Structural Evolution of Retinal Chromophore in Early Intermediates of Inward and Outward Proton-Pumping Rhodopsins

Area of Science:

• Structural biology
• Biophysics
• Membrane protein function

Background:

• Proton-pumping rhodopsins are seven-transmembrane proteins with a retinal chromophore.
• They are crucial for understanding unidirectional ion transport mechanisms.
• Outward and inward proton-pumping rhodopsins share similar structures but opposite transport directions.

Purpose of the Study:

• To elucidate the chromophore structures in early intermediates of inward and outward proton-pumping rhodopsins.
• To determine the factors controlling proton transport direction in these rhodopsins.

Main Methods:

• Spectroscopic analysis of chromophore structures in early intermediates.
• Measurement of proton-pumping activities.
• Correlation of chromophore structural data with protein function.

Main Results:

• The Schiff base hydrogen bond strengthens in the L intermediate for both inward and outward pumps.
• Proton release direction during the L-to-M transition is dictated by negative charges near the Schiff base.
• Retinal chromophore structure does not determine proton release direction, contrary to previous hypotheses.

Conclusions:

• The number of negative charges on the extracellular side of the Schiff base determines proton transport direction.
• This finding contrasts with the notion that chromophore configuration dictates proton uptake direction.
• The study clarifies the key factors governing proton transport directionality in rhodopsins.