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Asymmetric Functional Conversion of Eubacterial Light-driven Ion Pumps.

Keiichi Inoue1, Yurika Nomura2, Hideki Kandori3

  • 1From the Department of Frontier Materials and OptoBioTechnology Research Center, Nagoya Institute of Technology, Showa-ku, Nagoya 466-8555, Japan and PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.

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Summary

Researchers successfully converted ion pumps by reversing evolutionary changes, demonstrating that ancestral functions are retained and can be reactivated. This provides insights into rhodopsin evolution and ion transport mechanisms.

Keywords:
biophysicschloride transportmutantphotoreceptorproton pumprhodopsinsodium transport

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Area of Science:

  • Biochemistry and Molecular Biology
  • Microbiology
  • Structural Biology

Background:

  • Eubacteria possess novel ion-pumping rhodopsins, including outward Na(+) and inward Cl(-) pumps, alongside known light-driven proton pumps.
  • These rhodopsins, like bacteriorhodopsin (BR) and halorhodopsin (HR), convert light energy into electrochemical potential.
  • Conserved motifs (DTE, NDQ, NTQ) in helix C are hypothesized as functional determinants for H(+), Na(+), and Cl(-) pumps, respectively.

Purpose of the Study:

  • To investigate the role of conserved motifs in determining the ion specificity of rhodopsin pumps.
  • To achieve functional interconversion between different types of ion-pumping rhodopsins through site-directed mutagenesis.
  • To understand the evolutionary trajectory and ancestral states of ion-pumping rhodopsins.

Main Methods:

  • Site-directed mutagenesis was employed to alter specific amino acid residues within the conserved motifs of ion-pumping rhodopsins.
  • Functional assays were performed to assess the ion transport activity (H(+), Na(+), Cl(-)) of the mutated rhodopsins.
  • Phylogenetic analysis was conducted to reconstruct the evolutionary relationships among different rhodopsin pump families.

Main Results:

  • Successful functional conversion of Na(+) pump to H(+) pump (Na(+) → H(+)) was achieved by introducing the proton-pumping motif.
  • Na(+) → Cl(-) and Cl(-) → H(+) conversions were successful with the introduction of characteristic motifs and additional mutations.
  • Conversions in the reverse direction (H(+) → Na(+), H(+) → Cl(-), Cl(-) → Na(+)) were unsuccessful with limited mutagenesis, suggesting evolutionary constraints.

Conclusions:

  • Functional interconversion of ion-pumping rhodopsins is dependent on the direction of evolutionary changes; reversing these changes facilitates conversion.
  • Ancestral functional mechanisms are retained even after acquiring new functions, and can be reactivated by specific mutations.
  • The evolution of ion-pumping rhodopsins likely proceeded from a common H(+) pump ancestor, with subsequent divergence into Cl(-) and Na(+) pumps.