Silver staining of proteins on electroblotting membranes and intensification of silver staining of proteins separated by polyacrylamide gel electrophoresis

An Integrated Approach to Improve the Assay Performance of Quantum Dot-Based Lateral Flow Immunoassays by Using Silver Deposition

Traditional quantum dot-based lateral flow immunoassay (QD-LFIA) is limited to signal loss in part by the blinking, photobleaching and oxidative quenching of QD probes. Inspired by the good application of silver deposition on QD surfaces in tissue imaging, and in the context of improving the assay performance without compromising the simplicity and practicality, we report that introducing the QD-silver combination to the LFIA system, has the advantages of accuracy improvement, signal enhancement and user friendliness promotion, but maintains the cost-effective property and commercial accessibility of QD-LFIA. The effect was shown by using CdSe/ZnS QD-LFIA coupled with anti-sodium pentachlorophenate antibody, which provided a 4-fold improvement in the signal, a 2.5-fold improvement in the detection limit and a zero false-negative rate for sodium pentachlorophenate analysis in chicken samples. The proposed LFIA integrates the possibilities of colorimetric and fluorometric detection with different detection limits (fluorometric at 10 ng/mL and colorimetric at 4 ng/mL) and with acceptable detection times (fluorometric at 12 min and colorimetric at 27 min). The current results indicate that this QD-silver combined LFIA is complementary to conventional fluorescence LFIA and could be an inexpensive, versatile, and sensitive alternative.

Genetic diversity and population structure of Polygonatum cyrtonema Hua in China using SSR markers

Polygonatum cyrtonema Hua is a perennial herbaceous plant of the Polygonatum genus, belonging to the Liliaceae family, with significant medicinal and nutritional value. In China, this species is a traditional medicinal and edible herb with a long history of application and is widely appreciated by the people. However, as the demand for medicinal herbs continues to grow, excessive harvesting has led to the depletion of wild resources and the risk of genetic erosion. In addition, the chaotic cultivation of varieties and the lack of high quality germplasm resources have led to inconsistent quality of medical materials. Therefore, it is urgent to conduct genetic diversity evaluation of this species and establish a sound conservation plan. This study assessed the genetic diversity and population structure of 96 samples collected from seven regions in China using the simple sequence repeat (SSR) molecular marker technology. In this study, a total of 60 alleles (Na) were detected across the 10 polymorphic SSR markers used, with an average of 6.0 alleles generated per locus. The values of polymorphic information content (PIC) values ranged from 0.3396 to 0.8794, with an average value of 0.6430. The average value of the effective number of alleles (Ne) was 2.761, and the average value of the Shannon's information index (I) was 1.196. The population structure analysis indicates that the Polygonatum cyrtonema Hua germplasm can be classified into three subpopulations (JZ, QY, JD) at the molecular level, which corresponds to the previous subgroups identified based on individual plant phenotypic traits. Analysis of Molecular Variance (AMOVA) showed that 74% of the genetic variation was between individuals within populations in different regions. The phylogenetic analysis of the 96 germplasm samples divided them into three main populations. The QY and JD subpopulations are largely clustered together, which could be attributed to their mountainous distribution and the local climate environment. The genetic differentiation coefficient (Fst) value was low at 0.065, indicating relatively low population differentiation. The ratio of the genetic differentiation coefficient (Fst) between the JZ population and the other two populations (QY and JD) is much higher than the ratio between the QY and JD populations. Based on the clustering results and the ratio of the genetic differentiation coefficient (Fst), it can be inferred that the genetic relationship between the QY and JD subpopulations is closer, with a certain degree of genetic differentiation from the JZ subpopulation. This study supports the conservation of germplasm resources of Polygonatum cyrtonema Hua in China and provides new parental material for germplasm genetic improvement and breeding programs.

The Rice Plastidial Phosphorylase Participates Directly in Both Sink and Source Processes

The plastidial starch phosphorylase (Pho1) functions in starch metabolism. A distinctive structural feature of the higher Pho1 is a 50–82-amino-acid long peptide (L50–L82), which is absent in phosphorylases from non-plant organisms. To study the function of the rice Pho1 L80 peptide, we complemented a pho1− rice mutant (BMF136) with the wild-type Pho1 gene or with a Pho1 gene lacking the L80 region (Pho1ΔL80). While expression of Pho1 in BMF136 restored normal wild-type phenotype, the introduction of Pho1ΔL80 enhanced the growth rate and plant productivity above wild-type levels. Mass spectrometry analysis of proteins captured by anti-Pho1 showed the surprising presence of PsaC, the terminal electron acceptor/donor subunit of photosystem I (PSI). This unexpected interaction was substantiated by reciprocal immobilized protein pull-down assays of seedling extracts and supported by the presence of Pho1 on isolated PSI complexes resolved by blue-native gels. Spectrophotometric studies showed that Pho1ΔL80 plants exhibited modified PSI and enhanced CO2 assimilation properties. Collectively, these findings indicate that the higher plant Pho1 has dual roles as a potential modulator of source and sink processes.

Targeting myelin lipid metabolism as a potential therapeutic strategy in a model of CMT1A neuropathy

In patients with Charcot-Marie-Tooth disease 1A (CMT1A), peripheral nerves display aberrant myelination during postnatal development, followed by slowly progressive demyelination and axonal loss during adult life. Here, we show that myelinating Schwann cells in a rat model of CMT1A exhibit a developmental defect that includes reduced transcription of genes required for myelin lipid biosynthesis. Consequently, lipid incorporation into myelin is reduced, leading to an overall distorted stoichiometry of myelin proteins and lipids with ultrastructural changes of the myelin sheath. Substitution of phosphatidylcholine and phosphatidylethanolamine in the diet is sufficient to overcome the myelination deficit of affected Schwann cells in vivo. This treatment rescues the number of myelinated axons in the peripheral nerves of the CMT rats and leads to a marked amelioration of neuropathic symptoms. We propose that lipid supplementation is an easily translatable potential therapeutic approach in CMT1A and possibly other dysmyelinating neuropathies.

Rice Endosperm Starch Phosphorylase (Pho1) Assembles with Disproportionating Enzyme (Dpe1) to Form a Protein Complex That Enhances Synthesis of Malto-oligosaccharides

Starch synthesis in cereal grain endosperm is dependent on the concerted actions of many enzymes. The starch plastidial phosphorylase (Pho1) plays an important role in the initiation of starch synthesis and in the maturation of starch granule in developing rice seeds. Prior evidence has suggested that the rice enzyme, OsPho1, may have a physical/functional interaction with other starch biosynthetic enzymes. Pulldown experiments showed that OsPho1 as well as OsPho1 devoid of its L80 region, a peptide unique to higher plant phosphorylases, captures disproportionating enzyme (OsDpe1). Interaction of the latter enzyme form with OsDpe1 indicates that the putative regulatory L80 is not responsible for multienzyme assembly. This heterotypic enzyme complex, determined at a molar ratio of 1:1, was validated by reciprocal co-immunoprecipitation studies of native seed proteins and by co-elution chromatographic and co-migration electrophoretic patterns of these enzymes in rice seed extracts. The OsPho1-OsDpe1 complex utilized a broader range of substrates for enhanced synthesis of larger maltooligosaccharides than each individual enzyme and significantly elevated the substrate affinities of OsPho1 at 30 °C. Moreover, the assembly with OsDpe1 enables OsPho1 to utilize products of transglycosylation reactions involving G1 and G3, sugars that it cannot catalyze directly.

The hypolipidemic effect and antithrombotic activity of Mucuna pruriens protein hydrolysates

Hydrolysates and peptide fractions (PF) obtained fromM. pruriensprotein concentrates with commercial and digestive enzymatic systems were studied for their hypolipidemic and antithrombotic activities.

MALDI-TOF MS and CD spectral analysis for identification and structure prediction of a purified, novel, organic solvent stable, fibrinolytic metalloprotease from Bacillus cereus B80

The ability to predict protein function from structure is becoming increasingly important; hence, elucidation and determination of protein structure become the major steps in proteomics. The present study was undertaken for identification of metalloprotease produced by Bacillus cereus B80 and recognition of characteristics that can be industrially exploited. The enzyme was purified in three steps combining precipitation and chromatographic methods resulting in 33.5% recovery with 13.1-fold purification of enzyme which was detected as a single band with a molecular mass of 26 kDa approximately in SDS-PAGE and zymogram. The MALDI-TOF MS showed that the enzyme exhibited 70-93% similarity with zinc metalloproteases from various strains Bacillus sp. specifically from Bacillus cereus group. The sequence alignment revealed the presence of zinc-binding region VVVHEMCHMV in the most conserved C terminus region. Secondary structure of the enzyme was obtained by CD spectra and I-TASSER. The enzyme kinetics revealed a Michaelis constant (Km) of 0.140 μmol/ml and Vmax of 2.11 μmol/min. The application studies showed that the enzyme was able to hydrolyze various proteins with highest affinity towards casein followed by BSA and gelatin. The enzyme exhibited strong fibrinolytic, collagenolytic, and gelatinolytic properties and stability in various organic solvents.

Interaction of the molecular chaperone DNAJB6 with growing amyloid-beta 42 (Aβ42) aggregates leads to sub-stoichiometric inhibition of amyloid formation

The human molecular chaperone protein DNAJB6 was recently found to inhibit the formation of amyloid fibrils from polyglutamine peptides associated with neurodegenerative disorders such as Huntington disease. We show in the present study that DNAJB6 also inhibits amyloid formation by an even more aggregation-prone peptide (the amyloid-beta peptide, Aβ42, implicated in Alzheimer disease) in a highly efficient manner. By monitoring fibril formation using Thioflavin T fluorescence and far-UV CD spectroscopy, we have found that the aggregation of Aβ42 is retarded by DNAJB6 in a concentration-dependent manner, extending to very low sub-stoichiometric molar ratios of chaperone to peptide. Quantitative kinetic analysis and immunochemistry studies suggest that the high inhibitory efficiency is due to the interactions of the chaperone with aggregated forms of Aβ42 rather than the monomeric form of the peptide. This interaction prevents the growth of such species to longer fibrils and inhibits the formation of new amyloid fibrils through both primary and secondary nucleation. A low dissociation rate of DNAJB6 from Aβ42 aggregates leads to its incorporation into growing fibrils and hence to its gradual depletion from solution with time. When DNAJB6 is eventually depleted, fibril proliferation takes place, but the inhibitory activity can be prolonged by introducing DNAJB6 at regular intervals during the aggregation reaction. These results reveal the highly efficacious mode of action of this molecular chaperone against protein aggregation, and demonstrate that the role of molecular chaperones can involve interactions with multiple aggregated species leading to the inhibition of both principal nucleation pathways through which aggregates are able to form.

Structure elucidation of ACE-inhibitory and antithrombotic peptides isolated from mackerel skin gelatine hydrolysates

BACKGROUND

The fish-processing industry generates significant amounts of waste and by-products that are usually discarded. This study investigated the preparation of bioactive gelatine peptides from fish skin. Gelatine was extracted from mackerel (Scomber scombrus) skin and hydrolysed by pepsin for 1, 2, 6 and 24 h. All hydrolysates were screened for antioxidant, ACE-inhibitory and antithrombotic activities.

RESULTS

Gelatine peptides obtained after 24 h of hydrolysis exhibited the highest antioxidant activity (DPPH reduction ∼80%, FRAP ∼130 µmol Trolox equivalent L(-1) ). These hydrolysates had high ACE-inhibitory activity (>70%) and were able to significantly (P < 0.05) inhibit platelet aggregation by about 30%, corresponding to moderate antithrombotic activity.

CONCLUSION

The bioactive properties were mainly due to the presence of low-molecular-weight peptides of 337 and 423 Da.

Identifying the emerging human pathogen Scedosporium prolificans by using a species-specific monoclonal antibody that binds to the melanin biosynthetic enzyme tetrahydroxynaphthalene reductase

The dematiaceous (melanized) fungus Scedosporium prolificans is an emerging and frequently fatal pathogen of immunocompromised humans and which, along with the closely related fungi Pseudallescheria boydii, Scedosporium apiospermum and S. aurantiacum in the Pseudallescheria-Scedosporium complex, is a contributing aetiology to tsunami lung and central nervous system infections in near-drowning victims who have aspirated water laden with spores. At present, the natural habitat of the fungus is largely unknown, and accurate detection methods are needed to identify environmental reservoirs of infectious propagules. In this study, we report the development of a monoclonal antibody (mAb) (CA4) specific to S. prolificans, which does not cross-react with closely related fungi in the Pseudallescheria-Scedosporium complex or with a wide range of mould and yeast species pathogenic to humans. Using genome sequencing of a soil isolate and targeted gene disruption of the CA4 antigen-encoding gene, we show that mAb CA4 binds to the melanin-biosynthetic enzyme tetrahydroxynaphthalene reductase. Enzyme-deficient mutants produce orange-brown or green-brown spore suspensions compared with the black spore suspension of the wild-type strain. Using mAb CA4 and a mAb (HG12) specific to the related fungi P. boydii, P. apiosperma, S. apiospermum and S. aurantiacum, we demonstrate how the mAbs can be used in combination with a semiselective isolation procedure to track these opportunistic pathogens in environmental samples containing mixed populations of human pathogenic fungi. Specificity of mAb CA4 was confirmed by sequencing of the internally transcribed spacer 1 (ITS1)-5.8S-ITS2 rRNA-encoding regions of fungi isolated from estuarine muds.

Identification of an O-linked repetitive glycan chain of the polar flagellum flagellin of Azospirillum brasilense Sp7

This is the first report to have identified an O-linked repetitive glycan in bacterial flagellin, a structural protein of the flagellum. Studies by sugar analysis, Smith degradation, (1)H and (13)C NMR spectroscopy, and mass spectrometry showed that the glycan chains of the polar flagellum flagellin of the plant-growth-promoting rhizobacterium Azospirillum brasilense Sp7 are represented by a polysaccharide with a molecular mass of 7.7 kDa, which has a branched tetrasaccharide repeating unit of the following structure:

Engineered Trx2p industrial yeast strain protects glycolysis and fermentation proteins from oxidative carbonylation during biomass propagation

BACKGROUND

In the yeast biomass production process, protein carbonylation has severe adverse effects since it diminishes biomass yield and profitability of industrial production plants. However, this significant detriment of yeast performance can be alleviated by increasing thioredoxins levels. Thioredoxins are important antioxidant defenses implicated in many functions in cells, and their primordial functions include scavenging of reactive oxygen species that produce dramatic and irreversible alterations such as protein carbonylation.

RESULTS

In this work we have found several proteins specifically protected by yeast Thioredoxin 2 (Trx2p). Bidimensional electrophoresis and carbonylated protein identification from TRX-deficient and TRX-overexpressing cells revealed that glycolysis and fermentation-related proteins are specific targets of Trx2p protection. Indeed, the TRX2 overexpressing strain presented increased activity of the central carbon metabolism enzymes. Interestingly, Trx2p specifically preserved alcohol dehydrogenase I (Adh1p) from carbonylation, decreased oligomer aggregates and increased its enzymatic activity.

CONCLUSIONS

The identified proteins suggest that the fermentative capacity detriment observed under industrial conditions in T73 wine commercial strain results from the oxidative carbonylation of specific glycolytic and fermentation enzymes. Indeed, increased thioredoxin levels enhance the performance of key fermentation enzymes such as Adh1p, which consequently increases fermentative capacity.

Differences in two-dimensional gel electrophoresis patterns of skeletal muscle myosin light chain isoforms between Bos taurus, Sus scrofa and selected poultry species

BACKGROUND

In this study the interspecies differences in two-dimensional electrophoresis patterns of skeletal muscle myosin light chain (MLC) isoforms between Bos taurus (cattle), Sus scrofa (pig), Gallus gallus (chicken), Meleagris gallopavo (turkey), Anas platyrhynchos (duck) and Anser anser (goose) were characterised on the basis of specific properties of MLCs associated with their structure and mobility in gel.

RESULTS

Two-dimensional electrophoresis separations revealed species-specific differences in the molecular weight and pI of individual MLC isoforms (MLC1f, MLC2f and MLC3f). In the case of closely related animal species such as goose and duck or turkey and chicken, significant differences occurred in MLC1f. For MLC2f, differences between cattle and turkey and between pig and chicken were around 1 and 0.3 kDa respectively. It appeared from the comparison of amino acid sequences that even MLCs with only 2% difference in sequences have different electrophoretic mobilities.

CONCLUSION

Interspecies differences in skeletal MLC isoforms appeared between cattle, pig, chicken, turkey, duck and goose. The slight changes observed in the course of the aging process confirmed that these proteins are relatively little susceptible to proteolytic enzymes during meat aging.

Application of capillary isoelectric focusing and peptide mass fingerprinting in carbohydrate-deficient transferrin detection

Carbohydrate-deficient transferrin (CDT) is a specific biomarker of alcohol abuse and is widely used in clinical diagnosis to detect and follow up excessive alcohol consumption. However, false %CDT results still exist in CDT detection, because of interference from genetic variants and the lack of standardization in CDT analysis. Therefore, it is still very important to find a method with high sensitivity and high accuracy for CDT detection. Here, we compared the detection sensitivity and accuracy of pI values based methods [isoelectric focusing on polyacrylamide gel electrophoresis (IEF-PAGE) and capillary isoelectric focusing (CIEF)] with hydrophobic characteristic based methods [reversed-phase high-performance liquid chromatography (RP-HPLC)] on CDT detection. Moreover, we investigated the potential of peptide mass fingerprinting (PMF), a method based on the mass spectrometry to identify human transferrin (HTf) variants including CDT isoforms and genetic variants, based on their specific peptide masses. Results indicated that PMF can identify HTf variants including CDT isoforms and genetic variants based on their specific peptides, and CIEF showed higher sensitivity detection of HTf variants than RP-HPLC and IEF-PAGE did. Accordingly, we suggest that PMF is suitable for identifying CDT with high accuracy, and CIEF has potential for detection of CDT and genetic variants with high sensitivity; moreover, they are both worth further investigation in clinical diagnosis.

Autoantibodies to the proteasome in monosymptomatic optic neuritis may predict progression to multiple sclerosis

OBJECTIVE

Proteasome autoantibodies (PAB) have been found in multiple sclerosis (MS) patient sera and cerebrospinal fluid (CSF). Presence of PAB could thus be a possible diagnostic marker for MS. We investigated whether PAB serum status in acute monosymptomatic optic neuritis (ON) and MS differed significantly from that of healthy controls, and whether or not PAB status is predictive of later MS development in patients with ON.

MATERIAL AND METHODS

Sera from ON patients, MS patients and healthy donors were analysed retrospectively using ELISA. Subsequently, a small group of PAB-positive samples were subjected to SDS-PAGE, immunoblotting and 2-D PAGE.

RESULTS

We found that 20 % (6/30) ON patients, 47 % (22/47) MS patients and 9 % (7/81) controls tested PAB positive using ELISA analysis. High PAB levels were found in 2 (4 %) MS patients, 1 (3 %) ON patient and 2 (3 %) controls. PAB positivity in ELISA was confirmed by immunoblotting. Separation of proteasome subunits by 2D PAGE followed by immunoblotting revealed no particular PAB subunit preference.

CONCLUSIONS

A retrospective search in available patient files revealed that 6 of 6 (100.0 %) PAB-positive ON patients developed MS over time. Eight of 24 (33 %) PAB-negative ON patients developed MS over time and 47 % (14/30) of all ON patients developed MS. A series of patient CSF was analysed by ELISA to assess the possible correlation between PAB status of concurrent serum and CSF samples, but no correlation was found. However, the results from the six PAB-positive ON patients could potentially be of prognostic value.

Chemical cross-linking of the chloroplast localized small heat-shock protein, Hsp21, and the model substrate citrate synthase

The molecular mechanism whereby the small heat-shock protein (sHsp) chaperones interact with and prevent aggregation of other proteins is not fully understood. We have characterized the sHsp-substrate protein interaction at normal and increased temperatures utilizing a model substrate protein, citrate synthase (CS), widely used in chaperone assays, and a dodecameric plant sHsp, Hsp21, by chemical cross-linking with 3,3'-Dithiobis[sulfosuccinimidylpropionate] (DTSSP) and mass spectrometric peptide mapping. In the absence of CS, the cross-linker captured Hsp21 in dodecameric form, even at increased temperature (47 degrees C). In the presence of equimolar amounts of CS, no Hsp21 dodecamer was captured, indicating a substrate-induced Hsp21 dodecamer dissociation by equimolar amounts of CS. Cross-linked Hsp21-Hsp21 dipeptides indicated an exposure of the Hsp21 C-terminal tails and substrate-binding sites normally covered by the C terminus. Cross-linked Hsp21-CS dipeptides mapped to several sites on the surface of the CS dimer, indicating that there are numerous weak and short-lived interactions between Hsp21 and CS, even at normal temperatures. The N-terminal arms especially interacted with a motif in the CS dimer, which is absent in thermostable forms of CS. The cross-linking data suggest that the presence of substrate rather than temperature influences the conformation of Hsp21.

Scanning Electrochemical Microscopy as a Readout Tool for Protein Electrophoresis

Scanning electrochemical microscopy (SECM) was used to image silver-stained proteins on a poly(vinylidene difluoride) membrane. The method is based on measuring the current at a scanning microelectrode in the feedback mode. The electrochemical feedback is caused by the redox-mediated etching of the isolated 5 - 10-nm-diameter silver nanoparticles formed during the staining process. Several parameters, such as the redox mediator and the staining protocol, were optimized to ensure a high resolution and a low detection limit, i.e., 0.5 ng of bovine serum albumin (4 x 10(-14) mol) distributed on an area of 1 mm(2) (4 x 10(-16) mol x cm(-2)). Images of beta-lactoglobulin A and myoglobin bands after gel electrophoretic separation and electroblotting were obtained in order to demonstrate that SECM can be employed as a sensitive and quantitative readout method for detection of proteins after gel electrophoresis. An additional advantage is that the silver staining can be removed, allowing further downstream mass spectrometry analysis.

A mass spectrometry compatible silver staining method for protein incorporating a new silver sensitizer in sodium dodecyl sulfate-polyacrylamide electrophoresis gels

A quick, sensitive, and MALDI-TOF MS compatible silver staining method, namely Eriochrome black T (EBT)-silver method, is described. The method can detect 0.05-0.2 ng protein within 60 min in SDS-PAGE gels. EBT dye was used as a silver ion sensitizer having reducing power for silver ions.

Proteins in human brain cortex are modified by oxidation, glycoxidation, and lipoxidation. Effects of Alzheimer disease and identification of lipoxidation targets

Diverse oxidative pathways, such as direct oxidation of amino acids, glycoxidation, and lipoxidation could contribute to Alzheimer disease pathogenesis. A global survey for the amount of structurally characterized probes for these reactions is lacking and could overcome the lack of specificity derived from measurement of 2,4-dinitrophenylhydrazine reactive carbonyls. Consequently we analyzed (i) the presence and concentrations of glutamic and aminoadipic semialdehydes, N(epsilon)-(carboxymethyl)-lysine, N(epsilon)-(carboxyethyl)-lysine, and N(epsilon)-(malondialdehyde)-lysine by means of gas chromatography/mass spectrometry, (ii) the biological response through expression of the receptor for advanced glycation end products, (iii) the fatty acid composition in brain samples from Alzheimer disease patients and age-matched controls, and (iv) the targets of N(epsilon)-(malondialdehyde)-lysine formation in brain cortex by proteomic techniques. Alzheimer disease was associated with significant, although heterogeneous, increases in the concentrations of all evaluated markers. Alzheimer disease samples presented increases in expression of the receptor for advanced glycation end products with high molecular heterogeneity. Samples from Alzheimer disease patients also showed content of docosahexaenoic acid, which increased lipid peroxidizability. In accordance, N(epsilon)-(malondialdehyde)-lysine formation targeted important proteins for both glial and neuronal homeostasis such as neurofilament L, alpha-tubulin, glial fibrillary acidic protein, ubiquinol-cytochrome c reductase complex protein I, and the beta chain of ATP synthase. These data support an important role for lipid peroxidation-derived protein modifications in Alzheimer disease pathogenesis.

Efficacy and compatibility with mass spectrometry of methods for elution of proteins from sodium dodecyl sulfate–polyacrylamide gels and polyvinyldifluoride membranes

The resolving power of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) combined with isoelectric focusing in two-dimensional gel electrophoresis has made it one of the most important techniques for resolving complex mixtures, and it is of great importance for proteome mapping projects. As a result of this, methods for postelectrophoretic protein characterization are of great interest as exemplified by in situ protease digestion combined with mass spectrometry (MS), which is the method of choice for identification of proteins. In this study we have developed and compared methods for recovering intact proteins from polyacrylamide gels and electroblotting membranes to define efficient methods compatible with MS. These methods complement in situ digestion protocols and allow determination of the molecular mass of whole proteins separated by SDS-PAGE. Passive elution of proteins from SDS-PAGE gels was efficient only in the presence of SDS, whereas electroelution was achieved using buffers without SDS. Surface-enhanced laser desorption/ionization MS (SELDI-MS) analysis of proteins eluted in the presence of SDS was possible using ion exchange ProteinChip arrays for concentration of sample and removal of SDS. Comparison of different electroblotting methods verified that the different membranes and buffers were equally efficient for transfer of proteins in the range 20-100 kDa. Elution from polyvinyldifluoride membranes was most efficient using either concentrated solutions of trifluoroacetic acid (TFA) or combinations of 8M urea and 1% Triton X-100, 1% Tween 20, or 40% isopropanol. The same result was obtained using nitrocellulose membranes, except that these were incompatible with organic solvent and TFA. Elution by TFA was compatible with matrix-assisted laser desorption/ionization MS (MALDI-MS) but was complicated by a high degree of trifluoroacetylation of the proteins. Alternatively, elution by 8M urea+1% Triton X-100, 1% Tween 20, or 40% isopropanol was compatible with both SELDI-MS and MALDI-MS. Eluted proteins were identified in MS experiments by intact mass determination, by peptide mapping, and by MS/MS analysis.

Interaction of C1q with the receptor calreticulin requires a conformational change in C1q

The interaction between C1q and the chaperone calreticulin was studied under various conditions. When both proteins were present in equal amounts in solution, no interaction could be demonstrated. However, C1q immobilized on a hydrophobic surface, exposed to heat-treatment or bound to immunoglobulins (Igs) showed a strong, rapid and specific binding of calreticulin. The interaction appeared to be a two-step process, and the initial phase of interaction was sensitive to high concentrations of salt but not to a physiological salt concentration. The following strong binding was insensitive to salt and extremes of pH but sensitive to strongly denaturing agents (urea and guanidine). The sensitivity to salt during the initial phase of interaction was practically identical to that observed when calreticulin was bound to type V collagen. Binding between C1q and calreticulin could be inhibited by serum amyloid P component and by proteinase K-digested ovalbumin, and the binding of calreticulin to proteinase K-digested ovalbumin was shown to be inhibited by C1q. The data indicate that C1q binds stably to the peptide-binding site of calreticulin and that the initial binding of calreticulin to C1q involves the collagen-like domain of the C1q molecule. In conclusion, our results suggest calreticulin as a potential receptor for an altered conformation of C1q as occurs during binding to Igs. Thus, the chaperone and protein-scavenging function of calreticulin may extend from the endoplasmic reticulum to the topologically equivalent cell surface, where it may contribute to the elimination of immune complexes and apoptotic cells.