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Rhodopsin from Haloquadratum walsbyi is a light-driven magnesium transporter.

Ling-Ning Ko1, Guo Zhen Lim1, Jui-Chien Chen1

  • 1Department of Biochemical Science and Technology, College of Life Science, National Taiwan University, Taipei, Taiwan, ROC.

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|May 14, 2025
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Haloquadratum walsbyi middle rhodopsin (HwMR) senses magnesium ion (Mg2+) concentration. Structural and functional studies reveal HwMR acts as an inward magnesium transporter, utilizing D84 and T216 residues.

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Area of Science:

  • Microbial rhodopsins
  • Membrane transport proteins
  • Halophilic archaea

Background:

  • Middle rhodopsin (HwMR) from Haloquadratum walsbyi is a functionally uncharacterized microbial rhodopsin.
  • HwMR is found in an archaeon thriving in extreme 2M MgCl2 conditions.
  • It shares conserved residues with proton pumps and phototaxis sensors but performs neither function.

Purpose of the Study:

  • To elucidate the function and Mg2+ sensing mechanism of HwMR.
  • To determine the structural basis of HwMR's interaction with Mg2+.
  • To propose a model for Mg2+ transport by HwMR.

Main Methods:

  • Mg2+ titration assays and absorption spectroscopy to study Mg2+ sensing.
  • X-ray crystallography to determine the atomic structures of wild-type HwMR and D84N mutant.
  • Omit maps analysis to identify Mg2+ binding sites.
  • Cell-based light-driven conductivity assays to investigate transport function.

Main Results:

  • HwMR senses environmental Mg2+ concentration through the D84 residue, indicated by spectral shifts and altered photocycle kinetics.
  • X-ray crystallography revealed D84 as a primary Mg2+ binding site and T216 as a secondary site on the cytoplasmic side.
  • The D84N mutant lacked these Mg2+ binding sites.
  • Conductivity assays supported HwMR's role as an inward magnesium transporter.

Conclusions:

  • HwMR functions as a magnesium transporter, distinct from known proton pumps or phototaxis sensors.
  • D84 is identified as the key Mg2+ binding residue, and T216 as a stabilizing residue in the transport pathway.
  • A sequential model for Mg2+ transport via HwMR is proposed, highlighting its unique role in halophilic archaea.