THE AB INITIO SOLUTION OF THE HALORHODOPSIN AND OMP F PORIN MEMBRANE PROTEINS AT 6Å FROM ELECTRON DIFFRACTION PROJECTION DATA USING THE MAXIMUM ENTROPY-LIKELIHOOD METHOD IN THE MICE COMPUTER PROGRAM. C.J.Gilmore, W.N.Nicholson Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, Scotland U.K. and D.L.Dorset, Electron Diffraction Department, Hauptman-Woodward Institute, 73 High Street, Buffalo, New York 14203, U.S.A.

Using the combination of maximum entropy and likelihood (Bricogne (1984) Acta Cryst. A40, 410-445) in the MICE computer program (Gilmore, Bricogne & Bannister (1990) Acta Cryst. A46, 297-308), an ab initio phase determination was carried out at low resolution (6Å) for two dissimilar membrane proteins, the Omp F porin from the outer membrane of E. coli (which is largely beta-sheet) and halorhodopsin (which is largely alpha-helix). Surprisingly for a structure of this complexity and at this resolution, accurate phase information was found for the most likely solutions which enabled potential maps to be calculated that contained most of the essential structural details at 10deg. resolution of these macromolecules in projection without the need for any image derived phases. The mean phase errors for the porin structure were less than 10^deg., whilst those from halorhodopsin were less than 20^deg.. The calculations were remarkably easy using the MICE program more or less in default mode. A comparison with the use of the Sayre equation and phase annealing as an ab initio phasing procedure is made. (Dorset (1995). Proc. Natl. Acad. Sci. USA 92, 10074-10078, Dorset, Kopp, Fryer & Tivol (1995) Ultramicroscopy 57, 59-89.) Both methods have their strengths and weaknesses which will be discussed.

Research funded in part by grants from the National Institute of General Medical Sciences (GM-46733) and the Human Frontier Science Program.