Transmembrane location of retinal in bacteriorhodopsin by neutron diffraction

Area of Science:

• Structural biology
• Biophysics
• Membrane protein research

Background:

• Bacteriorhodopsin is a light-driven proton pump.
• Understanding its chromophore's location is key to its function.
• Previous studies lacked precise localization within the membrane.

Purpose of the Study:

• To determine the precise transmembrane location of the bacteriorhodopsin chromophore.
• To map the orientation of the chromophore within the purple membrane.
• To utilize neutron diffraction with selectively deuterated retinal analogs.

Main Methods:

• Neutron diffraction on oriented purple membrane stacks.
• Synthesis of selectively deuterated retinal analogs (retinal-d11 and retinal-d5).
• Analysis of diffraction patterns using Fourier difference methods and D2O-H2O exchange for phasing.

Main Results:

• The cyclohexene ring (D11) is located 9.9 +/- 1.2 Å from the membrane surface.
• The Schiff base end (D5) is located 16.6 +/- 1.5 Å from the membrane surface.
• The chromophore is tilted ~40 degrees relative to the membrane plane, with the Schiff base near the center.

Conclusions:

• The chromophore is asymmetrically positioned within the bacteriorhodopsin transmembrane domain.
• The determined orientation supports models of proton translocation.
• Neutron diffraction with isotopic labeling provides high-resolution structural insights into membrane proteins.