The 9 Å projection structure of cytochrome b6f complex determined by electron crystallography

Thin three-dimensional crystals of the cytochrome b(6)f complex from the unicellular algae Chlamydomonas reinhardtii have been grown by BioBeads-mediated detergent removal from a mixture of protein and lipid solubilized in Hecameg. Frozen-hydrated crystals, exhibiting p22(1)2(1) plane group symmetry, were studied by electron crystallography and a projection map at 9 Angstrom resolution was calculated. The crystals (unit cell dimensions of a = 173.5 Angstrom, b = 70.0 Angstrom and gamma = 90.0 degrees) showed the presence of dimers, and within each monomer 14 domains of electron density were observed. The combination of the projection map obtained from ice-embedded crystals of cytochrome b(6)f with a previous map obtained from negatively stained samples brings new insight in the organization of the complex. For example, it distinguishes some peaks and/or domains that are only extramembrane or transmembrane, and reveals the possible localization of single-stranded transmembrane alpha-helices (Pet subunits). Furthermore, the cross-correlation of our projection map from frozen hydrated samples with the atomic model of the transmembrane part of the cytochrome bc(1) complex has allowed us to localize the cytochrome b(6) at the dinner interface and to reveal structural differences between the two complexes. (C) 1999 Academic Press.