Volume 31, No 4, Apr 2021

ISSN: 1001-0602 
EISSN: 1748-7838 2018 
impact factor 17.848* 
(Clarivate Analytics, 2019)

Volume 31 Issue 4, April 2021: 383-394   |  Open Access

ORIGINAL ARTICLES

Structural mechanism of cooperative activation of the human calcium-sensing receptor by Ca2+ ions and L-tryptophan

Shenglong Ling¹ , Pan Shi¹ , Sanling Liu¹ , Xianyu Meng¹ , Yingxin Zhou¹ , Wenjing Sun¹ , Shenghai Chang² , Xing Zhang² , Longhua Zhang¹ , Chaowei Shi¹ , Demeng Sun^1,* , Lei Liu^3,* , Changlin Tian^1,4,5,*

¹Hefei National Laboratory of Physical Sciences at Microscale, Anhui Laboratory of Advanced Photonic Science and Technology, and School of Life Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China
²School of Medicine, Zhejiang University, Hangzhou, Zhejiang 310011, China
³Tsinghua-Peking Joint Center for Life Sciences, Ministry of Education Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology, Department of Chemistry, Tsinghua University, Beijing 100084, China
⁴High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei, Anhui 230030, China
⁵Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203, China
These authors contributed equally: Shenglong Ling, Pan Shi, Sanling Liu, Xianyu Meng. Correspondence: Demeng Sun(dmsun@ustc.edu.cn)Lei Liu(lliu@mail.tsinghua.edu.cn)Changlin Tian(cltian@ustc.edu.cn)

The human calcium-sensing receptor (CaSR) is a class C G protein-coupled receptor (GPCR) responsible for maintaining Ca2+ homeostasis in the blood. The general consensus is that extracellular Ca2+ is the principal agonist of CaSR. Aliphatic and aromatic L-amino acids, such as L-Phe and L-Trp, increase the sensitivity of CaSR towards Ca2+ and are considered allosteric activators. Crystal structures of the extracellular domain (ECD) of CaSR dimer have demonstrated Ca2+ and L-Trp binding sites and conformational changes of the ECD upon Ca2+/L-Trp binding. However, it remains to be understood at the structural level how Ca2+/L-Trp binding to the ECD leads to conformational changes in transmembrane domains (TMDs) and consequent CaSR activation. Here, we determined the structures of full-length human CaSR in the inactive state, Ca2+- or L-Trp-bound states, and Ca2+/L-Trp-bound active state using single-particle cryo-electron microscopy. Structural studies demonstrate that L-Trp binding induces the closure of the Venus flytrap (VFT) domain of CaSR, bringing the receptor into an intermediate active state. Ca2+ binding relays the conformational changes from the VFT domains to the TMDs, consequently inducing close contact between the two TMDs of dimeric CaSR, activating the receptor. Importantly, our structural and functional studies reveal that Ca2+ ions and L-Trp activate CaSR cooperatively. Amino acids are not able to activate CaSR alone, but can promote the receptor activation in the presence of Ca2+. Our data provide complementary insights into the activation of class C GPCRs and may aid in the development of novel drugs targeting CaSR.

https://doi.org/10.1038/s41422-021-00474-0

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