Cryo-electron crystallography of two sub-complexes of bovine complex I reveals the relationship between the membrane and peripheral arms

NADH:ubiquinone oxidoreductase (complex I) is the first and largest enzyme of the mitochondrial respiratory chain. The low-resolution structure of the complex is known from electron microscopy studies. The general shape of the complex is in the form of an L, with one arm in the membrane and the other peripheral. We have purified complex I from beef heart mitochondria and reconstituted the enzyme into lipid bilayers. Under different conditions, several two-dimensional crystal forms were obtained. Crystals belonging to space groups p2221 and c12 (unit cell 488 Å x 79 Å) were obtained at 22°C and contained only the membrane fragment of complex I similar to hydrophobic subcomplex Iβ but lacking the ND5 subunit. A crystal form with larger unit cell (534 Å x 81 Å, space group c12) produced at 4°C contained both the peripheral and membrane arms of the enzyme, except that ND5 was missing. Projection maps from frozen hydrated samples were calculated for all crystal forms. By comparing two different c12 crystal forms, extra electron density in the projection map of large crystal form was assigned to the peripheral arm of the enzyme. One of the features of the map is a deep, channel-like, cleft next to peripheral arm. Comparison with available structures of the intact enzyme indicates that large hydrophobic subunit ND5 is situated at the distal end of the membrane domain. Possible locations of sub-unit ND4 and of other subunits in the membrane domain are proposed. Implications of our findings for the mechanism of proton pumping by complex I are discussed. (C) 2000 Academic Press.