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'.