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Calcium Binding to TAT Rhodopsin.

Teppei Sugimoto1, Kota Katayama1,2, Hideki Kandori1,2

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Calcium ions bind to microbial rhodopsin, stabilizing a deprotonated retinal Schiff base. This binding, mediated by specific amino acids, alters protein structure and light absorption properties.

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Area of Science:

  • Biochemistry
  • Structural Biology
  • Microbiology

Background:

  • Rhodopsins are photoreceptive proteins utilizing a retinal chromophore.
  • Normally, the retinal Schiff base is protonated and stabilized by negative counterions.
  • Cation binding near the retinal chromophore is typically disfavored due to electrostatic repulsion.

Purpose of the Study:

  • To investigate calcium ion (Ca2+) binding to a wild-type microbial rhodopsin.
  • To understand the conditions and mechanisms of Ca2+ interaction with the retinal chromophore.
  • To identify the specific residues involved in Ca2+ binding and its functional consequences.

Main Methods:

  • Spectroscopic analysis of TAT rhodopsin from marine bacteria.
  • Site-directed mutagenesis to probe amino acid function.
  • Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy.

Main Results:

  • Ca2+ binding was observed with a neutral retinal chromophore and a deprotonated Schiff base.
  • Increasing Ca2+ concentration shifted the equilibrium towards the deprotonated state (Kd = 0.17 mM).
  • Mutagenesis identified E54 and D227 as the Ca2+ binding site, with ATR-FTIR showing structural changes.

Conclusions:

  • Ca2+ can bind to microbial rhodopsin, stabilizing a deprotonated Schiff base.
  • Specific acidic residues (E54, D227) are crucial for Ca2+ binding.
  • Ca2+ binding induces secondary structural changes in the rhodopsin, affecting its properties.