The Role of Arrestin-1 Middle Loop in Rhodopsin Binding

Arrestins preferentially bind active phosphorylated G protein-coupled receptors (GPCRs). The middle loop, highly conserved in all arrestin subtypes, is localized in the central crest on the GPCR-binding side. Upon receptor binding, it directly interacts with bound GPCR and demonstrates the largest movement of any arrestin element in the structures of the complexes. Comprehensive mutagenesis of the middle loop of rhodopsin-specific arrestin-1 suggests that it primarily serves as a suppressor of binding to non-preferred forms of the receptor. Several mutations in the middle loop increase the binding to unphosphorylated light-activated rhodopsin severalfold, which makes them candidates for improving enhanced phosphorylation-independent arrestins. The data also suggest that enhanced forms of arrestin do not bind GPCRs exactly like the wild-type protein. Thus, the structures of the arrestin-receptor complexes, in all of which different enhanced arrestin mutants and reengineered receptors were used, must be interpreted with caution.

Automated summary

Arrestins preferentially bind phosphorylated G protein-coupled receptors (GPCRs) through their conserved middle loop, which directly interacts with the bound GPCR. Mutagenesis studies reveal that the middle loop primarily acts as a suppressor of binding to non-preferred forms of the receptor. Certain mutations in the middle loop enhance the binding to unphosphorylated light-activated rhodopsin, making them potential candidates for improving phosphorylation-independent arrestins. However, enhanced forms of arrestin do not bind GPCRs exactly like the wild-type protein, indicating the need for caution when interpreting the structures of arrestin-receptor complexes with different enhanced arrestin mutants and reengineered receptors.