Linking regions between helices in bacteriorhodopsin revealed

Area of Science:

• Structural Biology
• Electron Microscopy
• Biophysics

Background:

• Three-dimensional electron microscopy (3D EM) relies on combining tilted views, but high-angle tilting (>60 degrees) causes distortion, limiting observable diffraction data.
• This data gap creates a "missing cone" in reciprocal space, leading to predictable distortions and reduced resolution in the direction perpendicular to the sample grid.
• Previous analyses of bacteriorhodopsin using 3D EM revealed its seven helical rods but failed to visualize the polypeptide segments linking these helices due to this distortion.

Purpose of the Study:

• To develop and test a constrained density map modification and refinement method to mitigate distortions caused by missing data in 3D EM.
• To apply this method to bacteriorhodopsin structure determination to visualize the locations of linking regions between alpha-helices.
• To refine the overall structural model of bacteriorhodopsin, including helical region uniformity and linker segment placement.

Main Methods:

• Developed a constrained density map modification and refinement approach to address missing data in 3D EM.
• Tested the method on model cases before applying it to experimental data.
• Applied the method to electron microscopic structural data of bacteriorhodopsin.

Main Results:

• The refined maps revealed the location of at least five extrahelical polypeptide segments in bacteriorhodopsin, previously not visible.
• The number of possible helix interconnections was reduced from 5,040 to five consistent possibilities.
• Helical regions showed more uniform and cylindrical density, with increased length (35-45 Å), and three models placed the chromophore-binding helix consistently with neutron diffraction data.

Conclusions:

• Constrained density map refinement effectively reduces distortions and visualizes previously unobserved structural features in 3D EM.
• The method provides insights into the connectivity of bacteriorhodopsin's helices and the location of its linking segments.
• The refined structural models offer a more accurate representation of bacteriorhodopsin, consistent with multiple experimental techniques.