2Fo-Fc electron density map of retinal at 1.55 A resolution


The protein:  bacteriorhodopsin (BR) is a photon-driven ion pump.  BR is a seven-helical transmembrane protein with a retinal co-factor.  It is found in the bacterium H. salinarium where it converts light to a proton gradient which in turn is used by a second membrane protein called ATP synthase to generate chemical energy in the form of ATP.  ATP is then used by the cell to drive a multitude of vital processes.

The method:  cubic lipid phase (CLP) crystallization, developed by E. Landau & G. Rosenbusch (PNAS, 1996).  The detergent-solubilized membrane protein is added to the CLP matrix where it partitions into the hydrophobic phase.  Since the hydrophobic phase has bilayer topology, the protein is assumed to be a lot "happier" in it than in detergent micelles.  Addition of a precipitant causes crystal nucleation & growth which is facilitated by controlled lateral diffusion in the three-dimensional bicontinuous matrix.

The crystals:  purple crystals appear after a few days, usually hundreds of them in a single set-up.  The crystals form small hexagonal plates about 80 um x 80 um x 15 um.  They are very stable and can easily be transported in the CLP matrix.  The layers in the a/b plane of the 3-dimensional BR crystals formed in CLP are very similar to the naturally occurring 2-D crystals sheets of BR in purple membranes.  The space group is P63 with a=b=61 Å, c=108 Å and two trimers per unit cell.  They diffract to 2.3 Å at the home lab, to 2.05 Å at beam line 9-1 at the Stanford Synchrotron (SSRL), and most recently to 1.40 Å (10 sigma spots) at beamline 5.0.2 at the Advanced Light Source (ALS) and to 1.33 Å at the microfocus beamline (ID13) at the European Synchrotron Research Facility (ESRF).  The P63 CLP crystals are heavily merohedrally twinned and thus require special consideration during refinement.


The papers:

Proton Transfer Pathways in Bacteriorhodopsin at 2.3 Angstrom Resolution.
H Luecke, H-T Richter, JK Lanyi (1998) Science 280, 1934-1937.

Structure of Bacteriorhodopsin at 1.55 Angstrom Resolution.
H Luecke, B Schobert, H-T Richter, JP Cartailler, JK Lanyi (1999) J. Mol. Biol. 291, 899-911.


The atomic models:

1brx.pdb (released on Aug 25, 1998): 2.3 Å light-adapted wild-type structure described in the 1998 Science paper.

1c3w.pdb (released on Jul 31, 1999): 1.55 Å light-adapted wild-type structure described in the 1999 JMB paper.