Uncovering the molecular mode of action of the antimalarial drug atovaquone using a bacterial system.

Atovaquone is an antiparasitic drug that selectively inhibits electron transport through the parasite mitochondrial cytochrome bc1 complex and collapses the mitochondrial membrane potential at concentrations far lower than those at which the mammalian system is affected. Because this molecule represents a new class of antimicrobial agents, we seek a deeper understanding of its mode of action. To that end, we employed site-directed mutagenesis of a bacterial cytochrome b, combined with biophysical and biochemical measurements. A large scale domain movement involving the iron-sulfur protein subunit is required for electron transfer from cytochrome b-bound ubihydroquinone to cytochrome c1 of the cytochrome bc1 complex. Here, we show that atovaquone blocks this domain movement by locking the iron-sulfur subunit in its cytochrome b-binding conformation. Based on our malaria atovaquone resistance data, a series of cytochrome b mutants was produced that were predicted to have either enhanced or reduced sensitivity to atovaquone. Mutations altering the bacterial cytochrome b at its ef loop to more closely resemble Plasmodium cytochrome b increased the sensitivity of the cytochrome bc1 complex to atovaquone. A mutation within the ef loop that is associated with resistant malaria parasites rendered the complex resistant to atovaquone, thereby providing direct proof that the mutation causes atovaquone resistance. This mutation resulted in a 10-fold reduction in the in vitro activity of the cytochrome bc1 complex, suggesting that it may exert a cost on efficiency of the cytochrome bc1 complex.

A novel human cancer vaccine elicits cellular responses to the tumor-associated antigen, human chorionic gonadotropin beta.

PURPOSE: The oncofetal antigen, human chorionic gonadotropin beta subunit (hCGbeta), is expressed by a number of carcinomas and is a prognostic indicator in renal, colorectal, bladder, and pancreatic cancers. We describe the development of a novel antibody-based dendritic cell (DC)-targeted cancer vaccine capable of eliciting cellular immune responses directed against hCGbeta. EXPERIMENTAL DESIGN: The tumor-associated antigen hCGbeta was coupled genetically to a human anti-DC antibody (B11). The resulting fusion protein (B11-hCGbeta) was evaluated for its ability to promote tumor antigen-specific cellular immune responses in a human in vitro model. Monocyte-derived human DCs from normal donors were exposed to purified B11-hCGbeta, activated with CD40 ligand, mixed with autologous lymphocytes, and tested for their ability to promote hCGbeta-specific proliferative and cytotoxic T-lymphocyte responses. RESULTS: B11-hCGbeta was found to be a soluble, well-defined, and readily purified product that specifically recognized the human mannose receptor via the B11 antibody portion of the fusion protein. B11-hCGbeta functionally promoted the uptake and processing of tumor antigen by DCs, which led to the generation of tumor-specific HLA class I and class II-restricted T-cell responses, including CTLs capable of killing human cancer cell lines expressing hCGbeta. CONCLUSIONS: Although other hCG vaccines have been shown to be capable of eliciting antibody responses to hCGbeta, this is the first time that cellular immune responses to hCGbeta have been induced by a vaccine in a human system. This DC-targeted hCGbeta vaccine holds promise for the management of a number of cancers and merits additional clinical development.

X-Ray Structure of Rhodobacter Capsulatus Cytochrome bc (1): Comparison with its Mitochondrial and Chloroplast Counterparts.

Ubihydroquinone: cytochrome (cyt)c oxidoreductase, or cyt bc (1), is a widespread, membrane integral enzyme that plays a crucial role during photosynthesis and respiration. It is one of the major contributors of the electrochemical proton gradient, which is subsequently used for ATP synthesis. The simplest form of the cyt bc (1) is found in bacteria, and it contains only the three ubiquitously conserved catalytic subunits: the Fe-S protein, cyt b and cyt c (1). Here we present a preliminary X-ray structure of Rhodobacter capsulatus cyt bc (1) at 3.8 A and compare it to the available structures of its homologues from mitochondria and chloroplast. Using the bacterial enzyme structure, we highlight the structural similarities and differences that are found among the three catalytic subunits between the members of this family of enzymes. In addition, we discuss the locations of currently known critical mutations, and their implications in terms of the cyt bc (1) catalysis.

The dithiol:disulfide oxidoreductases DsbA and DsbB of Rhodobacter capsulatus are not directly involved in cytochrome c biogenesis, but their inactivation restores the cytochrome c biogenesis defect of CcdA-null mutants.

The cytoplasmic membrane protein CcdA and its homologues in other species, such as DsbD of Escherichia coli, are thought to supply the reducing equivalents required for the biogenesis of c-type cytochromes that occurs in the periplasm of gram-negative bacteria. CcdA-null mutants of the facultative phototroph Rhodobacter capsulatus are unable to grow under photosynthetic conditions (Ps(-)) and do not produce any active cytochrome c oxidase (Nadi(-)) due to a pleiotropic cytochrome c deficiency. However, under photosynthetic or respiratory growth conditions, these mutants revert frequently to yield Ps(+) Nadi(+) colonies that produce c-type cytochromes despite the absence of CcdA. Complementation of a CcdA-null mutant for the Ps(+) growth phenotype was attempted by using a genomic library constructed with chromosomal DNA from a revertant. No complementation was observed, but plasmids that rescued a CcdA-null mutant for photosynthetic growth by homologous recombination were recovered. Analysis of one such plasmid revealed that the rescue ability was mediated by open reading frame 3149, encoding the dithiol:disulfide oxidoreductase DsbA. DNA sequence data revealed that the dsbA allele on the rescuing plasmid contained a frameshift mutation expected to produce a truncated, nonfunctional DsbA. Indeed, a dsbA ccdA double mutant was shown to be Ps(+) Nadi(+), establishing that in R. capsulatus the inactivation of dsbA suppresses the c-type cytochrome deficiency due to the absence of ccdA. Next, the ability of the wild-type dsbA allele to suppress the Ps(+) growth phenotype of the dsbA ccdA double mutant was exploited to isolate dsbA-independent ccdA revertants. Sequence analysis revealed that these revertants carried mutations in dsbB and that their Ps(+) phenotypes could be suppressed by the wild-type allele of dsbB. As with dsbA, a dsbB ccdA double mutant was also Ps(+) Nadi(+) and produced c-type cytochromes. Therefore, the absence of either DsbA or DsbB restores c-type cytochrome biogenesis in the absence of CcdA. Finally, it was also found that the DsbA-null and DsbB-null single mutants of R. capsulatus are Ps(+) and produce c-type cytochromes, unlike their E. coli counterparts, but are impaired for growth under respiratory conditions. This finding demonstrates that in R. capsulatus the dithiol:disulfide oxidoreductases DsbA and DsbB are not essential for cytochrome c biogenesis even though they are important for respiration under certain conditions.

The [2Fe-2S] cluster E(m) as an indicator of the iron-sulfur subunit position in the ubihydroquinone oxidation site of the cytochrome bc1 complex.

Recent crystallographic and kinetic data have revealed the crucial role of the large scale domain movement of the iron-sulfur subunit [2Fe-2S] cluster domain during the ubihydroquinone oxidation reaction catalyzed by the cytochrome bc(1) complex. Previously, the electron paramagnetic resonance signature of the [2Fe-2S] cluster and its redox midpoint potential (E(m)) value have been used extensively to characterize the interactions of the [2Fe-2S] cluster with the occupants of the ubihydroquinone oxidation (Q(o)) catalytic site. In this work we analyze these interactions in various iron-sulfur subunit mutants that carry mutations in its flexible hinge region. We show that the E(m) increases of the iron-sulfur subunit [2Fe-2S] cluster induced either by these mutations or by the addition of stigmatellin do not act synergistically. Moreover, the E(m) increases disappear in the presence of class I inhibitors like myxothiazol. Because various inhibitors are known to affect the location of the iron-sulfur subunit cluster domain, the measured E(m) value of the [2Fe-2S] cluster therefore reflects its equilibrium position in the Q(o) site. We also demonstrate the existence in this site of a location where the E(m) of the cluster is increased by about 150 mV and discuss its possible implications in term of Q(o) site catalysis and energetics.