Direct phase determination in protein electron crystallography: Aquaporin channel-forming integral membrane protein

The location of helix sites in the projected structure of the aquaporin channel-forming integral membrane protein from bovine red blood cells was determined by multisolution direct methods to a mean accuracy of +/-1.9 Angstrom, based on hk0 electron diffraction data extending to 6 Angstrom. The structure was assumed to be composed of pseudo-atoms, corresponding to the helix cross sections, and after re-scaling, normalized structure factors were used to order Sigma(n) triples according to the A values. Initial phases were found by symbolic addition with algebraic unknowns. Probable solutions could be isolated by an overall Luzzati test for density flatness and restrictions on local density extremes. The best solution was identified by matching Patterson functions, generated from the trial map density sites, to the one calculated from observed intensities.