The Machado-Joseph disease-associated expanded form of ataxin-3: Overexpression, purification, and preliminary biophysical and structural characterization.

An expansion of the polyglutamine (polyQ) tract within the deubiquitinase ataxin-3 protein is believed to play a role in a neurodegenerative disorder. Ataxin-3 contains a Josephin catalytic domain and a polyQ tract that renders it intrinsically prone to aggregate, and thus full-length protein is difficult to characterize structurally by high-resolution methods. We established a robust protocol for expression and purification of wild-type and expanded ataxin-3, presenting 19Q and 74Q, respectively. Both proteins are monodisperse as assessed by analytical size exclusion chromatography. Initial biophysical characterization was performed, with apparent transition melting temperature of expanded ataxin-3 lower than the wild-type counterpart. We further characterize the molecular envelope of wild-type and expanded polyQ tract in ataxin-3 using small angle X-ray scattering (SAXS). Characterization of protein-protein interactions between ataxin-3 and newly identified binding partners will benefit from our protocol.

New insights about flocculation process in sodium caseinate-stabilized emulsions.

Flocculation process was studied in emulsions formulated with 10wt.% sunflower oil, 2, 5 or 7.5wt.% NaCas, and with or without addition of sucrose (0, 5, 10, 15, 20 or 30wt.%). Two different processing conditions were used to prepare emulsions: ultraturrax homogenization or further homogenization by ultrasound. Emulsions with droplets with diameters above (coarse) or below (fine) 1µm were obtained. Emulsions were analyzed for droplet size distribution by static light scattering (SLS), stability by Turbiscan, and structure by confocal laser scanning microscopy (CLSM) and small angle X-ray scattering (SAXS). SAXS data were fitted by a theoretical model that considered a system composed of poly dispersed spheres with repulsive interaction and presence of aggregates. Flocculation behavior was caused by the self-assembly properties of NaCas, but the process was more closely related to interfacial protein content than micelles concentration in the aqueous phase. The results indicated that casein aggregation was strongly affected by disaccharide addition, hydrophobic interaction of the emulsion droplets, and interactions among interfacial protein molecules. The structural changes detected in the protein micelles in different environments allowed understanding the macroscopic physical behavior observed in concentrated NaCas emulsions.

Structural analysis of intermolecular interactions in the kinesin adaptor complex fasciculation and elongation protein zeta 1/ short coiled-coil protein (FEZ1/SCOCO).

Cytoskeleton and protein trafficking processes, including vesicle transport to synapses, are key processes in neuronal differentiation and axon outgrowth. The human protein FEZ1 (fasciculation and elongation protein zeta 1 / UNC-76, in C. elegans), SCOCO (short coiled-coil protein / UNC-69) and kinesins (e.g. kinesin heavy chain / UNC116) are involved in these processes. Exploiting the feature of FEZ1 protein as a bivalent adapter of transport mediated by kinesins and FEZ1 protein interaction with SCOCO (proteins involved in the same path of axonal growth), we investigated the structural aspects of intermolecular interactions involved in this complex formation by NMR (Nuclear Magnetic Resonance), cross-linking coupled with mass spectrometry (MS), SAXS (Small Angle X-ray Scattering) and molecular modelling. The topology of homodimerization was accessed through NMR (Nuclear Magnetic Resonance) studies of the region involved in this process, corresponding to FEZ1 (92-194). Through studies involving the protein in its monomeric configuration (reduced) and dimeric state, we propose that homodimerization occurs with FEZ1 chains oriented in an anti-parallel topology. We demonstrate that the interaction interface of FEZ1 and SCOCO defined by MS and computational modelling is in accordance with that previously demonstrated for UNC-76 and UNC-69. SAXS and literature data support a heterotetrameric complex model. These data provide details about the interaction interfaces probably involved in the transport machinery assembly and open perspectives to understand and interfere in this assembly and its involvement in neuronal differentiation and axon outgrowth.

FERM domain interaction with myosin negatively regulates FAK in cardiomyocyte hypertrophy.

Focal adhesion kinase (FAK) regulates cellular processes that affect several aspects of development and disease. The FAK N-terminal FERM (4.1 protein-ezrin-radixin-moesin homology) domain, a compact clover-leaf structure, binds partner proteins and mediates intramolecular regulatory interactions. Combined chemical cross-linking coupled to MS, small-angle X-ray scattering, computational docking and mutational analyses showed that the FAK FERM domain has a molecular cleft (~998 Å(2)) that interacts with sarcomeric myosin, resulting in FAK inhibition. Accordingly, mutations in a unique short amino acid sequence of the FERM myosin cleft, FP-1, impaired the interaction with myosin and enhanced FAK activity in cardiomyocytes. An FP-1 decoy peptide selectively inhibited myosin interaction and increased FAK activity, promoting cardiomyocyte hypertrophy through activation of the AKT-mammalian target of rapamycin pathway. Our findings uncover an inhibitory interaction between the FAK FERM domain and sarcomeric myosin that presents potential opportunities to modulate the cardiac hypertrophic response through changes in FAK activity.

Central domain deletions affect the SAXS solution structure and function of yeast Hsp40 proteins Sis1 and Ydj1.

BACKGROUND: Ydj1 and Sis1 are structurally and functionally distinct Hsp40 proteins of the yeast cytosol. Sis1 is an essential gene whereas the ydj1 gene is essential for growth at elevated temperatures and cannot complement sis1 gene deletion. Truncated polypeptides capable of complementing the sis1 gene deletion comprise the J-domain of either Sis1 or Ydj1 connected to the G/F region of Sis1 (but not Ydj1). Sis1 mutants in which the G/F was deleted but G/M maintained were capable of complementing the sis1 gene deletion. RESULTS: To investigate the relevance of central domains on the structure and function of Ydj1 and Sis1 we prepared Sis1 constructs deleting specific domains. The mutants had decreased affinity for heated luciferase but were equally capable of stimulating ATPase activity of Hsp70. Detailed low resolution structures were obtained and the overall flexibility of Hsp40 and its mutants were assessed using SAXS methods. Deletion of either the G/M or the G/M plus CTDI domains had little impact on the quaternary structure of Sis1 analyzed by the SAXS technique. However, deletion of the ZFLR-CTDI changed the relative position of the J-domains in Ydj1 in such a way that they ended up resembling that of Sis1. The results revealed that the G/F and G/M regions are not the only flexible domains. All model structures exhibit a common clamp-like conformation. CONCLUSIONS: Our results suggest that the central domains, previously appointed as important features for substrate binding, are also relevant keeping the J-domains in their specific relative positions. The clamp-like architecture observed seems also to be favorable to the interactions of Hsp40 with Hsp70.

Human Nek6 is a monomeric mostly globular kinase with an unfolded short N-terminal domain.

BACKGROUND: The NIMA-related kinases (Neks) are widespread among eukaryotes. In mammalians they represent an evolutionarily conserved family of 11 serine/threonine kinases, with 40-45% amino acid sequence identity to the Aspergillus nidulans mitotic regulator NIMA within their catalytic domains. Neks have cell cycle-related functions and were recently described as related to pathologies, particularly cancer, consisting in potential chemotherapeutic targets. Human Nek6, -7 and -9 are involved in the control of mitotic spindle formation, acting together in a mitotic kinase cascade, but their mechanism of regulation remain elusive. RESULTS: In this study we performed a biophysical and structural characterization of human Nek6 with the aim of obtaining its low resolution and homology models. SAXS experiments showed that hNek6 is a monomer of a mostly globular, though slightly elongated shape. Comparative molecular modeling together with disorder prediction analysis also revealed a flexible disordered N-terminal domain for hNek6, which we found to be important to mediate interactions with diverse partners. SEC-MALS experiments showed that hNek6 conformation is dependent on its activation/phosphorylation status, a higher phosphorylation degree corresponding to a bigger Stokes radius. Circular dichroism spectroscopy confirmed our in silico predictions of secondary structure content and thermal stability shift assays revealed a slightly higher stability of wild-type hNek6 compared to the activation loop mutant hNek6(S206A). CONCLUSIONS: Our data present the first low resolution 3D structure of hNek6 protein in solution. SAXS, comparative modeling and SEC-MALS analysis revealed that hNek6 is a monomeric kinase of slightly elongated shape and a short unfolded N-terminal domain.

Solution structure of the human signaling protein RACK1.

BACKGROUND: The adaptor protein RACK1 (receptor of activated kinase 1) was originally identified as an anchoring protein for protein kinase C. RACK1 is a 36 kDa protein, and is composed of seven WD repeats which mediate its protein-protein interactions. RACK1 is ubiquitously expressed and has been implicated in diverse cellular processes involving: protein translation regulation, neuropathological processes, cellular stress, and tissue development. RESULTS: In this study we performed a biophysical analysis of human RACK1 with the aim of obtaining low resolution structural information. Small angle X-ray scattering (SAXS) experiments demonstrated that human RACK1 is globular and monomeric in solution and its low resolution structure is strikingly similar to that of an homology model previously calculated by us and to the crystallographic structure of RACK1 isoform A from Arabidopsis thaliana. Both sedimentation velocity and sedimentation equilibrium analytical ultracentrifugation techniques showed that RACK1 is predominantly a monomer of around 37 kDa in solution, but also presents small amounts of oligomeric species. Moreover, hydrodynamic data suggested that RACK1 has a slightly asymmetric shape. The interaction of RACK1 and Ki-1/57 was tested by sedimentation equilibrium. The results suggested that the association between RACK1 and Ki-1/57(122-413) follows a stoichiometry of 1:1. The binding constant (KB) observed for RACK1-Ki-1/57(122-413) interaction was of around (1.5 +/- 0.2) x 10(6) M(-1) and resulted in a dissociation constant (KD) of (0.7 +/- 0.1) x 10(-6) M. Moreover, the fluorescence data also suggests that the interaction may occur in a cooperative fashion. CONCLUSION: Our SAXS and analytical ultracentrifugation experiments indicated that RACK1 is predominantly a monomer in solution. RACK1 and Ki-1/57(122-413) interact strongly under the tested conditions.

Low-resolution structural studies of human Stanniocalcin-1.

BACKGROUND: Stanniocalcins (STCs) represent small glycoprotein hormones, found in all vertebrates, which have been functionally implicated in Calcium homeostasis. However, recent data from mammalian systems indicated that they may be also involved in embryogenesis, tumorigenesis and in the context of the latter especially in angiogenesis. Human STC1 is a 247 amino acids protein with a predicted molecular mass of 27 kDa, but preliminary data suggested its di- or multimerization. The latter in conjunction with alternative splicing and/or post-translational modification gives rise to forms described as STC50 and "big STC", which molecular weights range from 56 to 135 kDa. RESULTS: In this study we performed a biochemical and structural analysis of STC1 with the aim of obtaining low resolution structural information about the human STC1, since structural information in this protein family is scarce. We expressed STC1 in both E. coli and insect cells using the baculo virus system with a C-terminal 6 x His fusion tag. From the latter we obtained reasonable amounts of soluble protein. Circular dichroism analysis showed STC1 as a well structured protein with 52% of alpha-helical content. Mass spectroscopy analysis of the recombinant protein allowed to assign the five intramolecular disulfide bridges as well as the dimerization Cys202, thereby confirming the conservation of the disulfide pattern previously described for fish STC1. SAXS data also clearly demonstrated that STC1 adopts a dimeric, slightly elongated structure in solution. CONCLUSION: Our data reveal the first low resolution, structural information for human STC1. Theoretical predictions and circular dichroism spectroscopy both suggested that STC1 has a high content of alpha-helices and SAXS experiments revealed that STC1 is a dimer of slightly elongated shape in solution. The dimerization was confirmed by mass spectrometry as was the highly conserved disulfide pattern, which is identical to that found in fish STC1.

Human FEZ1 has characteristics of a natively unfolded protein and dimerizes in solution.

The fasciculation and elongation protein Zeta 1 (FEZ1) is the mammalian orthologue of the Caenorhabditis elegans protein UNC-76, which is necessary for axon growth. Human FEZ1 interacts with Protein Kinase C (PKC) and several regulatory proteins involved in functions ranging from microtubule associated transport to transcriptional regulation. Theoretical prediction, circular dichroism, fluorescence spectroscopy, and limited proteolysis of recombinant FEZ1 suggest that it contains disordered regions, especially in its N-terminal region, and that it may belong to the group of natively unfolded proteins. Small angle X-ray scattering experiments indicated a mainly disordered conformation, proved that FEZ1 is a dimer of elongated shape and provided overall dimensional parameters for the protein. In vitro pull down experiments confirmed these results and demonstrated that dimerization involves the N-terminus. Ab-initio 3D low resolution models of the full-length conformation of the dimeric constructs 6xHis-FEZ1(1-392) and 6xHis-FEZ1(1-227) were obtained. Furthermore, we performed in vitro phosphorylation assays of FEZ1 with PKC. The phosphorylation occurred mainly in its C-terminal region, and does not cause any significant conformational changes, but nonetheless inhibited its interaction with the FEZ1 interacting domain of the protein CLASP2 in vitro. The C terminus of FEZ1 has been reported to bind to several interacting proteins. This suggests that FEZ1 binding and transport function of interacting proteins may be subject to regulation by phosphorylation.

Structural and kinetic characterization of a maize aldose reductase.

The aldo-keto reductases (AKRs) are classified as oxidoreductases and are found in organisms from prokaryotes to eukaryotes. The AKR superfamily consists of more than 120 proteins that are distributed throughout 14 families. Very few plant AKRs have been characterized and their biological functions remain largely unknown. Previous work suggests that AKRs may participate in stress tolerance by detoxifying reactive aldehyde species. In maize endosperm, the presence of an aldose reductase (AR; EC 1.1.1.21) enzyme has also been hypothesized based on the extensive metabolism of sorbitol. This manuscript identifies and characterizes an AKR from maize (Zea mays L.) with features of an AR. The cDNA clone, classified as AKR4C7, was expressed as a recombinant His-tag fusion protein in Escherichia coli. The product was purified by immobilized metal affinity chromatography followed by anion exchange chromatography. Circular dichroism spectrometry and SAXS analysis indicated that the AKR4C7 protein was stable, remained folded throughout the purification process, and formed monomers of a globular shape, with a molecular envelope similar to human AR. Maize AKR4C7 could utilize dl-glyceraldehyde and some pentoses as substrates. Although the maize AKR4C7 was able to convert sorbitol to glucose, the low affinity for this substrate indicated that AKR4C7 was probably a minimal contributor to sorbitol metabolism in maize seeds. Polyclonal antisera raised against AKR4C7 recognized at least three AR-like polypeptides in maize kernels, consistent with the presence of a small gene family. Diverse functions may have evolved for maize AKRs in association with specific physiological requirements of kernel development.

Human regulatory protein Ki-1/57 has characteristics of an intrinsically unstructured protein.

The human protein Ki-1/57 was first identified through the cross reactivity of the anti-CD30 monoclonal antibody Ki-1, in Hodgkin lymphoma cells. The expression of Ki-1/57 in diverse cancer cells and its phosphorylation in peripheral blood leukocytes after mitogenic activation suggested its possible role in cell signaling. Ki-1/57 interacts with several other regulatory proteins involved in cellular signaling, transcriptional regulation and RNA metabolism, suggesting it may have pleiotropic functions. In a previous spectroscopic analysis, we observed a low content of secondary structure for Ki-1/57 constructs. Here, Circular dichroism experiments, in vitro RNA binding analysis, and limited proteolysis assays of recombinant Ki-1/57(122-413) and proteolysis assays of endogenous full length protein from human HEK293 cells suggested that Ki-1/57 has characteristics of an intrinsically unstructured protein. Small-angle X-ray scattering (SAXS) experiments were performed with the C-terminal fragment Ki-1/57(122-413). These results indicated an elongated shape and a partially unstructured conformation of the molecule in solution, confirming the characteristics of an intrinsically unstructured protein. Experimental curves together with ab initio modeling approaches revealed an extended and flexible molecule in solution. An elongated shape was also observed by analytical gel filtration. Furthermore, sedimentation velocity analysis suggested that Ki-1/57 is a highly asymmetric protein. These findings may explain the functional plasticity of Ki-1/57, as suggested by the wide array of proteins with which it is capable of interacting in yeast two-hybrid interaction assays.

Conserved central domains control the quaternary structure of type I and type II Hsp40 molecular chaperones.

Heat shock protein (Hsp)40s play an essential role in protein metabolism by regulating the polypeptide binding and release cycle of Hsp70. The Hsp40 family is large, and specialized family members direct Hsp70 to perform highly specific tasks. Type I and Type II Hsp40s, such as yeast Ydj1 and Sis1, are homodimers that dictate functions of cytosolic Hsp70, but how they do so is unclear. Type I Hsp40s contain a conserved, centrally located cysteine-rich domain that is replaced by a glycine- and methionine-rich region in Type II Hsp40s, but the mechanism by which these unique domains influence Hsp40 structure and function is unknown. This is the case because high-resolution structures of full-length forms of these Hsp40s have not been solved. To fill this void, we built low-resolution models of the quaternary structure of Ydj1 and Sis1 with information obtained from biophysical measurements of protein shape, small-angle X-ray scattering, and ab initio protein modeling. Low-resolution models were also calculated for the chimeric Hsp40s YSY and SYS, in which the central domains of Ydj1 and Sis1 were exchanged. Similar to their human homologs, Ydj1 and Sis1 each has a unique shape with major structural differences apparently being the orientation of the J domains relative to the long axis of the dimers. Central domain swapping in YSY and SYS correlates with the switched ability of YSY and SYS to perform unique functions of Sis1 and Ydj1, respectively. Models for the mechanism by which the conserved cysteine-rich domain and glycine- and methionine-rich region confer structural and functional specificity to Type I and Type II Hsp40s are discussed.

Structural characterization and low-resolution model of BJ-48, a thrombin-like enzyme from Bothrops jararacussu venom.

Thrombin-like enzymes (TLEs) are important components of snake venoms due to their involvement in coagulopathies occurring on envenoming. Structural characterization of this group of serine proteases is of utmost importance for better understanding their unique properties. However, the high carbohydrate content of some members of this group prevents successful crystallization for structural determination. Circumventing this difficulty, the structure of BJ-48, a highly glycosylated TLE from Bothrops jararacussu venom, was studied in solution. At pH 8.0, where the enzyme displays maximum activity, BJ-48 has a radius of gyration (Rg) of 37 A and a maximum dimension (D(max)) of 130 A as measured by small-angle X-ray scattering (SAXS) and a Stokes radius (SR) of 50 A according to dynamic light scattering (DLS) data. At the naturally more acidic pH (6.0) of the B. jararacussu venom BJ-48 behaves as a more compact particle as evidenced by SAXS (R(g)=27.9 A and D(max)=82 A) and DLS (SR=30 A) data. In addition, Kratky plot analysis indicates a rigid shape at pH 8.0 and a flexible shape at pH 6.0. On the other hand, the center of mass of intrinsic fluorescence was not changed while varying pH, possibly indicating the absence of fluorescent amino acids in the regions affected by pH variation. Circular dichroism experiments carried out with BJ-48 indicate a substantially random coiled secondary structure that is not affected by pH. Low-resolution model of BJ-48 presented a prolate elongated shape at pH 8.0 and a U-shape at 6.0. BJ-48 tertiary structure at pH 6.0 was maintained on heating up to 52 degrees C and was completely lost at 75 degrees C. The possible existence of two pH-induced folding states for BJ-48 and its importance for the biological role and stability of this enzyme was discussed.

L-tryptophan transport through a hydrophobic liquid membrane using AOT micelles: dynamics of the process as revealed by small angle X-ray scattering.

Hydrophobic liquid membranes have a high technological potential in many fields of separation science. The dynamics of these systems is very complex and still not fully understood. In this work we studied the effect of the incorporation of cationic and anionic L-tryptophan at pH 1.8 and 10.0, respectively, in Aerosol-OT reverse micelles performing small angle X-ray scattering experiments. The use of a synchrotron radiation source allowed efficient in situ data acquisition. Several insights on L-tryptophan transport dynamics through hydrophobic membranes containing AOT could be obtained from these SAXS experiments, such as amino acid site localization and changes in the reverse micelle sizes.

Effects of gamma radiation on beta-lactoglobulin: oligomerization and aggregation.

The conformational changes and aggregation process of beta-lactoglobulin (beta-LG) subjected to gamma irradiation are presented. Beta-LG in solutions of different protein concentrations (3 and 10 mg/ml) and in solid state with different water activities (a(w)) (0.22; 0.53; 0.74) was irradiated using a Cobalt-60 radiation source at dose level of 1-50 kGy. Small-angle X-ray scattering (SAXS) was used to study the conformational changes of beta-LG due to the irradiation treatment. The irradiated protein was also examined by high performance size exclusion chromatography (HPSEC) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under nonreducing and reducing conditions and fluorescence. SAXS analysis showed that the structural conformation of irradiated beta-LG in solid state at different a(w) and dose level was essentially the same as the nonirradiated beta-LG. The scattering data also showed that the irradiation of beta-LG in solution promoted the formation of oligomers. Interestingly, from the data analysis and model building, it could be shown that the formed oligomers are linear molecules, built by linear combinations of beta-LG dimers (tetramers, hexamers, etc). The formation of oligomers was also evidenced by SDS-PAGE analysis and HPSEC chromatograms, in which products with higher molecular mass than that of the dimeric beta-LG were detected. Formation of intermolecular cross-linking between tyrosyl radicals are proposed to be at least partially responsible for this occurrence. From the results it could be shown that the samples irradiated in solution presented some conformational changes under gamma irradiation, resulting in well ordered oligomers and aggregates formed by cross-linking of beta-LG dimers subunits, while the samples irradiated in the solid state were not modified.

A new member of the aldo-keto reductase family from the plant pathogen Xylella fastidiosa.

The Xylella fastidiosa genome program generated a large number of gene sequences that belong to pathogenicity, virulence and adaptation categories from this important plant pathogen. One of these genes (XF1729) encodes a protein similar to a superfamily of aldo-keto reductase together with a number of structurally and functionally related NADPH-dependent oxidoreductases. In this work, the similar sequence XF1729 from X. fastidiosa was cloned onto the pET32Xa/LIC vector in order to overexpress a recombinant His-tag fusion protein in Escherichia coli BL21(DE3). The expressed protein in the soluble fraction was purified by immobilized metal affinity chromatography (agarose-IDA-Ni resin). Secondary structure contents were verified by circular dichroism spectroscopy. Small angle X-ray scattering (SAXS) measurements furnish general structural parameters and provide a strong indication that the protein has a monomeric form in solution. Also, ab initio calculations show that the protein has some similarities with a previously crystallized aldo-keto reductase protein. The recombinant XF1729 purified to homogeneity catalyzed the reduction of dl-glyceraldehyde (K(cat) 2.26s(-1), Km 8.20+/-0.98 mM) and 2-nitrobenzaldehyde (K(cat) 11.74 s(-1), Km 0.14+/-0.04 mM) in the presence of NADPH. The amino acid sequence deduced from XF1729 showed the highest identity (40% or higher) with several functional unknown proteins. Among the identified AKRs, we found approximately 29% of identity with YakC (AKR13), 30 and 28% with AKR11A and AKR11B, respectively. The results establish XF1729 as the new member of AKR family, AKR13B1. Finally, the first characterization by gel filtration chromatography assays indicates that the protein has an elongated shape, which generates an apparent higher molecular weight. The study of this protein is an effort to fight X. fastidiosa, which causes tremendous losses in many economically important plants.

Structural insights into the interaction between prion protein and nucleic acid.

The infectious agent of transmissible spongiform encephalopathies (TSE) is believed to comprise, at least in part, the prion protein (PrP). Other molecules can modulate the conversion of the normal PrP(C) into the pathological conformer (PrP(Sc)), but the identity and mechanisms of action of the key physiological factors remain unclear. PrP can bind to nucleic acids with relatively high affinity. Here, we report small-angle X-ray scattering (SAXS) and nuclear magnetic resonance spectroscopy measurements of the tight complex of PrP with an 18 bp DNA sequence. This double-stranded DNA sequence (E2DBS) binds with nanomolar affinity to the full-length recombinant mouse PrP. The SAXS data show that formation of the rPrP-DNA complex leads to larger values of the maximum dimension and radius of gyration. In addition, the SAXS studies reveal that the globular domain of PrP participates importantly in the formation of the complex. The changes in NMR HSQC spectra were clustered in two major regions: one in the disordered portion of the PrP and the other in the globular domain. Although interaction is mediated mainly through the PrP globular domain, the unstructured region is also recruited to the complex. This visualization of the complex provides insight into how oligonucleotides bind to PrP and opens new avenues to the design of compounds against prion diseases.

Thermotropic phase behavior of DPPC liposome systems in the presence of the anti-cancer agent 'Ellipticine'.

This letter presents our first results on the structural changes in DPPC multilamellar vesicles dispersed in water in the presence of the anti-cancer agent Ellipticine. The thermotropic phase transitions of the lamellar packing inside lipid vesicles were characterized in situ by small angle X ray diffraction. The results lead to the determination of a critical concentration value for drug loading on the vesicle system around 4% molar fraction of Ellipticine, an indication of the localization of the drug in the alkyl chains and the influence of the drug on the decreasing rate of the bilayer period after the main phase transition.

Tetramerization of the LexA repressor in solution: implications for gene regulation of the E.coli SOS system at acidic pH.

Structural changes on LexA repressor promoted by acidic pH have been investigated. Intense protein aggregation occurred around pH 4.0 but was not detected at pH values lower than pH 3.5. The center of spectral mass of the Trp increased 400 cm(-1) at pH 2.5 relatively to pH 7.2, an indication that LexA has undergone structural reorganization but not denaturation. The Trp fluorescence polarization of LexA at pH 2.5 indicated that its hydrodynamic volume was larger than its dimer at pH 7.2. 4,4'-Dianilino-1,1'-binaphthyl-5,5'- disulfonic acid (bis-ANS) experiments suggested that the residues in the hydrophobic clefts already present at the LexA structure at neutral pH had higher affinity to it at pH 2.5. A 100 kDa band corresponding to a tetramer was obtained when LexA was subject to pore-limiting native polyacrylamide gel electrophoresis at this pH. The existence of this tetrameric state was also confirmed by small angle X-ray scattering (SAXS) analysis at pH 2.5. 1D 1H NMR experiments suggested that it was composed of a mixture of folded and unfolded regions. Although 14,000-fold less stable than the dimeric LexA, it showed a tetramer-monomer dissociation at pH 2.5 from the hydrostatic pressure and urea curves. Albeit with half of the affinity obtained at pH 7.2 (Kaff of 170 nM), tetrameric LexA remained capable of binding recA operator sequence at pH 2.5. Moreover, different from the absence of binding to the negative control polyGC at neutral pH, LexA bound to this sequence with a Kaff value of 1415 nM at pH 2.5. A binding stoichiometry experiment at both pH 7.2 and pH 2.5 showed a [monomeric LexA]/[recA operator] ratio of 2:1. These results are discussed in relation to the activation of the Escherichia coli SOS regulon in response to environmental conditions resulting in acidic intracellular pH. Furthermore, oligomerization of LexA is proposed to be a possible regulation mechanism of this regulon.

Low resolution structure and stability studies of human GrpE#2, a mitochondrial nucleotide exchange factor.

GrpE acts as a nucleotide exchange factor for the Hsp70 chaperone system. Only one GrpE isoform is present in Escherichia coli, but for reasons not yet well understood, two GrpE isoforms have been found in mammalian mitochondria.Therefore, studies aimed at evaluating the physico-chemical characteristics of these proteins are important for the comprehension of the function of the Hsp70 chaperone system in different organisms. Here we report biophysical studies on human mitochondrial GrpE isoform 2. Small angle X-ray scattering measurements of human GrpE isoform 2 showed that this protein has a quaternary structure which is similar to those of human GrpE isoform 1 and E. coli GrpE: a dimer with a cruciform elongated shape. However, mitochondrial isoforms differed from each other regarding chemical and thermal denaturation profiles. This fact, combined with results of distinct expression patterns previously reported, point out that these proteins may have different response to external stimuli. Our results also indicate that human GrpE isoform 2 is more similar to the GrpE from E. coli than to human GrpE isoform 1. These results are relevant because differences in the conformation of Hsp70 co-chaperones are considered to be one of the reasons for functional diversity of this system.