The molecular chaperone, Atp12p, from Homo sapiens. In vitro studies with purified wild type and mutant (E240K) proteins

Factors affecting safety performance in the construction industry: an empirical study using structural equation modelling

The Saudi construction industry is among the largest in the region - and, for workers, among the most dangerous industries. The importance of this study is assisting to reduce hazards, sources of risk and perceptions of safety in the construction sector. Using a quantitative survey measure administered to a small (n = 276) sample of individuals, this study aimed to contribute to empirical understandings of safety performance in this unique context. A multivariate safety performance model was developed to ensure compatibility with the structure of the survey measure. The survey data revealed a strong consensus expressing negative views of every safety dimension and variable tested, with only tiny minorities selecting positively valenced responses. To test the descriptive power of the model as a whole, a structural equation modelling technique was used to assess the correspondence between the relationships constituting the model and their significance relative to empirical data.

Expression of mammalian mitochondrial F1-ATPase in Escherichia coli depends on two chaperone factors, AF1 and AF2

F₁‐ATPase (F₁) is a multisubunit water‐soluble domain of F_oF_1‐ATP synthase and is a rotary enzyme by itself. Earlier genetic studies using yeast suggested that two factors, Atp11p and Atp12p, contribute to F₁ assembly. Here, we show that their mammalian counterparts, AF1 and AF2, are essential and sufficient for efficient production of recombinant bovine mitochondrial F₁ in Escherichia coli cells. Intactness of the function and conformation of the E. coli‐expressed bovine F₁ was verified by rotation analysis and crystallization. This expression system opens a way for the previously unattempted mutation study of mammalian mitochondrial F₁.

Defining the pathogenesis of the human Atp12p W94R mutation using a Saccharomyces cerevisiae yeast model

Studies in yeast have shown that a deficiency in Atp12p prevents assembly of the extrinsic domain (F(1)) of complex V and renders cells unable to make ATP through oxidative phosphorylation. De Meirleir et al. (De Meirleir, L., Seneca, S., Lissens, W., De Clercq, I., Eyskens, F., Gerlo, E., Smet, J., and Van Coster, R. (2004) J. Med. Genet. 41, 120-124) have reported that a homozygous missense mutation in the gene for human Atp12p (HuAtp12p), which replaces Trp-94 with Arg, was linked to the death of a 14-month-old patient. We have investigated the impact of the pathogenic W94R mutation on Atp12p structure/function. Plasmid-borne wild type human Atp12p rescues the respiratory defect of a yeast ATP12 deletion mutant (Deltaatp12). The W94R mutation alters the protein at the most highly conserved position in the Pfam sequence and renders HuAtp12p insoluble in the background of Deltaatp12. In contrast, the yeast protein harboring the corresponding mutation, ScAtp12p(W103R), is soluble in the background of Deltaatp12 but not in the background of Deltaatp12Deltafmc1, a strain that also lacks Fmc1p. Fmc1p is a yeast mitochondrial protein not found in higher eukaryotes. Tryptophan 94 (human) or 103 (yeast) is located in a positively charged region of Atp12p, and hence its mutation to arginine does not alter significantly the electrostatic properties of the protein. Instead, we provide evidence that the primary effect of the substitution is on the dynamic properties of Atp12p.

Chaperones of F1-ATPase

Mitochondrial F(1)-ATPase contains a hexamer of alternating alpha and beta subunits. The assembly of this structure requires two specialized chaperones, Atp11p and Atp12p, that bind transiently to beta and alpha. In the absence of Atp11p and Atp12p, the hexamer is not formed, and alpha and beta precipitate as large insoluble aggregates. An early model for the mechanism of chaperone-mediated F(1) assembly (Wang, Z. G., Sheluho, D., Gatti, D. L., and Ackerman, S. H. (2000) EMBO J. 19, 1486-1493) hypothesized that the chaperones themselves look very much like the alpha and beta subunits, and proposed an exchange of Atp11p for alpha and of Atp12p for beta; the driving force for the exchange was expected to be a higher affinity of alpha and beta for each other than for the respective chaperone partners. One important feature of this model was the prediction that as long as Atp11p is bound to beta and Atp12p is bound to alpha, the two F(1) subunits cannot interact at either the catalytic site or the noncatalytic site interface. Here we present the structures of Atp11p from Candida glabrata and Atp12p from Paracoccus denitrificans, and we show that some features of the Wang model are correct, namely that binding of the chaperones to alpha and beta prevents further interactions between these F(1) subunits. However, Atp11p and Atp12p do not resemble alpha or beta, and it is instead the F(1) gamma subunit that initiates the release of the chaperones from alpha and beta and their further assembly into the mature complex.

Drosophila frataxin: an iron chaperone during cellular Fe-S cluster bioassembly

Frataxin, a mitochondrial protein that is directly involved in regulating cellular iron homeostasis, has been suggested to serve as an iron chaperone during cellular Fe-S cluster biosynthesis. In humans, decreased amounts or impaired function of frataxin causes the autosomal recessive neurodegenerative disorder Friedreich's ataxia. Cellular production of Fe-S clusters is accomplished by the Fe cofactor assembly platform enzymes Isu (eukaryotes) and IscU (prokaryotes). In this report, we have characterized the overall stability and iron binding properties of the Drosophila frataxin homologue (Dfh). Dfh is highly folded with secondary structural elements consistent with the structurally characterized frataxin orthologs. While the melting temperature ( T M approximately 59 degrees C) and chemical stability ([urea] 50% approximately 2.4 M) of Drosophila frataxin, measured using circular dichroism (CD) and fluorescence spectroscopy, closely match values determined for the human ortholog, pure Dfh is more stable against autodegradation than both the human and yeast proteins. The ferrous iron binding affinity ( K d approximately 6.0 microM) and optimal metal to protein stoichiometry (1:1) for Dfh have been measured using isothermal titration calorimetry (ITC). Under anaerobic conditions with salt present, holo-Dfh is a stable iron-loaded protein monomer. Frataxin prevents reactive oxygen species-induced oxidative damage to DNA when presented with both Fe(II) and H 2O 2. Ferrous iron bound to Dfh is high-spin and held in a partially symmetric Fe-(O/N) 6 coordination environment, as determined by X-ray absorption spectroscopy (XAS). Extended X-ray absorption fine structure (EXAFS) simulations indicate the average Fe-O/N bond length in Dfh is 2.13 A, consistent with a ligand geometry constructed by water and carboxylate oxygens most likely supplied in part by surface-exposed conserved acidic residues located on helix 1 and strand 1 in the structurally characterized frataxin orthologs. The iron-dependent binding affinity ( K d approximately 0.21 microM) and optimal holo-Dfh to Isu monomer stoichiometry (1:1) have also been determined using ITC. Finally, frataxin mediates the delivery of Fe(II) to Isu, promoting Fe-S cluster assembly in vitro. The Dfh-assisted assembly of Fe-S clusters occurs with an observed kinetic rate constant ( k obs) of 0.096 min (-1).

Deficiency of mitochondrial ATP synthase of nuclear genetic origin

We present clinical and laboratory data from 14 cases with an isolated deficiency of the mitochondrial ATP synthase (7-30% of control) caused by nuclear genetic defects. A quantitative decrease of the ATP synthase complex was documented by Blue-Native electrophoresis and Western blotting and was supported by the diminished activity of oligomycin/aurovertin-sensitive ATP hydrolysis in fibroblasts (10 cases), muscle (6 of 7 cases), and liver (one case). All patients had neonatal onset and elevated plasma lactate levels. In 12 patients investigated 3-methyl-glutaconic aciduria was detected. Seven patients died, mostly within the first weeks of life and surviving patients showed psychomotor and various degrees of mental retardation. Eleven patients had hypertrophic cardiomyopathy; other clinical signs included hypotonia, hepatomegaly, facial dysmorphism and microcephaly. This phenotype markedly differs from the severe central nervous system changes of ATP synthase disorders caused by mitochondrial DNA mutations of the ATP6 gene presenting mostly as NARP and MILS.

Assembly factors of F1Fo-ATP synthase across genomes

Work with respiration-deficient strains of Saccharomyces cerevisiae has provided evidence that assembly of the mitochondrial ATP synthase is dependent on proteins that serve substrate-specific, chaperone-type functions: Atp10p, Atp11p, Atp12p, Atp22p, and Fmc1p. Atp11p and Atp12p mediate the formation of the F1 moiety via interaction with subunits F1-beta and F1-alpha, respectively. The role of Fmc1p is less clear. Atp10p and Atp22p are essential for the formation of the F(O) part, during which Atp10p assists in the incorporation of the F(O)-a subunit. Here we present a comprehensive analysis of ATP synthase assembly factors from all available genomes. The mechanism of the F1 assembly is preserved in all eukaryotic lineages that are capable of ATP synthesis via oxidative phosphorylation and requires Atp11p and Atp12p. Conversely, composition of the F(O) part as well as its assembly is more versatile. We found two distinct subtypes of the F(O)-a subunit, one of which seems to be dependent on the action of Atp10p while the other does not. Restricted occurrence of Fmc1p and Atp22p suggests the existence of lineage-specific assembly factors. Our phylogenetic data served as a source for comparative sequence analysis, which identified evolutionarily conserved residues, putative functional domains and their basic structural features for Atp10p, Atp11p, and Atp12p orthologs. These results provide the basis for detailed molecular analysis of the ATP synthase-specific chaperones.

The pathology of cellular anti-stress mechanisms: a new frontier

Exposure to stressors is an omnipresent variable for all living organisms, which have evolved anti-stress mechanisms to deal with the consequences of stress. The chaperoning systems are among these mechanisms, and their central components are the molecular chaperones that play important roles in protein biogenesis. Recent data suggest that failure of the chaperoning systems due to defective chaperones, for example, leads to pathology. Consequently, medical researchers and practitioners must now also consider the chaperoning systems, both as potentially major players in pathogenesis and as diagnostic-prognostic indicators.

Mitochondrial diseases and ATPase defects of nuclear origin

Dysfunctions of the F(1)F(o)-ATPase complex cause severe mitochondrial diseases affecting primarily the paediatric population. While in the maternally inherited ATPase defects due to mtDNA mutations in the ATP6 gene the enzyme is structurally and functionally modified, in ATPase defects of nuclear origin mitochondria contain a decreased amount of otherwise normal enzyme. In this case biosynthesis of ATPase is down-regulated due to a block at the early stage of enzyme assembly-formation of the F(1) catalytic part. The pathogenetic mechanism implicates dysfunction of Atp12 or other F(1)-specific assembly factors. For cellular energetics, however, the negative consequences may be quite similar irrespective of whether the ATPase dysfunction is of mitochondrial or nuclear origin.