Additive effect of nuclear and mitochondrial mutations in a patient with mitochondrial encephalomyopathy.

We describe the case of a woman in whom combination of a mitochondrial (MT-CYB) and a nuclear (SDHB) mutation was associated with clinical and metabolic features suggestive of a mitochondrial disorder. The mutations impaired overall energy metabolism in the patient's muscle and fibroblasts and increased cellular susceptibility to oxidative stress. To clarify the contribution of each mutation to the phenotype, mutant yeast strains were generated. A significant defect in strains carrying the Sdh2 mutation, either alone or in combination with the cytb variant, was observed. Our data suggest that the SDHB mutation was causative of the mitochondrial disorder in our patient with a possible cumulative contribution of the MT-CYB variant. To our knowledge, this is the first association of bi-genomic variants in the mtDNA and in a nuclear gene encoding a subunit of complex II.

Structural engineering of a phage lysin that targets gram-negative pathogens.

Bacterial pathogens are becoming increasingly resistant to antibiotics. As an alternative therapeutic strategy, phage therapy reagents containing purified viral lysins have been developed against gram-positive organisms but not against gram-negative organisms due to the inability of these types of drugs to cross the bacterial outer membrane. We solved the crystal structures of a Yersinia pestis outer membrane transporter called FyuA and a bacterial toxin called pesticin that targets this transporter. FyuA is a ß-barrel membrane protein belonging to the family of TonB dependent transporters, whereas pesticin is a soluble protein with two domains, one that binds to FyuA and another that is structurally similar to phage T4 lysozyme. The structure of pesticin allowed us to design a phage therapy reagent comprised of the FyuA binding domain of pesticin fused to the N-terminus of T4 lysozyme. This hybrid toxin kills specific Yersinia and pathogenic E. coli strains and, importantly, can evade the pesticin immunity protein (Pim) giving it a distinct advantage over pesticin. Furthermore, because FyuA is required for virulence and is more common in pathogenic bacteria, the hybrid toxin also has the advantage of targeting primarily disease-causing bacteria rather than indiscriminately eliminating natural gut flora.

Mapping the interactions between Escherichia coli TolQ transmembrane segments.

The tolQRAB-pal operon is conserved in Gram-negative genomes. The TolQRA proteins of Escherichia coli form an inner membrane complex in which TolQR uses the proton-motive force to regulate TolA conformation and the in vivo interaction of TolA C-terminal region with the outer membrane Pal lipoprotein. The stoichiometry of the TolQ, TolR, and TolA has been estimated and suggests that 4-6 TolQ molecules are associated in the complex, thus involving interactions between the transmembrane helices (TMHs) of TolQ, TolR, and TolA. It has been proposed that an ion channel forms at the interface between two TolQ and one TolR TMHs involving the TolR-Asp(23), TolQ-Thr(145), and TolQ-Thr(178) residues. To define the organization of the three TMHs of TolQ, we constructed epitope-tagged versions of TolQ. Immunodetection of in vivo and in vitro chemically cross-linked TolQ proteins showed that TolQ exists as multimers in the complex. To understand how TolQ multimerizes, we initiated a cysteine-scanning study. Results of single and tandem cysteine substitution suggest a dynamic model of helix interactions in which the hairpin formed by the two last TMHs of TolQ change conformation, whereas the first TMH of TolQ forms intramolecular interactions.

Is cytochrome b glutamic acid 272 a quinol binding residue in the bc1 complex of Saccharomyces cerevisiae?

The mitochondrial bc1 complex catalyzes the oxidation of ubiquinol and the reduction of cytochrome (cyt) c coupled to a vectorial translocation of protons across the membrane. On the basis of the three-dimensional structures of the bc1 complex in the presence of the inhibitor stigmatellin, it was assumed that the substrate quinol binding involves the cyt b glutamate residue E272 and the histidine 181 on the Rieske protein. Although extensive mutagenesis of glutamate E272 has been carried out, different experimental results were recently obtained, and different conclusions were drawn to explain its role in the bifurcated electron/proton transfer at the QO site. This residue is not totally conserved during evolution. We show in this study that replacement of E272 with apolar residues proline and valine naturally present in some organisms did not abolish the bc1 activity, although it slowed down the kinetics of electron transfer. The Km value for the binding of the substrate quinol was not modified, and the EPR data showed that the quinone/quinol binding still occurred in the mutants. Binding of stigmatellin was retained; however, mutations E272P,V induced resistance toward the QO site inhibitor myxothiazol. The pH dependence of the bc1 activity was not modified in the absence of the glutamate E272. Our results suggest that this residue may not be involved in direct substrate binding or in its direct deprotonation. Revertants were selected from the respiratory deficient mutant E272P. The observed suppressor mutations introduced polar residues serine and threonine at position 272. The data lead us to suggest that E272 may be involved in a later step on the proton exit pathway via the interaction with a water molecule.