Three-dimensional structure of halorhodopsin at 7 A resolution

Area of Science:

• Structural biology
• Membrane protein structure
• Biophysics

Background:

• Halorhodopsin (HR) is a light-driven chloride pump found in Halobacterium.
• HR functions similarly to bacteriorhodopsin (BR), a related proton pump.
• Overproducing strains of Halobacterium spontaneously form crystalline patches of HR in their cell membranes.

Purpose of the Study:

• To determine the three-dimensional structure of halorhodopsin (HR).
• To compare the structure of HR with that of bacteriorhodopsin (BR).
• To understand the structural basis for HR's function as a chloride pump.

Main Methods:

• Electron cryo-microscopy of tilted specimens was used to analyze the 3D structure.
• Image analysis involved anisotropic scaling and correction for the phase contrast transfer function.
• Amplitudes and phases were obtained from images due to small crystal size, precluding electron diffraction.

Main Results:

• The 3D structure of HR was determined with space group p42(1)2 and a lattice constant of 102 Å.
• HR exhibits a seven-transmembrane helix arrangement, analogous to BR.
• HR's structure shows a slight rotation relative to BR and a wider cytoplasmic half-channel, consistent with chloride transport.

Conclusions:

• The determined structure of HR provides insights into its function as a light-driven chloride pump.
• The structural similarities and differences between HR and BR highlight their distinct transport mechanisms.
• The employed image analysis techniques are valuable for determining structures from small crystals without prior homologous information.