Understanding the structure and functional mechanisms of cyanobacterial halorhodopsin has become increasingly important, given the report that Synechocystis halorhodopsin (SyHR), a homolog of the cyanobacterial halorhodopsin from Mastigocladopsis repens (MrHR), can take up divalent ions, such as SO42−, as well as chloride ions. Here, the crystal structure of MrHR, containing a unique “TSD” chloride ion conduction motif, was determined as a homotrimer at a resolution of 1.9 Å. The detailed structure of MrHR revealed a unique trimeric topology of the light-driven chloride pump, with peculiar coordination of two water molecules and hydrogen-mediated bonds near the TSD motif, as well as a short B–C loop. Structural and functional analyses of MrHR revealed key residues responsible for the anion selectivity of cyanobacterial halorhodopsin and the involvement of two chloride ion-binding sites in the ion conduction pathway. Alanine mutant of Asn63, Pro118, and Glu182 locating in the anion inlet induce multifunctional uptake of chloride, nitrate, and sulfate ions. Moreover, the structure of N63A/P118A provides information on how SyHR promotes divalent ion transport. Our findings significantly advance the structural understanding of microbial rhodopsins with different motifs. They also provide insight into the general structural framework underlying the molecular mechanisms of the cyanobacterial chloride pump containing SyHR, the only molecule known to transport both sulfate and chloride ions.
Bibliographical noteFunding Information:
This work was supported by Researcher Program ( NRF-2019M3E5D6063903 , 2017M3A9F6029753 , 2018K2A9A2A06024227 to W. Lee and NRF-2016R1A6A3A04010213 to J.H. Yun) through the National Research Foundation of Korea , and by the Platform Project for Supporting Drug Discovery and Life Science Research (BINDS) from AMED , under Grant Number JP18am0101076 (to S.Y. Park), JSPS KAKENHI grant numbers JP19H05779 (to S.Y. Park), and Takeda Science Foundation (to S.Y. Park). The grant to H. Liu is also acknowledged ( NSFC No. U1930402). The computational work was supported by the Tianhe-2JK computing time award at the Beijing Computational Research Center (CSRC). The work was supported in part by the Brain Korea 21 (BK21) PLUS program; J.H.P. and Z.J. are fellowship awardees of the BK21 PLUS program.
© 2020 The Authors
All Science Journal Classification (ASJC) codes
- Structural Biology
- Molecular Biology