Crystallization of mitochondrial cytochrome b-c1 complex from gel with or without reduced pressure

Structure and function of cytochrome bc complexes

The cytochrome bc complexes represent a phylogenetically diverse group of complexes of electron-transferring membrane proteins, most familiarly represented by the mitochondrial and bacterial bc1 complexes and the chloroplast and cyanobacterial b6f complex. All these complexes couple electron transfer to proton translocation across a closed lipid bilayer membrane, conserving the free energy released by the oxidation-reduction process in the form of an electrochemical proton gradient across the membrane. Recent exciting developments include the application of site-directed mutagenesis to define the role of conserved residues, and the emergence over the past five years of X-ray structures for several mitochondrial complexes, and for two important domains of the b6f complex.

Activation of a matrix processing peptidase from the crystalline cytochrome bc1 complex of bovine heart mitochondria

No mitochondrial processing peptidase (MPP) activity is detected in crystalline bovine heart mitochondrial cytochrome bc1 complex, which possesses full electron transfer activity. However, when the complex is treated with increasing concentrations of Triton X-100 at 37 degreesC, the electron transfer activity decreases, whereas peptidase activity increases. Maximum MPP activity is obtained when the electron transfer activity in the complex is completely inactivated with 1.5 mM of Triton X-100. This result supports our suggestion that the lack of MPP activity in the mammalian cytochrome bc1 complex is because of binding of an inhibitor polypeptide to the active site of MPP located at the interface of core subunits I and II. This suggestion is based on the three-dimensional structural information for the bc1 complex and the sequence homology between subunits of MPP and the core subunits of the beef complex. Triton X-100, at concentrations that disrupt the structural integrity of the bc1 complex as indicated by the loss of its electron transfer activity, weakens the binding of inhibitor polypeptide to the active site of MPP in core subunits, thus activating MPP. The Triton X-100-activated MPP is pH-, buffer system-, ionic strength-, and temperature-dependent. Maximum activity is observed with an assay mixture containing 15 mM Tris-HCl buffer at neutral pH (6.5-8.5) and at 37 degreesC. Activated MPP is completely inhibited by metal ion chelators such as EDTA and o-phenanthroline and partially inhibited by myxothiazol (58%), ferricyanide (28%), and dithiothreitol (81%). The metal ion chelator-inhibited activity can be partially restored by the addition of divalent cations such as Zn2+ (68%), Mg2+ (44%), Mn2+ (54%), Co2+ (62%), and Fe2+ (92%), indicating that metal ion is required for MPP activity. The cleavage site specificity of activated MPP depends more on the length of amino acid sequence from the mature protein portion and less on the presequence portion, when a synthetic peptide composed of NH2-terminal residues of a mature protein and the COOH-terminal residues of its presequence is used as a substrate.

Crystal structure of the cytochrome bc1 complex from bovine heart mitochondria

On the basis of x-ray diffraction data to a resolution of 2.9 angstroms, atomic models of most protein components of the bovine cytochrome bc1 complex were built, including core 1, core 2, cytochrome b, subunit 6, subunit 7, a carboxyl-terminal fragment of cytochrome c1, and an amino-terminal fragment of the iron-sulfur protein. The positions of the four iron centers within the bc1 complex and the binding sites of the two specific respiratory inhibitors antimycin A and myxothiazol were identified. The membrane-spanning region of each bc1 complex monomer consists of 13 transmembrane helices, eight of which belong to cytochrome b. Closely interacting monomers are arranged as symmetric dimers and form cavities through which the inhibitor binding pockets can be accessed. The proteins core 1 and core 2 are structurally similar to each other and consist of two domains of roughly equal size and identical folding topology.

Crystallization and preliminary structure of beef heart mitochondrial cytochrome-bc1 complex

The method reported for isolation of ubiquinol-cytochrome-c reductase complex from submitochondrial particles was modified to yield a preparation for crystallization. The cytochrome bc1 complex was first crystallized in large thin plate form and diffracts X-rays to 7 A resolution in the presence of mother liquor. This crystalline complex was enzymatically active and contains ten protein subunits. It had 33 mol phospholipid and 0.6 mol ubiquinone per mol protein. With slightly modified crystallization conditions, different crystal forms were obtained. Crystals grown in the presence of 20% glycerol diffracted X-rays up to 2.9 A resolution using a synchrotron source. Four heavy atom derivatives have been obtained. The 3-D structure of the cytochrome bc1 complex was solved to 3.4 A resolution. Crystalline cytochrome bc1 complex is a dimer: most of the masses of core proteins I and II protrudes from the matrix side of the membrane, whereas the cytochrome b protein is located mainly within the membrane. There are 13 transmembrane helices in each monomer. Most of the mass of cytochrome c1 and iron-sulfur protein including their redox centers are located on the cytoplasmic side of the membrane. The distances between these redox centers have been determined, and several electron transfer inhibitor binding sites in the complex have been located.

Immunoelectron microscopy and epitope mapping with monoclonal antibodies suggest the existence of an additional N-terminal transmembrane helix in the cytochrome b subunit of bacterial ubiquinol:cytochrome-c oxidoreductases

The topology of the ubiquinol:cytochrome-c oxidoreductase (cytochrome bc1 complex) from Paracoccus denitrificans was investigated by immunoelectron microscopy with sequence-specific murine monoclonal antibodies. Epitope mapping with synthetic peptides and enzymic proteolytic cleavage of the cytochrome bc1 complex were employed to localize precisely the respective antibody epitopes on the subunits of this membrane protein complex. Localization of defined epitopes on the cytochrome bc1 complex by immunoelectron microscopy clearly demonstrates that the N-terminus of the cytochrome b subunit is exposed to the periplasmic space. This finding is in agreement with a nine-transmembrane-helices topology model (I-IX) as predicted before for cytochrome b. However, due to other published evidence we favour the existence of an additional transmembrane helix (helix 0) complementing a more recently published eight-helices model (A-C,cd, D-H), at least for prokaryotes.