TAT Rhodopsin Is an Ultraviolet-Dependent Environmental pH Sensor

Area of Science:

• Biochemistry
• Photochemistry
• Microbiology

Background:

• Rhodopsins typically have high Schiff base pKa for visible light absorption.
• TAT rhodopsin exhibits both protonated and unprotonated Schiff base forms at physiological pH.
• The protonated form shows rapid K intermediate decay, questioning its function.

Purpose of the Study:

• Investigate the molecular properties of protonated and unprotonated TAT rhodopsin Schiff base forms.
• Determine the photocycle dynamics and functional role of TAT rhodopsin.
• Elucidate the mechanism of TAT rhodopsin's interaction with environmental pH.

Main Methods:

• Spectroscopic studies of TAT rhodopsin in various microenvironments (detergents, nanodiscs, liposomes).
• Low-temperature photochemical analysis to study photoisomerization and proton transfer.
• Electrophysiological measurements to assess ion transport and pH sensing capabilities.

Main Results:

• Protonated TAT rhodopsin showed no photointermediate formation beyond 10^-5 s.
• Unprotonated TAT rhodopsin exhibited a long photocycle (15 s) with unusual proton transfer at 77 K.
• TAT rhodopsin lacks ion transport activity but senses extracellular pH, particularly at alkaline conditions.

Conclusions:

• TAT rhodopsin functions as a UV-dependent environmental pH sensor in marine bacteria.
• At high pH, UV/blue light absorption by the unprotonated form generates long-lived intermediates, potentially driving signal transduction.
• The protonated form likely dissipates light energy as heat at acidic pH, indicating a specialized sensory role.