The functions of G-protein-coupled receptors (GPCRs) are primarily mediated by two major families of signal transducers, G proteins and arrestins. The arrestin family contains four members, while visual arrestins (Arr1 and Arr4) are exclusively involved in signal transduction in vision through binding to rhodopsin. The mechanism of visual arrestin binding to rhodopsin has been illustrated by XU Huaqiang's team with the aid of the femtosecond X-ray laser method as early as 2015. The non-visual arrestins (Arr2 and Arr3) are required for regulating signal transduction of many non-visual GPCRs. Binding of Arr2 and Arr3 to GPCRs not only blocks G protein binding but also mediates receptor endocytosis and numerous G protein-independent signaling pathways. Because of the promiscuous binding and highly dynamic complex assembly, the structure determination of an arrestin complex with a non-visual GPCR is technically challenging and is a long-sought goal in the field of GPCR structural biology.

To meet this challenge, researchers from XU Huaqiang's team firstly identified NTSR1 as the model receptor after several rounds of screening. After exploring various factors systematically for 8 years, researchers developed a serial of strategies to enhance the interaction between Arr2 and NTSR1 and to improve the stability of complex, including fusing the wild type human NTSR1 with the human 3A mutant Arr2 at its C-terminus, adding BRIL to the N-terminus of the receptor to increase the complex expression, co-expressing GRK5, a GPCR kinase which phosphorylated the receptor to promote arrestin binding, fusing Fab30 and incubating with a reported PAM ML314, etc. The structure of Arr2-NTSR1 was finally determined, revealing an overall assembly that is strikingly different from the visual arrestin-rhodopsin complex by a 90° rotation of Arr2 relative to the receptor. In this new configuration, intracellular loop 3 (ICL3) and transmembrane helix 6 of the receptor are oriented toward the N-terminal domain of the arrestin, making it possible for GPCRs that lack the C-terminal tail to couple Arr2 through their ICL3. Molecular dynamics simulation and crosslinking data further support the assembly of the Arr2-NTSR1 complex. Sequence analysis and homology modeling suggest that the Arr2-NTSR1 complex structure may provide an alternative template for modeling arrestin-GPCR interactions.

The research, hosted jointly by XU Huaqiang's group and YU Xuekui's group of Shanghai Institute of Materia Medica, Chinese Academy of Sciences, as well as the CONG Yao's group of National Center for Protein Science (Shanghai), was published online in Cell Research on November 27, 2019. The study is another important breakthrough for XU Huaqiang's team, following landmark researches in the field published in the journal Nature (2015) and Cell (2017).

Schematic illustration of the overall assembly that is strikingly different from the visual arrestin-rhodopsin complex by a 90°rotation of Arr2 relative to the receptor. (Image by YIN Wanchao)

Article link: https://www.nature.com/articles/s41422-019-0256-2

Contact

H. Eric XU
Shanghai Institute of Materia Medica
E-mail: eric.xu@simm.ac.cn