Impact of the presequence of a mitochondrium-targeted precursor, preadrenodoxin, on folding, catalytic activity, and stability of the protein in vitro

Human Frataxin Folds Via an Intermediate State. Role of the C-Terminal Region

The aim of this study is to investigate the folding reaction of human frataxin, whose deficiency causes the neurodegenerative disease Friedreich's Ataxia (FRDA). The characterization of different conformational states would provide knowledge about how frataxin can be stabilized without altering its functionality. Wild-type human frataxin and a set of mutants, including two highly destabilized FRDA-associated variants were studied by urea-induced folding/unfolding in a rapid mixing device and followed by circular dichroism. The analysis clearly indicates the existence of an intermediate state (I) in the folding route with significant secondary structure content but relatively low compactness, compared with the native ensemble. However, at high NaCl concentrations I-state gains substantial compaction, and the unfolding barrier is strongly affected, revealing the importance of electrostatics in the folding mechanism. The role of the C-terminal region (CTR), the key determinant of frataxin stability, was also studied. Simulations consistently with experiments revealed that this stretch is essentially unstructured, in the most compact transition state ensemble (TSE2). The complete truncation of the CTR drastically destabilizes the native state without altering TSE2. Results presented here shed light on the folding mechanism of frataxin, opening the possibility of mutating it to generate hyperstable variants without altering their folding kinetics.

Mitochondrial protein import and the genesis of steroidogenic mitochondria

The principal site of regulation of steroid hormone biosynthesis is the transfer of cholesterol from the outer to inner mitochondrial membrane. Hormonal stimulation of steroidogenic cells promotes this mitochondrial lipid import through a multi-protein complex, termed the transduceosome, spanning the two membranes. The transduceosome complex is assembled from multiple proteins, such as the steroidogenic acute regulatory (STAR) protein and translocator protein (TSPO), and requires their targeting to the mitochondria for transduceosome function. The vast majority of mitochondrial proteins, including those participating in cholesterol import, are encoded in the nucleus. Their subsequent mitochondrial incorporation is performed through a series of protein import machineries located in the outer and inner mitochondrial membranes. Here we review our current knowledge of the mitochondrial cholesterol import machinery of the transduceosome. This is complemented with descriptions of mitochondrial protein import machineries and mechanisms by which these machineries assemble the transduceosome in steroidogenic mitochondria.

Effect of signal peptide on the stability and folding kinetics of maltose binding protein

While the role of the signal sequence in targeting proteins to specific subcellular compartments is well characterized, there are fewer studies that characterize its effects on the stability and folding kinetics of the protein. We report a detailed characterization of the folding kinetics and thermodynamic stabilities of maltose binding protein (MBP) and its precursor form, preMBP. Isothermal GdmCl and urea denaturation as a function of temperature and thermal denaturation studies have been carried out to compare stabilities of the two proteins. preMBP was found to be destabilized by about 2-6 kcal/mol (20-40%) with respect to MBP. Rapid cleavage of the signal peptide by various proteases shows that the signal peptide is accessible in the native form of preMBP. The observed rate constant of the major slow phase in folding was decreased 5-fold in preMBP relative to MBP. The rate constants of unfolding were similar at 25 degrees C, but preMBP also exhibited a large burst phase change in unfolding that was absent in MBP. At 10 degrees C, preMBP exhibited a higher unfolding rate than MBP as well as a large burst phase. The appreciable destabilization of MBP by signal peptide is functionally relevant, because it enhances the likelihood of finding the protein in an unfolded translocation-competent form and may influence the interactions of the protein with the translocation machinery. Destabilization is likely to result from favorable interactions between the hydrophobic signal peptide and other hydrophobic regions that are exposed in the unfolded state.

Exceptional stability of a [2Fe-2S] ferredoxin from hyperthermophilic bacterium Aquifex aeolicus

Aquifex aeolicus is the only hyperthermophile that is known to contain a plant- and mammalian-type [2Fe-2S] ferredoxin (Aae Fd1). This unique protein contains two cysteines, in addition to the four that act as ligands of the [2Fe-2S] cluster, which form a disulfide bridge. We have investigated the stability of Aae Fd1 with (wild-type) and without (C87A variant) the disulfide bond, with respect to pH, thermal and chemical perturbation, and compared the results to those for the mesophilic [2Fe-2S] ferredoxin from spinach. Unfolding reactions of all three proteins are irreversible due to cluster decomposition in the unfolded state. Wild-type and C87A Aae Fd1 proteins are extremely stable: unfolding at 20 degrees C requires high concentrations of the chemical denaturant and long incubation times. Moreover, their thermal-unfolding midpoints are 40-50 degrees higher than that for spinach ferredoxin (pH 7). The stability of the Aae Fd1 protein is significantly lower at pH 2.5 than pH 7 and 10, suggesting that ionic interactions play a role in structural integrity. Interestingly, the iron-sulfur cluster in C87A Aae Fd1 rearranges into a transient species with absorption bands at 520 and 610 nm, presumably a linear three-iron cluster, in the high-pH unfolded state.

Cellular surface display of dimeric Adx and whole cell P450-mediated steroid synthesis on E. coli

Bovine adrenodoxin (Adx) was expressed on the surface of Escherichia coli as a monomeric fusion protein with the translocation unit of the AIDA-I autotransporter. The fusion protein remained anchored in the outer membrane by the beta-barrel of the autotransporter. Dimeric Adx molecules were formed spontaneously on the bacterial surface with high efficiencies. Adx dimers could be activated to biological function by chemical incorporation of the [2Fe-2S] cluster. By adding purified adrenodoxin reductase and P450 CYP11A1, a whole cell biocatalyst system was obtained, which effectively synthesized pregnenolone from cholesterol. Addition of artificial membrane constituents or detergents, which was indispensable before to get functional steroidal P450 enzymes, was not necessary. The whole cell activity (0.21 nmol x h(-1) x nmol(-1) CYP11A1) was in the same range as obtained earlier for reconstitution assays. The whole cell system developed here is an easy to handle, stable tool for the expression of membrane-associated P450 enzymes without the need of microsome preparation or reconstitution of artificial membrane vesicles. Moreover, it is the first report on functional dimer formation of a protein anchored on the surface of E. coli after being transported as a monomer. This seems to be a special feature of the autotransporter translocation unit, containing a beta-barrel, motile in the outer membrane and opens a new dimension for the surface display of multimeric proteins.

GroEL-assisted refolding of adrenodoxin during chemical cluster insertion

Chemical reconstitution of recombinant bovine adrenal mitochondrial apoadrenodoxin was carried out in the presence of the nonhomologous chaperone protein GroEL and of the cochaperone GroES, both in the presence and in the absence of ATP. The approach used here was different from the one characterizing studies on chaperone activity, as we used an adrenodoxin apoprotein, devoid of the cluster iron and sulfide, rather than a denaturant-unfolded form of the protein, and catalytic amounts of the chaperone proteins. A possible scaffolding role for two bacterial sulfur transferases, namely, rhodanese from Azotobacter vinelandii and a rhodanese-like sulfurtransferase from Escherichia coli, was also investigated in the absence of the enzyme substrates. The extent and the rate of adrenodoxin refolding following cluster insertion was measured by spectroscopy and by monitoring the activity recovery in a NADPH-cytochrome c reduction assay. These measurements were carried out on the unresolved reaction mixture and on the adrenodoxin-containing fraction obtained by HPLC fractionation of the reconstitution mixture at different reaction times. The rate and extent of cluster insertion and activity recovery were substantially improved by addition of GroEL and increased with increasing the GroEL/apoadrenodoxin ratio. GroES and ATP had no effect by themselves, and did not enhance the effect of GroEL. A. vinelandii rhodanese, the E. coli sulfurtransferase, and bovine serum albumin had no effect on the rate and yield of chemical reconstitution. The accelerated chemical reconstitution of apoadrenoxin in the presence of GroEL is therefore attributable to a scaffolding effect of this protein.

Adrenodoxin: structure, stability, and electron transfer properties

Adrenodoxin is an iron-sulfur protein that belongs to the broad family of the [2Fe-2S]-type ferredoxins found in plants, animals and bacteria. Its primary function as a soluble electron carrier between the NADPH-dependent adrenodoxin reductase and several cytochromes P450 makes it an irreplaceable component of the steroid hormones biosynthesis in the adrenal mitochondria of vertebrates. This review intends to summarize current knowledge about structure, function, and biochemical behavior of this electron transferring protein. We discuss the recently solved first crystal structure of the vertebrate-type ferredoxin, the truncated adrenodoxin Adx(4-108), that offers the unique opportunity for better understanding of the structure-function relationships and stabilization of this protein, as well as of the molecular architecture of [2Fe-2S] ferredoxins in general. The aim of this review is also to discuss molecular requirements for the formation of the electron transfer complex. Essential comparison between bacterial putidaredoxin and mammalian adrenodoxin will be provided. These proteins have similar tertiary structure, but show remarkable specificity for interactions only with their own cognate cytochrome P450. The discussion will be largely centered on the protein-protein recognition and kinetics of adrenodoxin dependent reactions.

Rapid degradation of the presequence of the f1beta precursor of the ATP synthase inside mitochondria

We have investigated the fate of the presequence of an overexpressed protein derived from the precursor of the F(1)beta subunit of ATP synthase after import and processing in mitochondria. Our studies revealed a rapid degradation of the presequence inside mitochondria catalysed by matrix-located protease(s). In contrast, the mature portion of the precursor was not degraded. This is the first experimental evidence of the rapid degradation of a mitochondrial presequence in organello after in vitro import and processing.

GroEL-assisted and -unassisted refolding of mature and precursor adrenodoxin: the role of the precursor sequence

We have performed refolding studies on a [2Fe-2S] protein, adrenodoxin (Adx), and its precursor form, preadrenodoxin. In vitro, mature Adx is expressed as a soluble active form in Escherichia coli, but precursor Adx is expressed in inclusion bodies. Both mature and precursor Adx refolded spontaneously from their denatured forms and the recovery levels of enzyme activities were 40 and 37% for mature and precursor Adx, respectively. Furthermore, the interaction between GroEL- and Gdn-HCl-denatured mature and precursor forms was investigated. In the case of mature Adx, the activity was increased in the presence of either GroEL, GroES, or bovine serum albumin and the refolding of mature Adx is a nonspecific process. However, the GroEL-mediated reaction is specific for precursor Adx under the experimental conditions used here. A higher electron transfer activity is obtained after ATP addition to the GroEL-containing refolding mixture, and GroEL-precursor complexes were found by gel chromatography studies. Our observation suggests that the small single-domain protein Adx (mature form) folded independently of the chaperonin GroEL. The contribution of the chaperonin complexes to the folding is toward the aggregation-sensitive precursor Adx, which in vitro folded 1.3- to 1.4-fold slower than mature Adx. This demonstrates that the presequence is responsible for the formation of inclusion bodies and for the in vitro recognition motif for GroEL binding.