Cation-Specific Conformations in a Dual-Function Ion-Pumping Microbial Rhodopsin.

Area of Science:

• Biochemistry
• Molecular Biology
• Microbiology

Background:

• Dokdonia eikasta rhodopsin pump (DeNaR) is a light-driven ion transporter.
• DeNaR exhibits novel light-driven sodium ion pumping in NaCl solutions, unlike other rhodopsins.

Purpose of the Study:

• Investigate cation-dependent conformational changes in DeNaR.
• Determine if helical movements in DeNaR are conserved or altered by Na(+) or K(+).

Main Methods:

• Visible absorption spectroscopy to analyze the photoactive site.
• Electron paramagnetic resonance (EPR) spectroscopy to assess conformational changes and helix repositioning.
• EPR spin-spin dipolar coupling to study light-induced helix movements.

Main Results:

• DeNaR shows an 8 nm shift in absorption maxima between Na(+) and K(+), indicating cation influence on the retinylidene site.
• EPR reveals repositioning of helices F and G in the presence of Na(+).
• Light-induced helix movements are similar to bacteriorhodopsin but altered by Na(+), including a novel clockwise rotation of helix F.

Conclusions:

• This study presents the first observation of a cation switch controlling microbial rhodopsin conformations.
• Specific Na(+) interactions with DeNaR's half-channels facilitate light-driven sodium ion translocation.