Structure and function of the mitochondrial bc1 complex. Properties of the complex in temperature-sensitive cor1 mutants

Dimeric and high-resolution structures of Chlamydomonas Photosystem I from a temperature-sensitive Photosystem II mutant

Water molecules play a pivotal functional role in photosynthesis, primarily as the substrate for Photosystem II (PSII). However, their importance and contribution to Photosystem I (PSI) activity remains obscure. Using a high-resolution cryogenic electron microscopy (cryo-EM) PSI structure from a Chlamydomonas reinhardtii temperature-sensitive photoautotrophic PSII mutant (TSP4), a conserved network of water molecules - dating back to cyanobacteria - was uncovered, mainly in the vicinity of the electron transport chain (ETC). The high-resolution structure illustrated that the water molecules served as a ligand in every chlorophyll that was missing a fifth magnesium coordination in the PSI core and in the light-harvesting complexes (LHC). The asymmetric distribution of the water molecules near the ETC branches modulated their electrostatic landscape, distinctly in the space between the quinones and FX. The data also disclosed the first observation of eukaryotic PSI oligomerisation through a low-resolution PSI dimer that was comprised of PSI-10LHC and PSI-8LHC.

Caspy et al. report the structure of PSI from a temperature-sensitive photoautotrophic PSII mutant of Chlamydomonas reinhardtii (TSP4), and report the distribution of conserved water molecules in the structure from cyanobacterial to higher plant PSI. They suggest that the asymmetric distribution of water molecules near the electron transfer chain modulates the electron transfer from quinones to FX.

New insights into the co-evolution of cytochrome c reductase and the mitochondrial processing peptidase

The mitochondrial processing peptidase (MPP) is a heterodimeric enzyme that forms part of the cytochrome c reductase complex from higher plants. Mitochondria from mammals and yeast contain two homologous enzymes: (i) an active MPP within the mitochondrial matrix and (ii) an inactive MPP within the cytochrome c reductase complex. To elucidate the evolution of MPP, the cytochrome c reductase complexes from lower plants were isolated and tested for processing activity. Mitochondria were prepared from the staghorn fern Platycerium bifurcatum, from the horsetail Equisetum arvense, and from the colorless algae Polytomella, and cytochrome c reductase complexes were purified by a micro-isolation procedure based on Blue-native polyacrylamide gel electrophoresis and electroelution. This is the first report on the subunit composition of a respiratory enzyme complex from a fern or a horsetail. The cytochrome c reductase complexes from P. bifurcatum and E. arvense are shown to efficiently process mitochondrial precursor proteins, whereas the enzyme complex from Polytomella lacks proteolytic activity. An evolutionary model is suggested that assumes a correlation between the presence of an active MPP within the cytochrome c reductase complex and the occurrence of chloroplasts.

The COQ7 gene encodes a protein in saccharomyces cerevisiae necessary for ubiquinone biosynthesis

Ubiquinone (coenzyme Q) is a lipid that transports electrons in the respiratory chains of both prokaryotes and eukaryotes. Mutants of Saccharomyces cerevisiae deficient in ubiquinone biosynthesis fail to grow on nonfermentable carbon sources and have been classified into eight complementation groups (coq1 coq8; Tzagoloff, A., and Dieckmann, C. L.(1990) Microbiol. Rev. 54, 211-225). In this study we show that although yeast coq7 mutants lack detectable ubiquinone, the coq7 1 mutant does synthesize demethoxyubiquinone (2-hexaprenyl-3-methyl-6-methoxy-1,4-benzoquinone), a ubiquinone biosynthetic intermediate. The corresponding wild-type COQ7 gene was isolated, sequenced, and found to restore growth on nonfermentable carbon sources and the synthesis of ubiquinone. The sequence predicts a polypeptide of 272 amino acids which is 40% identical to a previously reported Caenorhabditis elegans open reading frame. Deletion of the chromosomal COQ7 gene generates respiration defective yeast mutants deficient in ubiquinone. Analysis of several coq7 deletion strains indicates that, unlike the coq7 1 mutant, demethoxyubiquinone is not produced. Both coq7 1 and coq7 deletion mutants, like other coq mutants, accumulate an early intermediate in the ubiquinone biosynthetic pathway, 3-hexaprenyl-4-hydroxybenzoate. The data suggest that the yeast COQ7 gene may encode a protein involved in one or more monoxygenase or hydroxylase steps of ubiquinone biosynthesis.

Structure-function relationships of the mitochondrial bc1 complex in temperature-sensitive mutants of the cytochrome b gene, impaired in the catalytic center N

Seven new structures of cytochrome b have been recently identified by isolating and sequencing revertants from cytochrome b respiratory deficient mutants (Coppée, J. Y., Brasseur, G., Brivet-Chevillotte, P., and Colson, A. M. (1994) J. Biol. Chem. 269, 4221-4226). These mutations are located in the center N domain (QN). All the revertants exhibited a modified heme b562 maximum, confirming that part of the NH2-terminal region is in the vicinity of the extramembranous loop between helices IV-V and heme b562. Based on measurements performed on the maximal activities occurring in each segment of the respiratory chain, the decrease observed in the NADH oxidase activities of several revertants was correlated with some bc1 complex activity impairments; this may also explain why a moderate decrease in bc1 complex activity does not limit the succinate oxidase activity. The decrease in the rate of reduction of cytochrome b via the center N pathway is responsible for the impairment of the bc1 complex activity of these revertants. The three double-mutated revertants (S206L/N208K or -Y; S206L/W30C) are temperature-sensitive in vivo, and their mitochondria like that of the original mutant S206L are thermosensitive in vitro. Isolating the W30C mutation does not yield a thermosensitive phenotype: the replacement of serine 206 by leucine is therefore responsible for the thermoinstability of these strains; this temperature sensitivity is reinforced by additional mutations N208K or N208Y, and not by W30C. These data suggest that serine 206 and asparagine 208 are involved in the thermostability of the protein. When bc1 complex activity is lost after incubating mitochondria at a nonpermissive temperature (37 degrees C), heme b is still present, but can no longer be reduced by physiological substrate. The progressive loss of bc1 complex activity seems to be initially linked to a change in the tertiary structure of cytochrome b, which occurs drastically at center N and much more slowly at center P, as shown by kinetic study on the two cytochrome b redox pathways.

The bifunctional cytochrome c reductase/processing peptidase complex from plant mitochondria

Cytochrome c reductase from potato has been extensively studied with respect to its catalytic activities, its subunit composition, and the biogenesis of individual subunits. Molecular characterization of all 10 subunits revealed that the high-molecular-weight subunits exhibit striking homologies with the components of the general mitochondrial processing peptidase (MPP) from fungi and mammals. Some of the other subunits show differences in the structure of their targeting signals or in their molecular composition when compared to their counterparts from heterotrophic organisms. The proteolytic activity of MPP was found in the cytochrome c reductase complexes from potato, spinach, and wheat, suggesting that the integration of the protease into this respiratory complex is a general feature of higher plants.

Are the 'core' proteins of the mitochondrial bc1 complex evolutionary relics of a processing protease?

The so-called 'core' proteins of the respiratory cytochrome bc1 complex and the two subunits of the mitochondrial processing peptidase (MPP) are structurally similar but their evolutionary relationship remains a mystery. Here, we present a model suggesting that the core proteins originated from an ancient proteolytic enzyme that was integrated into the bc1 complex during early stages of endosymbiosis.

Subunit structures of purified beef mitochondrial cytochrome bc1 complex from liver and heart

The existence of tissue-specific isozymes of cytochrome c oxidase has been widely documented. We have now studied if there are differences between subunits of mitochondrial bc1 complexes isolated from liver and heart. For this purpose, we have developed a method for the purification of an active ubiquinol-cytochrome c oxidoreductase from adult bovine liver that includes solubilization of submitochondrial particles with deoxycholate, ammonium acetate fractionation, resolubilization with dodecyl maltoside, and ion exchange chromatography. The electrophoretic pattern of the liver preparation showed the presence of 11 subunits, with apparent molecular weights identical to the ones reported for the heart complex. Western blot analysis and isoelectric focusing followed by two-dimensional gels of bc1 complexes from liver and heart were compared, and no qualitative differences were observed. In addition, the high-molecular-weight subunits of the purified complexes from both tissues, subunits I, II, V, and VI, were isolated by PAGE in the presence of Coomasie Blue and subjected to limited proteolysis and to chemical digestion with cyanogen bromide and BNPS-skatol, and the peptide patterns were compared. Finally, two of the small-molecular-weight subunits from the liver complex were isolated (subunits VII and X), partially analyzed by amino terminal sequencing, and found to be identical with the reported sequence of their heart counterparts. The data suggest that, in contrast to the case of cytochrome c oxidase, bc1 complexes from liver and heart do not exhibit tissue-specific differences.