DISTORTION CORRECTION OF TUBULAR CRYSTALS - IMPROVEMENTS IN THE ACETYLCHOLINE-RECEPTOR STRUCTURE

Biological molecules often crystallize either as tubes: having helical symmetry, or as two-dimensional sheets. Both sorts of crystal are pot entially suitable for structure determination to atomic resolution by electron crystallography, but their lattice distortions must first be corrected. We have developed a procedure for tubular crystals, based o n independent alignment of very short segments against a reference str ucture, that allows accurate determination and correction of distortio ns in all three dimensions. Application of this procedure to images us ed. previously to determine the 9 Angstrom structure of the acetylchol ine receptor showed that about half of the signal loss caused by the d istortions arises from effects correctable in the image plane (bending , changes in scale) and half from effects requiring out-of-plane corre ction (variations in tilt and in twist around the tube axis). By divid ing the tubes into short segments (of lengths about equal to their dia meter) it became possible to recover almost all of this loss without r educing appreciably the accuracy in the segmental alignments. The sign al retention improved by only 10% at low resolution (20 Angstrom), but by progressively greater amounts at higher resolutions, up to similar to 40% at 9 Angstrom. As a result the finer structural details were m ore clearly resolved. With images of better electron-optical quality, much greater gains in signal retention should be obtained.