The 9 angstrom projection structure of cytochrome b(6)f complex determinedby electron crystallography

Thin three-dimensional crystals of the cytochrome b(6)f complex from the un icellular algae Chlamydomonas reinhardtii have been grown by BioBeads-media ted detergent removal from a mixture of protein and lipid solubilized in He cameg. Frozen-hydrated crystals, exhibiting p22(1)2(1) plane group symmetry , were studied by electron crystallography and a projection map at 9 Angstr om resolution was calculated. The crystals (unit cell dimensions of a = 173 .5 Angstrom, b = 70.0 Angstrom and gamma = 90.0 degrees) showed the presenc e of dimers, and within each monomer 14 domains of electron density were ob served. The combination of the projection map obtained from ice-embedded cr ystals of cytochrome b(6)f with a previous map obtained from negatively sta ined samples brings new insight in the organization of the complex. For exa mple, it distinguishes some peaks and/or domains that are only extramembran e or transmembrane, and reveals the possible localization of single-strande d transmembrane alpha-helices (Pet subunits). Furthermore, the cross-correl ation of our projection map from frozen hydrated samples with the atomic mo del of the transmembrane part of the cytochrome bc(1) complex has allowed u s to localize the cytochrome b(6) at the dinner interface and to reveal str uctural differences between the two complexes. (C) 1999 Academic Press.