The structure of the polar core mutant R175E and its functional implications

Abstract

Mutation of arginine 175 to glutamic acid (R175E), a central residue in the polar core and previously predicted as the 'phosphosensor', leads to a constitutively active arrestin that is able to terminate phototransduction by binding to non-phosphorylated, light-activated rhodopsin. Crystal structure of a R175E mutant arrestin at 2.7 Å resolution reveals significant differences compared to the basal state reported in full-length arrestin structures. Most striking differences are disruption of hydrogen bond network in the polar core, and three-element interaction (between ß-strand I, a-helix I, and the C-tail), including disordering of several residues in the receptor-binding finger loop and the C-terminus (residues 361-404). Additionally, R175E structure shows a 7.5° rotation of the amino and carboxy-terminal domains relative to each other. Comparison of the crystal structures of basal arrestin and R175E mutant provides insights into the mechanism of arrestin activation, where the latter likely represents an intermediate activation state prior to formation of the high-affinity complex with the G protein-coupled receptor.