The pseudo-atom approach to phase determination in protein electron crystallography--noncentrosymmetric projections.
From an idea proposed by David Harker [Acta Cryst. (1953), 6, 731-736], the assembly of globular subunits in a protein can be treated as pseudo-atoms for normalizaTion of observed electron diffraction intensities. As demonstrated with published data from native or deoxycholate-treated bacteriorhodopsin, a multisolution approach via the Sayre-Hughes equation can then generate phase solutions to 6 A resolution that compare quite favorably with those determined earlier by phase extension. The major problem in such determinations is identification of the best set, especially if no lower-resolution images of the protein are available. (However, 15 to 10 A resolution image-derived phases could be used as a reference set to identify the correct solution). A viable option may be to compare Patterson maps, calculated from trial map peak positions, to the experimental autocorrelation function. Trial phase determinations for the Omp F porin from E. coli outer membrane, on the other hand, are somewhat less successful because the beta-sheet secondary structure is less well modeled by an array of 'globs'.
Dorset
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Electron Diffraction Department, Hauptman-Woodward Medical Research Institute, Inc., Buffalo, New York 14203-1196, USA. dorset@hwi.buffalo.edu
D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry; D001436:Bacteriorhodopsins / Q000737:chemistry*; D003461:Crystallography / Q000379:methods*; D003840:Deoxycholic Acid / Q000494:pharmacology; D004583:Electrons; D008565:Membrane Proteins / Q000737:chemistry; D018272:Porins / Q000737:chemistry
D001425:Bacterial Outer Membrane Proteins; D008565:Membrane Proteins; D018272:Porins; D003840:Deoxycholic Acid; D001436:Bacteriorhodopsins