Flash-induced voltage changes in halorhodopsin from Natronobacterium pharaonis

Area of Science:

• Photobiology
• Biophysics
• Membrane Proteins

Background:

• Halorhodopsin is a light-driven chloride pump.
• Its photocycle and ion transport mechanisms are crucial for understanding cellular energy transduction.
• Previous models for halorhodopsin photocycling require refinement.

Purpose of the Study:

• To investigate the kinetic components of halorhodopsin's flash-induced voltage response.
• To elucidate the role of chloride ions in halorhodopsin's photoelectrical properties.
• To challenge existing photocycle models based on photovoltage data.

Main Methods:

• Flash-induced voltage response measurements.
• Varying NaCl concentrations to assess chloride dependence.
• pH modulation to study spectral forms.

Main Results:

• Two kinetic components were identified in the voltage response: a fast (approx. 1 µs) and a slow (approx. 1-2.5 ms) component.
• The slow component is independent of chloride concentration and linked to chloride translocation.
• Absence of chloride or decreased pH alters the response polarity and kinetics, suggesting different functional states.

Conclusions:

• The photovoltage data suggest a revised photocycle model for halorhodopsin.
• The O-type intermediate may represent a non-chloride-transporting form of halorhodopsin.
• Halorhodopsin's chloride transport mechanism shares similarities with bacteriorhodopsin's proton transport.