two different interfaces, WT arrestin1 promotes self-association of the mutant with only one "damaged" interface by forming mixed oligomers. Thus, our data indicate that excess of monomeric arrestin1 induces apoptosis of rod photoreceptors in vivo.

Conclusions: : Arrestin1 is expressed in rods at very high level, at 0.8:1 ratio to rhodopsin. Cytotoxicity of the arrestin1 monomer explains why it acquired propensity to oligomerize, even though only the monomer is the active rhodopsin-binding species. The inability of cone arrestin4 to self-associate likely explains why cones express much more arrestin1 than their "own" arrestin4 subtype.

Keywords: photoreceptors • apoptosis/cell death • protein structure/function