Thermally induced aggregation of human transferrin receptor studied by light-scattering techniques

An affordable label-free ultrasensitive immunosensor based on gold nanoparticles deposited on glassy carbon electrode for the transferrin receptor detection

In recent decades, there has been a concerning and consistent rise in the incidence of cancer, posing a significant threat to human health and overall quality of life. The transferrin receptor (TfR) is one of the most crucial protein biomarkers observed to be overexpressed in various cancers. This study reports on the development of a novel voltammetric immunosensor for TfR detection. The electrochemical platform was made up of a glassy carbon electrode (GCE) functionalized with gold nanoparticles (AuNPs), on which anti-TfR was immobilized. The surface characteristics and electrochemical behaviors of the modified electrodes were comprehensively investigated through scanning electron microscopy, XPS, Raman spectroscopy FT-IR, electrochemical cyclic voltammetry and impedance spectroscopy. The developed immunosensor exhibited robust analytical performance with TfR fortified buffer solution, showing a linear range (LR) response from 0.01 to 3000 μg/mL, with a limit of detection (LOD) of 0.01 μg/mL and reproducibility (RSD <4 %). The fabricated sensor demonstrated high reproducibility and selectivity when subjected to testing with various types of interfering proteins. The immunosensor designed for TfR detection demonstrated several advantageous features, such as being cost-effective and requiring a small volume of test sample making it highly suitable for point-of-care applications.

Characterization of structural changes occurring in insulin at different time intervals at room temperature by surface-enhanced Raman spectroscopy

BACKGROUND

Insulin storage above the temperature recommended by food and drug administration (FDA) causes decrease in its functional efficacy due to degradation and aggregation of its protein based active pharmaceutical ingredient (API) that results poor glycemic control in diabetic patients. The aggregation of protein causes serious neurodegenerative diseases such as type-2 diabetes, Huntington disease, Parkinson's disease, and Alzheimer's disease. Surface-enhanced Raman spectroscopy (SERS) has been employed for the denaturation study of many proteins at the temperature above the recommendations of food and drug administration (FDA) (above 30 °C) which indicates potential of technique for such studies.

OBJECTIVE

SERS along with multivariate discriminating analysis techniques-based analysis of degradation of liquid pharmaceutical insulin protein after regular intervals of time at room temperature to analyze the structural changes in this protein during the storage of insulin pharmaceutical at room temperature.

METHODS

Silver nanoparticles (Ag-NPs) prepared by chemical reduction method are used as SERS active substrate for the surface enhancement of the insulin spectral signal. SERS spectral measurements of insulin were collected from eight different samples of insulin in the time range of 7 pm to 7 am first at fridge temperature (5 °C), second after half hour and next six with the time difference of 2 h each time at room temperature. The acquired SERS spectral data was preprocessed and analyzed. SERS structural transformations detection and discrimination potential in insulin was further confirmed by applying multivariate discriminating analysis techniques including principal component analysis (PCA) and Partial least square regression analysis (PLSR).

RESULTS

SERS significantly detects the structural changes produced in insulin even after 2 h of insulin placement at room temperature. PCA successfully differentiates the insulin spectral data obtained after regular intervals of time according to PC-1 (77 %) explained variance. Application of PLSR model provides quantitative confirmation of SERS efficiency, by providing insulin data regression coefficients plot, efficient prediction of time with calibration data set having 0.77 mean square absolute error of calibration (RMSAEC), validation data set with 0.80 mean square absolute error of prediction (RMSAEP) and 0.98 coefficient of determination (R2) for both calibration and validation data set.

CONCLUSION

SERS is proved as a highly sensitive and discriminating technique to detect and discriminate insulin structural changes after regular intervals of time at room temperature.

Targeted Delivery of Amoxicillin to C. trachomatis by the Transferrin Iron Acquisition Pathway

Weak intracellular penetration of antibiotics makes some infections difficult to treat. The Trojan horse strategy for targeted drug delivery is among the interesting routes being explored to overcome this therapeutic difficulty. Chlamydia trachomatis, as an obligate intracellular human pathogen, is responsible for both trachoma and sexually transmitted diseases. Chlamydia develops in a vacuole and is therefore protected by four membranes (plasma membrane, bacterial inclusion membrane, and bacterial membranes). In this work, the iron-transport protein, human serum-transferrin, was used as a Trojan horse for antibiotic delivery into the bacterial vacuole. Amoxicillin was grafted onto transferrin. The transferrin-amoxicillin construct was characterized by mass spectrometry and absorption spectroscopy. Its affinity for transferrin receptor 1, determined by fluorescence emission titration [K_affTf-amox = (1.3 ± 1.0) x 10⁸], is very close to that of transferrin [4.3 x 10⁸]. Transmission electron and confocal microscopies showed a co-localization of transferrin with the bacteria in the vacuole and were also used to evaluate the antibiotic capability of the construct. It is significantly more effective than amoxicillin alone. These promising results demonstrate targeted delivery of amoxicillin to suppress Chlamydia and are of interest for Chlamydiaceae and maybe other intracellular bacteria therapies.

Transferrin receptor-1 iron-acquisition pathway - synthesis, kinetics, thermodynamics and rapid cellular internalization of a holotransferrin-maghemite nanoparticle construct

BACKGROUND

Targeting nanoobjects via the iron-acquisition pathway is always reported slower than the transferrin/receptor endocytosis. Is there a remedy?

METHODS

Maghemite superparamagnetic and theragnostic nanoparticles (diameter 8.6nm) were synthesized, coated with 3-aminopropyltriethoxysilane (NP) and coupled to four holotransferrin (TFe2) by amide bonds (TFe2-NP). The constructs were characterized by X-ray diffraction, transmission electron microscopy, FTIR, X-ray Electron Spectroscopy, Inductively Coupled Plasma with Atomic Emission Spectrometry. The in-vitro protein/protein interaction of TFe2-NP with transferrin receptor-1 (R1) and endocytosis in HeLa cells were investigated spectrophotometrically, by fast T-jump kinetics and confocal microscopy.

RESULTS

In-vitro, R1 interacts with TFe2-NP with an overall dissociation constant KD=11nM. This interaction occurs in two steps: in the first, the C-lobe of the TFe2-NP interacts with R1 in 50μs: second-order rate constant, k1=6×10(10)M(-1)s(-1); first-order rate constant, k-1=9×10(4)s(-1); dissociation constant, K1d=1.5μM. In the second step, the protein/protein adduct undergoes a slow (10,000s) change in conformation to reach equilibrium. This mechanism is identical to that occurring with the free TFe2. In HeLa cells, TFe2-NP is internalized in the cytosol in less than 15min.

CONCLUSION

This is the first time that a nanoparticle-transferrin construct is shown to interact with R1 and is internalized in time scales similar to those of the free holotransferrin.

GENERAL SIGNIFICANCE

TFe2-NP behaves as free TFe2 and constitutes a model for rapidly targeting theragnostic devices via the main iron-acquisition pathway.

Detecting and tracking nonfluorescent nanoparticle probes in live cells

Precisely imaging and tracking dynamic biological processes in live cells are crucial for both fundamental research in life sciences and biomedical applications. Nonfluorescent nanoparticles are emerging as important optical probes in live-cell imaging because of their excellent photostability, large optical cross sections, and low cytotoxicity. Here, we provide a review of recent development in optical imaging of nonfluorescent nanoparticle probes and their applications in dynamic tracking and biosensing in live cells. A brief discussion on cytotoxicity of nanoparticle probes is also provided.

Stimuli-responsive properties of peptide-based copolymers studied via directional growth of self-assembled patterns on solid substrate

We studied the self-assembly of peptide-based ABA and CBC triblock-copolymers (obtained by bacterial expression) containing random coiled hydrophilic central B blocks flanked with helical A or C blocks. The A and C blocks were of different compositions with respect to the fraction of lysine residues which provided a higher pH sensitivity of the copolymer solutions. The interchain interactions of the copolymers driven by external stimuli (pH and temperature) were explored in the process of macromolecular self-assembling in the thin films of the copolymer solutions deposited on the solid substrate. The interactions involved in the macromolecular association affected the morphology of the developed patterns. The polypeptide of the B block was not involved in the formation of the secondary structures, while the A and C blocks demonstrated helical folding responsible for the intermolecular association. The mechanism of the responsive behavior of the copolymers is based on the reversible assembling of the helices into coiled-coil structures upon the change of pH or temperature. It was found that at low pH values, when electrostatic repulsion was strong and the A/C blocks unfolded, assembling yielded fractal dendrites. Increasing the pH resulted in the recovery of the helical conformation of the A/C blocks and caused a transition from the fractal to compact structures. An elevation of temperature resulted in the disruption of the dendritic structures. The reported here approach to the evaluation of the intermolecular interactions, based on the analysis of the dendritic patterns, provides a rapid and simple method for the characterization of complex processes of self-assembling biomacromolecules.

Mechanism of suppression of protein aggregation by α-crystallin

This review summarizes experimental data illuminating the mechanism of suppression of heat-induced protein aggregation by alpha-crystallin, one of the small heat shock proteins. The dynamic light scattering data show that the initial stage of thermal aggregation of proteins is the formation of the initial aggregates involving hundreds of molecules of the denatured protein. Further sticking of the starting aggregates proceeds in a regime of diffusion-limited cluster-cluster aggregation. The protective effect of alpha-crystallin is due to transition of the aggregation process to the regime of reaction-limited cluster-cluster aggregation, wherein the sticking probability for the colliding particles becomes lower than unity.

Surface fractals in liposome aggregation

In this work, the aggregation of charged liposomes induced by magnesium is investigated. Static and dynamic light scattering, Fourier-transform infrared spectroscopy, and cryotransmission electron microscopy are used as experimental techniques. In particular, multiple intracluster scattering is reduced to a negligible amount using a cross-correlation light scattering scheme. The analysis of the cluster structure, probed by means of static light scattering, reveals an evolution from surface fractals to mass fractals with increasing magnesium concentration. Cryotransmission electron microscopy micrographs of the aggregates are consistent with this interpretation. In addition, a comparative analysis of these results with those previously reported in the presence of calcium suggests that the different hydration energy between lipid vesicles when these divalent cations are present plays a fundamental role in the cluster morphology. This suggestion is also supported by infrared spectroscopy data. The kinetics of the aggregation processes is also analyzed through the time evolution of the mean diffusion coefficient of the aggregates.

Growth of lipid vesicle structures: from surface fractals to mass fractals

We study fractal vesicle aggregates whose morphology is conditioned by the interaction between the lipid vesicle membranes and calcium and magnesium ions. These morphologies are probed by means of static light scattering using a cross-correlation scheme that avoids the multiple intracluster scattering. In contrast to the branched structures induced by calcium, we report a singular surface- to mass-fractal transition controlled by the magnesium concentration. From infrared spectroscopy data we conclude that the specific dehydration of the lipid membranes due to these cations plays an essential role in short-range intervesicle interactions.

Aggregation of liposomes in presence of La3+: a study of the fractal dimension

A study of the fractal dimension of the aggregation of three different types of large unilamellar vesicles, formed by egg yolk phosphatidylcholine (EYPC), dimyristoyl-phosphocholine (DMPC), and dipalmitoyl-phosphocholine (DPPC), in the presence of La3+, is presented. Aggregate liposome fractal dimensions were calculated by two methods, aggregation kinetics, using the approaches diffusion-limited cluster aggregation (DLCA) and reaction-limited cluster aggregation (RLCA) and angle-scattering light dispersion. Electrophoretic measurements show a similar variation of the zeta potential (zeta potential) for EYPC and DPPC, with a small increase of initial positive values. However, the zeta potential of DMPC changes from a initial negative value to near zero with increasing La3+ concentration. The evolution of the aggregate sizes was followed by light scattering. DPPC and DMPC show a RLCA regimen growth at low La3+ concentrations and a DLCA regimen at higher concentrations. In the case of EYPC, the final size of aggregation strongly depends on La3+ concentration. The calculated fractal dimension is in the range 1.8 to 2.1.

Mechanism of thermal aggregation of rabbit muscle glyceraldehyde-3-phosphate dehydrogenase

Thermal denaturation and aggregation of rabbit muscle glyceraldehyde-3-phosphate dehydrogenase (GAPDH) have been studied using differential scanning calorimetry (DSC), dynamic light scattering (DLS), and analytical ultracentrifugation. The maximum of the protein thermal transition (T(m)) increased with increasing the protein concentration, suggesting that the denaturation process involves the stage of reversible dissociation of the enzyme tetramer into the oligomeric forms of lesser size. The dissociation of the enzyme tetramer was shown by sedimentation velocity at 45 degrees C. The DLS data support the mechanism of protein aggregation that involves a stage of the formation of the start aggregates followed by their sticking together. The hydrodynamic radius of the start aggregates remained constant in the temperature interval from 37 to 55 degrees C and was independent of the protein concentration (R(h,0) approximately 21 nm; 10 mM sodium phosphate, pH 7.5). A strict correlation between thermal aggregation of GAPDH registered by the increase in the light scattering intensity and protein denaturation characterized by DSC has been proved.

The study of amorphous aggregation of tobacco mosaic virus coat protein by dynamic light scattering

The kinetics of heat-induced and cetyltrimethylammonium bromide induced amorphous aggregation of tobacco mosaic virus coat protein in Na(+)/Na(+) phosphate buffer, pH 8.0, have been studied using dynamic light scattering. In the case of thermal aggregation (52 degrees C) the character of the dependence of the hydrodynamic radius (R(h)) on time indicates that at certain instant the population of aggregates is split into two components. The size of the aggregates of one kind remains practically constant in time, whereas the size of aggregates of other kind increases monotonously in time reaching the values characteristic of aggregates prone to precipitation (R(h)=900-1500 nm). The construction of the light scattering intensity versus R(h) plots shows that the large aggregates (the start aggregates) exist in the system at the instant the initial increase in the light scattering intensity is observed. For thermal aggregation the R(h) value for the start aggregates is independent of the protein concentration and equal to 21.6 nm. In the case of the surfactant-induced aggregation (at 25 degrees C) no splitting of the aggregates into two components is observed and the size of the start aggregates turns out to be much larger (107 nm) than on the thermal aggregation. The dependence of R(h) on time for both heat-induced aggregation and surfactant-induced aggregation after a lapse of time follows the power law indicating that the aggregation process proceeds in the kinetic regime of diffusion-limited cluster-cluster aggregation. Fractal dimension is close to 1.8. The molecular chaperone alpha-crystallin does not affect the kinetics of tobacco mosaic virus coat protein thermal aggregation.

Methods for separating nucleation and growth in protein crystallisation

The availability of high-quality crystals is crucial to the structure determination of proteins by X-ray diffraction. With the advent of structural genomics the pressure to produce crystals is greater than ever before. Finding favourable conditions for crystallisation is usually achieved by screening of the protein solution with numerous crystallising agents. Optimisation of the crystallisation conditions involves the manipulation of the crystallisation phase diagram with the aim of leading crystal growth in the direction that will produce the desired results. This article highlights recent advances in experimental methods for improving crystal size and quality by separating the nucleation and growth phases of crystallisation using the vapour diffusion and microbatch techniques.

Anion Binding and Controlled Aggregation of Human Interleukin-1 Receptor Antagonist

Highly concentrated human recombinant interleukin-1 receptor antagonist (IL-1ra) aggregates at elevated temperature without perturbation in its secondary structure. The protein aggregation can be suppressed depending on the buffer ionic strength and the type of anion present in the sample solution. Phosphate is an approximately 4-fold weaker suppressant than either citrate or pyrophosphate on the basis of the measured protein aggregation rates. This is in agreement with the strength of protein-anion interactions at the IL-1ra single anion-binding site as judged by the estimated dissociation constant values of 2.9 mM, 3.8 mM, and 13.7 mM for pyrophosphate, citrate, and phosphate, respectively. The strength of binding also correlates with the anion size and with the number of ionized groups available per molecule at a given pH. Affinity probing of IL-1ra with methyl acetyl phosphate (MAP) in combination with proteolytic digestion and mass spectral analysis show that an anion-binding site location on the IL-1ra surface is contributed by lysine-93 and lysine-96 of the loop 84-98 as well as by lysine-6 of the unstructured N-terminal region 1-7. The replacement of lysine-93 with alanine by site-directed mutagenesis results in dramatically suppressed IL-1ra aggregation. Furthermore, when the unstructured N-terminal region of IL-1ra is removed by limited proteolysis, a 2-fold increase in the time course of the aggregation lag phase is observed for the truncated protein. An anion-controlled mechanism of IL-1ra aggregation is proposed by which the anion competition for the protein cationic site prevents formation of intermolecular cation-pi interactions and, thus, interferes with the protein asymmetric self-association pathway.

Turning protein crystallisation from an art into a science

Protein crystallisation has gained a new strategic and commercial relevance in the post-genomic era because of its pivotal role in structural genomics. Producing high-quality crystals has always been a bottleneck to structure determination and, with the advent of proteomics, this problem is becoming increasingly acute. The task of producing suitable crystals may be tackled using two approaches. The first relies on empirical techniques that are based mainly on trial and error, and what is perceived to be the 'art' of crystallisation. The second approach is aimed at gaining an understanding of the fundamental principles that govern crystallisation; this knowledge may be applied to design experimental methodology for producing high-quality crystals of medical and industrial interest.

Jack bean urease (EC 3.5.1.5) aggregation monitored by dynamic and static light scattering

Aggregation of jack bean urease (JBU) is involved in many alterations of its biological properties, notably the ureolytic and entomotoxic activities. In order to investigate this phenomenon, protein aggregates were characterized by dynamic (DLS) and static light scattering (SLS) spectroscopies through determination of apparent hydrodynamic radii, the average molecular masses, radii of gyration and second virial coefficients. No effect of disulfide reducing agents on protein association was observed contrasting with previous reports implicating their function in the prevention of JBU aggregation. The influence of freeze-thawing cycles on protein aggregation was also investigated. Our results showed that after freeze-thawing cycles the native form of JBU with apparent hydrodynamic radius of 7 nm and radius of gyration of 12 nm is replaced by high-order oligomers and this aggregation is not reverted neither by dithiothreitol (DTT) treatment nor by high concentration of salts. Altogether the data help to understand the complex behavior of JBU in solution and may correlate with the diversity of biological properties of this enzyme.

Transferrin's mechanism of interaction with receptor 1

The kinetics and thermodynamics of the interactions of transferrin receptor 1 with holotransferrin and apotransferrin in neutral and mildly acidic media are investigated at 37 degrees C in the presence of CHAPS micelles. Receptor 1 interacts with CHAPS in a very fast kinetic step (<1 micros). This is followed in neutral media by the interaction with holotransferrin which occurs in two steps after receptor deprotonation, with a proton dissociation constant (K(1a)) of 10.0 +/- 1.5 nM. The first step is detected by the T-jump technique and is associated with a molecular interaction between the receptor and holotransferrin. It occurs with a first-order rate constant (k(-1)) of (1.6 +/- 0.2) x 10(4) s(-1), a second-order rate constant (k(1)) of (3.20 +/- 0.2) x 10(10) M(-1) s(-1), and a dissociation constant (K(1)) of 0.50 +/- 0.07 microM. This step is followed by a slow change in the conformation with a relaxation time (tau(2)) of 3400 +/- 400 s and an equilibrium constant (K(2)) of (4.6 +/- 1.0) x 10(-3) with an overall affinity of the receptor for holotransferrin [(K'1)(-1)] of (4.35 +/- 0.60) x 10(8) M(-1). Apotransferrin does not interact with receptor 1 in neutral media, between pH 4.9 and 6, it interacts with the receptor in two steps after a receptor deprotonation (K(2a) = 2.30 +/- 0.3 microM). The first step occurs in the range of 1000-3000 s. It is ascribed to a slow change in the conformation which rate-controls a fast interaction between apotransferrin and receptor 1 with an overall affinity constant [(K(3))(-1)] of (2.80 +/- 0.30) x 10(7) M(-1). These results imply that receptor 1 probably exists in at least two forms, the neutral species which interacts with holotransferrin and not with apotransferrin and the acidic species which interacts with apotransferrin. At first, the interaction of the neutral receptor with holotransferrin is extremely fast. It is followed by the slow change in conformation, which leads to an important stabilization of the thermodynamic structure. In the acidic media of the endosome, the interaction of apotransferrin with the acidic receptor is sufficiently strong and rate-controlled by a very slow change in conformation which allows recycling back to the plasma membrane.

Effect of environmental conditions on aggregation and fibril formation of barstar

The dependence on environmental conditions of the assembly of barstar into amyloid fibrils was investigated starting from the nonnative, partially folded state at low pH (A-state). The kinetics of this process was monitored by CD spectroscopy and static and dynamic light scattering. The morphology of the fibrils was visualized by electron microscopy, while the existence of the typical cross-beta structure substantiated by solution X-ray scattering. At room temperature, barstar in the A-state is unable to form amyloid fibrils, instead amorphous aggregation is observed at high ionic strength. Further destabilization of the structure is required to transform the polypeptide chain into an ensemble of conformations capable of forming amyloid fibrils. At moderate ionic strength (75 mM NaCl), the onset and the rate of fibril formation can be sensitively tuned by increasing the temperature. Two types of fibrils can be detected differing in their morphology, length distribution and characteristic far UV CD spectrum. The formation of the different types depends on the particular environmental conditions. The sequence of conversion: A-state-->fibril type I-->fibril type II appears to be irreversible. The transition into fibrils is most effective when the protein chain fulfills particular requirements concerning secondary structure, structural flexibility and tendency to cluster.

The aggregation behaviour of protein-coated particles: a light scattering study

The different mechanisms involved in the aggregation of spherical latex particles coated with bovine serum albumin (BSA) have been studied using static and dynamic light scattering. These techniques assess the fractal dimension of the aggregates and their mean hydrodynamic radius. Particles with different degrees of surface coverage have been prepared. The net charge of the covered particles has been modified by varying the pH of the aqueous phase. The aggregation rate was measured and used to determine the importance of the different aggregation mechanisms that are responsible for these types of flocculation processes. At low and intermediate degrees of surface coverage, bridging flocculation is the principal aggregation mechanism irrespective of the electrical state of the protein-particle complexes. At high degree of surface coverage, however, weak flocculation is important only when the BSA molecules are at their isoelectric point.

Aluminum exchange between citrate and human serum transferrin and interaction with transferrin receptor 1

The kinetics and thermodynamics of Al(III) exchange between aluminum citrate (AlL) and human serum transferrin were investigated in the 7.2-8.9 pH range. The C-site of human serum apotransferrin in interaction with bicarbonate removes Al(III) from Al citrate with an exchange equilibrium constant K1 = (2.0 +/- 0.6) x 10(-2); a direct second-order rate constant k1 = 45 +/- 3 M(-1) x s(-1); and a reverse second-order rate constant k(-1) = (2.3 +/- 0.5) x 10(3) M(-1) x s(-1). The newly formed aluminum-protein complex loses a single proton with proton dissociation constant K1a = (15 +/- 3) nM to yield a first kinetic intermediate. This intermediate then undergoes a modification in its conformation followed by two proton losses; first-order rate constant k2 = (4.20 +/- 0.02) x 10(-2) s(-1) to produce a second kinetic intermediate, which in turn undergoes a last slow modification in the conformation to yield the aluminum-loaded transferrin in its final state. This last process rate-controls Al(III) uptake by the N-site of the protein and is independent of the experimental parameters with a constant reciprocal relaxation time tau3(-1) = (6 +/- 1) x 10(-5) x s(-1). The affinities involved in aluminum uptake by serum transferrins are about 10 orders of magnitude lower than those involved in the uptake of iron. The interactions of iron-loaded transferrins with transferrin receptor 1 occur with average dissociation constants of 3 +/- 1 and 5 +/- 1 nM for the only C-site iron-loaded and of 6.0 +/- 0.6 and 7 +/- 0.5 nM for the iron-saturated ST in the absence or presence of CHAPS, respectively. No interaction is detected between receptor 1 and aluminum-saturated or mixed C-site iron-loaded/N-site aluminum-loaded transferrin under the same conditions. The fact that aluminum can be solubilized by serum transferrin in biological fluids does not necessarily imply that its transfer from the blood stream to cytoplasm follows the receptor-mediated pathway of iron transport by transferrins.

Escherichia coli RNA polymerase subunit omega and its N-terminal domain bind full-length beta' to facilitate incorporation into the alpha2beta subassembly

The omega subunit of Escherichia coli RNA polymerase, consisting of 90 amino acids, is present in stoichiometric amounts per molecule of core RNA polymerase (alpha2betabeta'). The presence of omega is necessary to restore denatured RNA polymerase in vitro to its fully functional form, and, in an omega-less strain of E. coli, GroEL appears to substitute for omega in the maturation of RNA polymerase. The X-ray structure of Thermus aquaticus core RNA polymerase suggests that two regions of omega latch on to beta' at its N-terminus and C-terminus. We show here that omega binds only the intact beta' subunit and not the beta' N-terminal domain or beta' C-terminal domain, implying that omega binding requires both these regions of beta'. We further show that omega can prevent the aggregation of beta' during its renaturation in vitro and that a V8-protease-resistant 52-amino-acid-long N-terminal domain of omega is sufficient for binding and renaturation of beta'. CD and functional assays show that this N-terminal fragment retains the structure of native omega and is able to enhance the reconstitution of core RNA polymerase. Reconstitution of core RNA polymerase from its individual subunits proceeds according to the steps alpha + alpha --> alpha2 + beta --> alpha2beta + beta' --> alpha2betabeta'. It is shown here that omega participates during the last stage of enzyme assembly when beta' associates with the alpha2beta subassembly.

Structural and Functional Stability of the Mature Transferrin Receptor from Human Placenta

The transferrin receptor (TfR) is a N- and O-glycosylated transmembrane protein mediating the cellular iron uptake by binding and internalization of diferric transferrin. In this study, rate constants and dissociation constants of 125I-ferri-transferrin binding to the human TfR were examined dependent on receptor glycan composition, pH, bivalent cations, and temperature. To do so, purified human placental TfR was noncovalently immobilized to polystyrene surfaces and subjected to alterations in various parameters. We found that transferrin binding was clearly dependent on a receptor pretreatment with buffers of various pH in that most of the TfR molecules irreversibly lost transferrin binding activity below pH 6.5. However, the dissociation constant of the remaining active binding sites was not affected. Similarly, we were able to define the thermal stability of the receptor as a function of transferrin binding ability. Binding of transferrin was completely lost provided that the receptor was pretreated at temperatures of at least 65 degrees C. Treatment with EDTA also caused an irreversible loss of transferrin binding activity, indicating that the functionally active conformation of the mature TfR depends on bivalent cations. In order to examine the role of the receptor glycans, we enzymatically removed the sialic acid residues, the hybrid and oligomannosidic N-glycans, or all types of N-glycans. In contrast to the parameters described above, all desialylated and N-deglycosylated TfR variants had exactly the same transferrin binding properties as the native TfR. To assess changes in the secondary structure of the receptor, circular dichroic spectra were recorded from TfR at pH 5.0, from heat pretreated receptor and from deglycosylated TfR. Since the receptor did not exhibit detectable changes in the CD spectrum of the deglycosylated receptor, it can be concluded that the N-linked carbohydrates of the mature, fully processed TfR are not essential for transferrin binding and conformational stability.

Colloidal properties of human transferrin receptor in detergent free solution

The colloidal properties of transferrin receptor, isolated from human placenta, in detergent free solution has been investigated by light scattering techniques and analytical ultracentrifugation. In detergent free solution at 293.2 K, hTfR forms stable aggregates with an apparent hydrodynamic radius of 17 nm. The molecular mass was determined by ultracentrifugation to lie between (1722+/-87) kDa (sedimentation equilibrium) and (1675+/-46) kDa (sedimentation velocity). This implies that the aggregates are build up from nine hTfR dimers. Based on model calculations, which are in good agreement with the experimental data, we propose a torus-like structure for the aggregates. Upon pH shift from pH 7.5 to 5.0 or removal of the N-linked carbohydrate chains, formation of larger aggregates is induced. These aggregates can be described in terms of porous fractal structures. We propose a simple model, which accounts for that behaviour assuming that the aggregation is mainly due to the reduction of negative surface charge.

A thermodynamic coupling mechanism for the disaggregation of a model peptide substrate by chaperone secB

Molecular chaperones prevent protein aggregation in vivo and in vitro. In a few cases, multichaperone systems are capable of dissociating aggregated state(s) of substrate proteins, although little is known of the mechanism of the process. SecB is a cytosolic chaperone, which forms part of the precursor protein translocation machinery in Escherichia coli. We have investigated the interaction of the B-chain of insulin with chaperone SecB by light scattering, pyrene excimer fluorescence, and electron spin resonance spectroscopy. We show that SecB prevents aggregation of the B-chain of insulin. We show that SecB is capable of dissociating soluble B-chain aggregates as monitored by pyrene fluorescence spectroscopy. The kinetics of dissociation of the B-chain aggregate by SecB has been investigated to understand the mechanism of dissociation. The data suggests that SecB does not act as a catalyst in dissociation of the aggregate to individual B-chains, rather it binds the small population of free B-chains with high affinity, thereby shifting the equilibrium from the ensemble of the aggregate toward the individual B-chains. Thus SecB can rescue aggregated, partially folded/misfolded states of target proteins by a thermodynamic coupling mechanism when the free energy of binding to SecB is greater than the stability of the aggregate. Pyrene excimer fluorescence and ESR methods have been used to gain insights on the bound state conformation of the B-chain to chaperone SecB. The data suggests that the B-chain is bound to SecB in a flexible extended state in a hydrophobic cleft on SecB and that the binding site accommodates approximately 10 residues of substrate.