"Early" protein synthesis of Lactobacillus delbrueckii ssp. bulgaricus in milk revealed by [35S] methionine labeling and two-dimensional gel electrophoresis

The proteomes of exponentially growing and stationary cells of Lactobacillus delbrueckii ssp. bulgaricus grown in rich medium (MRS) were separated by two-dimensional polyacrylamide gel electrophoresis (2-DE) and quantified after Coomassie staining. Stationary cells grown in MRS were inoculated in reconstituted skim milk, and "early" protein synthesis during the first 30 min of fermentation in milk was monitored by [35S]methionine labeling and 2-DE. In contrast to exponentially growing or stationary cells, the predominant "early" proteins were small (< 15 kDa) and of low pI (< 5.3). Quantification of the proteome of the "early" lag phase based on 47 "spots" revealed that only three "early" proteins accounted for more than 80% of the total label. They were identified as pI 4.7 and 4.9 isoforms of the heat-stable phosphoryl carrier protein (HPr) with 45.2 and 9.4% of total label, respectively, and an unknown protein called EPr1 ("early" protein 1) with 26.6% of total label. Although an N-terminal sequence of 19 amino acids was obtained, no homologs to EPr1 could be found. De novo synthesis of the 10 and 60 kDa heat shock proteins (GroES and GroEL) was considerably lower (0.04 and 0.9% of total label, respectively), indicating only low levels of stress. Synthesis of triosephosphate isomerase (Tpi) as marker for glycolytic enzymes reached only 0.08% of total label. Our results demonstrate that inoculation in milk, resulting in a change from glucose to lactose as carbon source, imposes only little need for synthesis of stress or glycolytic enzymes, as sufficient proteins are present in the stationary, MRS-grown cells. The high level of expression of the pI 4.7 isoform of HPr suggests a regulatory function of the presumed Ser-46 phosphorylated form of HPr.

Overexpression of trigger factor prevents aggregation of recombinant proteins in Escherichia coli

To examine the effects of overexpression of trigger factor (TF) on recombinant proteins produced in Escherichia coli, we constructed plasmids that permitted controlled expression of TF alone or together with the GroEL-GroES chaperones. The following three proteins that are prone to aggregation were tested as targets: mouse endostatin, human oxygen-regulated protein ORP150, and human lysozyme. The results revealed that TF overexpression had marked effects on the production of these proteins in soluble forms, presumably through facilitating correct folding. Whereas overexpression of TF alone was sufficient to prevent aggregation of endostatin, overexpression of TF together with GroEL-GroES was more effective for ORP150 and lysozyme, suggesting that TF and GroEL-GroES play synergistic roles in vivo. Although coexpression of the DnaK-DnaJ-GrpE chaperones was also effective for endostatin and ORP150, coexpression of TF and GroEL-GroES was more effective for lysozyme. These results attest to the usefulness of the present expression plasmids for improving protein production in E. coli.

Expression and purification of the transcription factor NtcA from the cyanobacterium Anabaena PCC 7120

The transcription factor NtcA from the cyanobacterium Anabaena PCC 7120 was heterologously expressed in Escherichia coli. In order to optimize the expression of NtcA, random silent mutations were introduced at the 5' end of the DNA encoding the protein. To get as high a yield of pure protein as possible, different strategies of expression as well as purification conditions were used. Under optimal expression conditions, a high-level expression clone of NtcA was coexpressed with GroEL-ES at 37 degrees C. A hexahistidine tag was added to the N-terminus of the protein in order to allow purification on an IMAC affinity column. Expression followed by one purification step using IMAC affinity chromatography gave a yield of 30-40 mg pure NtcA protein per liter of bacterial culture. Gel-shift experiments showed that the recombinant NtcA was active in binding a DNA sequence containing an NtcA-specific site.

Increased solubility of glutathione S-transferase-P16 (GST-p16) fusion protein by co-expression of chaperones groes and groel in Escherichia coli

Human cdk (cyclin dependent kinase) inhibitor p16 was fused with glutathione S-transferase (GST) and the GST-p16 fusion protein is under the control of T7 promoter. When expressed in E. coli BL21(DE3), most products existed in the form of insoluble inclusion bodies. When co-expressed with molecular chaperones E. coli GroESL, most GST-p16 products accumulated in the soluble form with a 5-6 fold increase in solubility. When coproduced with human protein disulfide isomerase (PDI), there was no improvement in the solubility of GST-p16 fusion protein.

Induced levels of heat shock proteins in a dnaK mutant of Lactococcus lactis

The bacterial heat shock response is characterized by the elevated expression of a number of chaperone complexes and proteases, including the DnaK-GrpE-DnaJ and the GroELS chaperone complexes. In order to investigate the importance of the DnaK chaperone complex for growth and heat shock response regulation in Lactococcus lactis, we have constructed two dnaK mutants with C-terminal deletions in dnaK. The minor deletion of 65 amino acids in the dnaKDelta2 mutant resulted in a slight temperature-sensitive phenotype. BK6, containing the larger deletion of 174 amino acids (dnaKDelta1), removing the major part of the inferred substrate binding site of the DnaK protein, exhibited a pronounced temperature-sensitive phenotype and showed altered regulation of the heat shock response. The expression of the heat shock proteins was increased at the normal growth temperature, measured as both protein synthesis rates and mRNA levels, indicating that DnaK could be involved in the regulation of the heat shock response in L. lactis. For Bacillus subtilis, it has been found (A. Mogk, G. Homuth, C. Scholz, L. Kim, F. X. Schmid, and W. Schumann, EMBO J. 16:4579-4590, 1997) that the activity of the heat shock repressor HrcA is dependent on the chaperone function of the GroELS complex and that a dnaK insertion mutant has no effect on the expression of the heat shock proteins. The present data from L. lactis suggest that the DnaK protein could be involved in the maturation of the homologous HrcA protein in this bacterium.

Enhanced secretory production of a single-chain antibody fragment from Bacillus subtilis by coproduction of molecular chaperones

Formation of inclusion bodies is a major limiting factor for secretory production of an antidigoxin single-chain antibody (SCA) fragment from Bacillus subtilis. To address this problem, three new strains with enhanced production of molecular chaperones were constructed. WB600BHM constitutively produces the major intracellular molecular chaperones in an appropriate ratio without any heat shock treatment. This strain reduced the formation of insoluble SCA by 45% and increased the secretory production yield by 60%. The second strain, WB600B[pEPP], overproduces an extracytoplasmic molecular chaperone, PrsA. An increase in the total yield of SCA was observed. The third strain, WB600BHM[pEPP], coproduces both intracellular and extracytoplasmic molecular chaperones. This led to a further reduction in inclusion body formation and a 2.5-fold increase in the secretory production yield. SCA fragments secreted by this strain were biologically active and showed affinity to digoxin comparable to the affinity of those secreted by strains without overproduction of molecular chaperones. Interestingly, accumulation of a pool of periplasmic SCA was observed in the PrsA-overproducing strains. This pool is suggested to represent the secreted folding intermediates in the process of achieving their final configuration.

Simultaneous overexpression of two stress proteins in rat cardiomyocytes and myogenic cells confers protection against ischemia-induced injury

BACKGROUND

Mitochondria are known to be a major target during ischemic cardiac injury. Previous studies have shown that in rodent myogenic cells and in the hearts of transgenic mice in which the heat shock or stress protein 70 is increased, there is a marked tolerance to ischemia/reperfusion injury. Two other heat shock proteins (HSP60 and HSP10) are known to form, within the mitochondria, a chaperonin complex that is important for mitochondrial protein folding and function. We were then interested in investigating whether increased expression of these two stress proteins is able to protect myogenic cells against ischemia/reperfusion injury.

METHODS AND RESULTS

We generated recombinant adenoviral vectors containing HSP60, HSP10, or a combination of the two genes. These adenoviral constructs overexpress significant amounts of these stress proteins in both rat neonatal cardiomyocytes and the myogenic H9 c2 cell line. Cells infected with an adenoviral construct overexpressing both HSP60 and HSP10 were found to be protected against simulated ischemia, whereas cells infected with adenoviral constructs overexpressing only HSP60 or HSP10 alone were not rendered tolerant to simulated ischemic injury.

CONCLUSIONS

These results suggest that the simultaneous expression of these two proteins that form a chaperonin complex in the mitochondria plays an important role in the survival of myogenic cells after ischemia/reperfusion injury.

Stable expression and rapid purification of Escherichia coli GroEL and GroES chaperonins

An Escherichia coli expression vector pRE (P. Reddy, A. Peterkofsky, and K. McKenney, 1989, Nucleic Acids Res. 17, 10473-10488), originally developed for the cloning and expression of lethal genes, was used for cloning and hyperexpression of GroEL and GroES genes. Regulated gene expression is achieved in the pRE vector under the tight control of the lambda PL promoter. Upon induction of the promoter, stable expression of GroEL to about 60% of the total cell protein was observed. Similarly, stable expression of GroES to about 40% of the total cell protein was achieved. GroES was found to be a heat-stable protein while GroEL was not. Both GroE chaperonins were purified in a single chromatographic step with a yield of about 100 mg GroEL and 25 mg GroES per liter of E. coli culture. GroE chaperonins purified by the protocols described here were active in the renaturation of urea-denatured rhodanese.

Trigger factor is induced upon cold shock and enhances viability of Escherichia coli at low temperatures

Trigger factor (TF) in Escherichia coli is a molecular chaperone with remarkable properties: it has prolyl-isomerase activity, associates with nascent polypeptides on ribosomes, binds to GroEL, enhances GroEL's affinity for unfolded proteins, and promotes degradation of certain polypeptides. Because the latter effects appeared larger at 20 degrees C, we studied the influence of temperature on TF expression. Unlike most chaperones (e.g., GroEL), which are heat-shock proteins (hsps), TF levels increased progressively as growth temperature decreased from 42 degrees C to 16 degrees C and even rose in cells stored at 4 degrees C. Upon temperature downshift from 37 degrees C to 10 degrees C or exposure to chloramphenicol, TF synthesis was induced, like that of many cold-shock proteins. We therefore tested if TF expression might be important for viability at low temperatures. When stored at 4 degrees C, E. coli lose viability at exponential rates. Cells with reduced TF content die faster, while cells overexpressing TF showed greater viability. Although TF overproduction protected against cold, it reduced viability at 50 degrees C, while TF deficiency enhanced viability at this temperature. By contrast, overproduction of GroEL/ES, or hsps generally, while protective against high temperatures, reduced viability at 4 degrees C, which may explain why expression of hsps is suppressed in the cold. Thus, TF represents an example of an E. coli protein which protects cells against low temperatures. Moreover, the differential induction of TF at low temperatures and hsps at high temperatures appears to provide selective protection against these opposite thermal extremes.

Induction of heat shock proteins DnaK, GroEL, and GroES by salt stress in Lactococcus lactis

The bacterium Lactococcus lactis has become a model organism in studies of growth physiology and membrane transport, as a result of its simple fermentative metabolism. It is also used as a model for studying the importance of specific genes and functions during life in excess nutrients, by comparison of prototrophic wild-type strains and auxotrophic domesticated (dairy) strains. In a study of the capacity of domesticated strains to perform directed responses toward various stress conditions, we have analyzed the heat and salt stress response in the established L. lactis subsp. cremoris laboratory strain MG1363, which was originally derived from a dairy strain. After two-dimensional separation of proteins, the DnaK, GroEL, and GroES heat shock proteins, the HrcA (Orf1) heat shock repressor, and the glycolytic enzymes pyruvate kinase, glyceral-dehyde-3-phosphate dehydrogenase, and phosphoglycerate kinase were identified by a combination of Western blotting and direct N-terminal amino acid sequencing of proteins from the gels. Of 400 to 500 visible proteins, 17 were induced more than twofold during heat stress. Two classes of heat stress proteins were identified from their temporal induction pattern. The fast-induced proteins (including DnaK) showed an abruptly increased rate of synthesis during the first 10 min, declining to intermediate levels after 15 min. GroEL and GroES, which also belong to this group, maintained a high rate of synthesis after 15 min. The class of slowly induced proteins exhibited a gradual increase in the rate of synthesis after the onset of stress. Unlike other organisms, all salt stress-induced proteins in L. lactis were also subjected to heat stress induction. DnaK, GroEL, and GroES showed similar temporal patterns of induction during salt stress, resembling the timing during heat stress although at a lower induction level. These data indicate an overlap between the heat shock and salt stress responses in L. lactis.

The chemokine repertoire of human dermal microvascular endothelial cells and its regulation by inflammatory cytokines

Activation of endothelium is a critical event during the initiation of inflammatory processes and is associated with the induction of cell adhesion molecules and cytokines. The latter include chemotactically active cytokines (chemokines) that promote leukocyte diapedesis from the circulation to sites of evolving inflammation. In this study we evaluated the chemokine repertoire of human endothelial cells derived from the skin (HDMECs) and regulation of these chemokines by cytokines. HDMECs and an immortalized human dermal microvascular endothelial cell line, HMEC-1, were investigated for the expression of C-X-C and C-C chemokines at mRNA and protein levels. Upon stimulation with interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha), both HDMECs and HMEC-1 expressed high levels of IL-8, GRO, and monocyte chemoattractant protein-1 (MCP-1). RANTES was only weakly induced; however, concomitant treatment with TNF-alpha and interferon-gamma (IFN-gamma) led to upregulation of RANTES, indicating a synergy between these two cytokines. The C-X-C chemokine IFN-inducible protein-10 was upregulated by IFN-gamma but not by other cytokines studied. Macrophage inflammatory protein-1alpha and beta, 1-309, and ENA-78 could not be induced. The chemokine repertoires of HDMECs and HMEC-1 were compared to those of human umbilical vein endothelium and found to be rather similar with the important exception that IFN-gamma and IL-4 up-regulated MCP-1 only in macrovascular endothelium. Our data indicate that HDMECs contribute to the dermal cytokine network by selective production of MCP-1, IL-8, GRO, RANTES, and IP-10, which may critically influence the site-specific recruitment of leukocyte subsets.

Examination of the Tn5 transposase overproduction phenotype in Escherichia coli and localization of a suppressor of transposase overproduction killing that is an allele of rpoH

Tn5 transposase (Tnp) overproduction is lethal to Escherichia coli. Tnp overproduction causes cell filamentation, abnormal chromosome segregation, and an increase in anucleated cell formation. There are two simple explanations for the observed phenotype: induction of the SOS response or of the heat shock response. The data presented here show that overproduction of Tnp neither induces an SOS response nor a strong heat shock response. However, our experiments do indicate that induction of some sigma32-programmed function(s) (either due to an rpoH mutation, a deletion of dnaK, or overproduction of sigma32) suppresses Tnp overproduction killing. This effect is not due to overproduction of DnaK, DnaJ, or GroELS. In addition, Tnp but not deltall Tnp (whose overproduction does not kill the host cells) associates with the inner cell membrane, suggesting a possible correlation between cell killing and Tnp membrane association. These observations will be discussed in the context of a model proposing that Tnp overproduction titrates an essential host factor(s) involved in an early cell division step and/or chromosome segregation.

Sequence and transcriptional analysis of groES and groEL genes from the thermophilic bacterium Clostridium thermocellum

The groESL operon from Clostridium thermocellum (Ct) has been isolated and sequenced, revealing two ORFs of 285 and 1626 nt, separated by 48 nt. The first ORF encoded a 94-aa 10.6-kDa GroES homologue; the second encoded a 541-aa polypeptide of 57.6 kDa, that exhibited 61% and 77% sequence identity with GroEL from Escherichia coli (Ec) and Clostridium acetobutylicum (Ca), respectively. A putative tsp, preceded by -10 and -35 consensus promoters, was identified upstream of groES. This was followed by an inverted repeat observed previously in bacterial heat shock genes. A 15-nt palindrome characteristic of a Rho-independent transcription terminator, was located downstream of groEL. The first nt of the groES translational start codon was preceded (7 nt) by a putative RBS (AGGAGG); a second RBS sequence was located 8 nt upstream of the groEL start. Production of GroE homologues by Ct was constitutive, but was enhanced significantly during a temperature upshift from 60 degrees C to 70 degrees C. The Ct GroEL, expressed in Ec as a fusion protein with GST, was purified, free of contaminating Ec GroEL.

Protein misfolding and inclusion body formation in recombinant Escherichia coli cells overexpressing Heat-shock proteins

PreS2-S'-beta-galactosidase, a three-domain fusion protein that aggregates extensively in the cytoplasm of Escherichia coli, was used to systematically investigate the effects of heat-shock protein (hsp) overproduction on protein misfolding and inclusion body formation. While the co-overexpression of the DnaK and DnaJ molecular chaperones led to a 3-6 fold increase in the recovery of enzymatically active preS2-S'-beta-galactosidase over a wide range of growth temperatures (30-42 degrees C), an increase in the concentration of the GroEL and GroES chaperonins had a significant effect at 30 degrees C only. Co-immunoprecipitation experiments confirmed that preS2-S'-beta-galactosidase formed a stable complex with DnaK, but not with GroEL, at 42 degrees C. When the intracellular concentration of chromosomal heat-shock proteins was increased by overproduction of the heat-shock transcription factor sigma 32, or by addition of 3% ethanol (v/v) to the growth medium, a 2-3 fold higher recovery of active enzyme was observed at 30 and 42 degrees C, but not at 37 degrees C. The overexpression of all heat-shock proteins or specific chaperone operons did not significantly affect the synthesis rates or stability of preS2-S'-beta-galactosidase and did not lead to the disaggregation of preformed inclusion bodies. Rather, the improvements in the recovery of soluble and active fusion protein resulted primarily from facilitated folding and assembly. Our findings suggest that titration of the DnaK-DnaJ early folding factors leads to the formation of preS2-S'-beta-galactosidase inclusion bodies.

Chaperonin-mediated assembly of wild-type and mutant subunits of human propionyl-CoA carboxylase expressed in Escherichia coli

We developed a bacterial expression system for the human alpha and beta cDNAs of propionyl-CoA carboxylase (PCC). These cDNAs (less the putative mitochondrial matrix targeting presequences) were co-expressed in Escherichia coli on one plasmid vector with each cDNA having its own IPTG-inducible promoter. Only negligible amounts of active PCC were measured despite the presence of both alpha and beta subunits as indicated by Western blot analysis and the almost complete biotinylation of the alpha subunit. Co-expression of this plasmid with a second plasmid vector over-expressing the E. coli chaperonin proteins, groES and groEL, resulted in a several hundred-fold increase in PCC specific activity, to a level comparable with that found in crude human liver extracts. PCC was partially purified on monomeric avidin affinity resin and the presence of both alpha and beta subunits was demonstrated, thereby confirming the assembly of both subunits into an active enzyme. Deficiency of either alpha PCC or beta PCC results in propionic acidemia, an autosomal recessive disorder. We used this expression system to characterize one missense mutation previously described in five Japanese alleles, namely C1283T (Thr428lle) in beta PCC. This mutation, when expressed in E.coli under the same conditions as that of wild-type PCC, had null activity, despite the presence of assembled alpha PCC and beta PCC subunits. This bacterial expression system can be useful for analysis of either alpha PCC or beta PCC mutations. Our findings indicated that the groES and groEL chaperonin proteins were essential for folding and assembly of the human PCC heteromeric subunits.

Suppression of ftsH mutant phenotypes by overproduction of molecular chaperones

Decreased intracellular levels of FtsH, a membrane-bound ATPase, led to retardation of growth and protein export, as well as to an abnormal translocation of alkaline phosphatase that had been attached to a cytoplasmic domain of a multispanning membrane protein, SecY. The last phenotype is designated Std (stop transfer defective). In this study, we examined the effects of overproduction of some molecular chaperones on the phenotypes of ftsH mutants. The growth retardation was partially suppressed by overproduction of GroEL/GroES (Hsp60/Hsp10) or HtpG (Hsp90), although these chaperones could not totally substitute for FtsH. Overproduction of HtpG specifically alleviated the Std phenotype, while that of GroEL/GroES alleviated the protein export defect of ftsH mutants. These results suggest that FtsH functions can be somehow compensated for when the cellular concentrations of some molecular chaperones increase.

Overproduction, purification and characterization of GroES and GroEL from thermophilic Bacillus stearothermophilus

To facilitate purification of the two chaperonins GroES and GroEL encoded by the thermophilic Bacillus stearothermophilus, an Escherichia coli strain was constructed in which the geoESL operon was replaced by that of B. stearothermophilus. This strain is perfectly viable, demonstrating that the B. stearothermophilus operon is functionally interchangeable with that of E. coli. To increase the amount of GroES, the groES gene was fused to an IPTG-inducible promoter. Both proteins GroES and GroEL, were purified from E. coli using the standard protocol with some modifications. This method should be applicable in all cases where a foreign groE operon can substitute that of E. coli. A preliminary characterization of GroEL, revealed that it has the same secondary structural elements as the E. coli homologue, but its thermodynamic stability is significantly increased.

Increase of solubility of foreign proteins in Escherichia coli by coproduction of the bacterial thioredoxin

Eukaryotic proteins are frequently produced in Escherichia coli as insoluble aggregates. This is one of the barriers to studies of macromolecular structure. We have examined the effect of coproduction of the E. coli thioredoxin (Trx) or E. coli chaperones GroESL on the solubility of various foreign proteins. The solubilities of all eight vertebrate proteins examined including transcription factors and kinases were increased dramatically by coproduction of Trx. Overproduction of E. coli chaperones GroESL increased the solubilities of four out of eight proteins examined. Although the tyrosine kinase Lck that was produced as an insoluble form and solubilized by urea treatment had a very low autophosphorylating activity, Lck produced in soluble form by coproduction of Trx had an efficient activity. These results suggest that the proteins produced in soluble form by coproduction of Trx have the native protein conformation. The mechanism by which coproduction of Trx increases the solubility of the foreign proteins is discussed.

The dnaKJ operon of Agrobacterium tumefaciens: transcriptional analysis and evidence for a new heat shock promoter

The dnaKJ operon of Agrobacterium tumefaciens was cloned and sequenced and was found to be highly homologous to previously analyzed dnaKJ operons. Transcription of this operon in A. tumefaciens was stimulated by heat shock as well as by exposure to ethanol and hydrogen peroxide. There were two transcripts representing the dnaKJ operon: one containing the dnaK and dnaJ genes and the second containing only the dnaK gene. Primer extension analysis indicated that transcription started from the same site in heat-shocked cells and in untreated cells. The upstream regulatory region of the dnaKJ operon of A. tumefaciens does not contain the highly conserved inverted repeat sequence previously found in the groESL operon of this bacterium, as well as in many other groE and dnaK operons. Sequence analysis of the promoter region of several groESL and dnaK operons from alpha-purple proteobacteria indicates the existence of a putative promoter sequence different from the known consensus promoter sequences recognized by the Escherichia coli vegetative or heat shock sigma factor. This promoter may constitute the heat shock promoter of these alpha-purple proteobacteria.

Affinity purification, overexpression, and characterization of chaperonin 10 homologues synthesized with and without N-terminal acetylation

Utilizing the ability of bacterial chaperonin 60 (GroEL) to functionally interact with chaperonin 10 (Cpn10) homologues in an ATP-dependent fashion, we have purified substantial amounts of mammalian, chloroplast, and thermophilic Cpn10 homologues from their natural host. In addition, large amounts of recombinant rat Cpn10 were produced in Escherichia coli and found to be identical to its authentic counterpart except for the lack of N-terminal acetylation. By comparing these two forms of Cpn10, it was found that acetylation does not influence the oligomeric structure of Cpn10 and is not essential for chaperone activity or mitochondrial import in vitro. In contrast, N-terminal acetylation proved crucial in the protection of Cpn10 against degradation by N-ethylmaleimide-sensitive proteases derived from organellar preparations of rat liver. The availability of large amounts of both affinity-purified and recombinant Cpn10 will facilitate not only further characterization of the eukaryotic folding machinery but also further scrutiny of the reported function of Cpn10 as early pregnancy factor.

Multicopy suppression of cold-sensitive sec mutations in Escherichia coli

Mutations in the secretory (sec) genes in Escherichia coli compromise protein translocation across the inner membrane and often confer conditional-lethal phenotypes. We have found that overproduction of the chaperonins GroES and GroEL from a multicopy plasmid suppresses a wide array of cold-sensitive sec mutations in E. coli. Suppression is accompanied by a stimulation of precursor protein translocation. This multicopy suppression does not bypass the Sec pathway because a deletion of secE is not suppressed under these conditions. Surprisingly, progressive deletion of the groE operon does not completely abolish the ability to suppress, indicating that the multicopy suppression of cold-sensitive sec mutations is not dependent on a functional groE operon. Indeed, overproduction of proteins unrelated to the process of protein export suppresses the secE501 cold-sensitive mutation, suggesting that protein overproduction, in and of itself, can confer mutations which compromise protein synthesis and the observation that low levels of protein synthesis inhibitors can suppress as well. In all cases, the mechanism of suppression is unrelated to the process of protein export. We suggest that the multicopy plasmids also suppress the sec mutations by compromising protein synthesis.

Expression in Escherichia coli, purification and functional activity of recombinant human chaperonin 10

We have recently reported the cloning of a cDNA coding for a stress inducible human chaperonin 10. The protein was shown to possess 100% identity with the bovine homologue and a single amino acid replacement (glycine to serine at position 52) compared to rat chaperonin 10. Here we report the heterologous expression of human chaperonin 10 in Escherichia coli, its purification and its functional characterization. The recombinant protein was purified to homogeneity as judged by different analytical techniques, and mass spectrometry analysis showed a MW of 10,801 Da in agreement with the predicted sequence. This molecular weight accounts for a protein which is not modified post-translationally. In fact, natural rat chaperonin 10 has been shown to be acetylated at the N-terminus, a feature suggested to be important for targeting and functional activity. Here we show that recombinant human chaperonin 10 is fully active in assisting the chaperonin 60 GroEL in the refolding of denatured yeast enolase, thereby showing that, at least in the present system, post-translational acetylation is not necessary for its activity.

Purification and characterization of recombinant human p50csk protein-tyrosine kinase from an Escherichia coli expression system overproducing the bacterial chaperones GroES and GroEL

An Escherichia coli expression system overproducing the bacterial chaperones GroES and GroEL was engineered and has been successfully used to produce large quantities of the recombinant human protein-tyrosine kinase p50csk. The co-overproduction of the two chaperones with p50csk results in increased solubility of the kinase and allows purification of milligram amounts of active enzyme. Analysis of the purified protein by SDS/polyacrylamide gel electrophoresis reveals a single band with an apparent molecular mass of 50 kDa, indicating that recombinant human p50csk has been purified to near homogeneity. The purified enzyme displays tyrosine kinase activity as measured by both autophosphorylation and phosphorylation of exogenous substrates. Biochemical properties, including in vitro substrate specificity and enzymatic characteristics of the enzyme, have been assessed and compared with those of members of the Src family of protein-tyrosine kinases. Results indicate that p50csk and p56lck have different substrate specificities and that p50csk and p60c-src have similar kinetic parameters. The successful production and purification of an enzymatically active form of p50csk will enable further characterization of this important kinase and allow clarification of its physiological role. In addition, the results suggest that the approach described may be generally applicable to improve the solubility of recombinant proteins which otherwise are produced in an insoluble form in E. coli.

Thermally-induced cell lysis in Escherichia coli K12

Escherichia coli cells exposed to high temperatures exhibit a progressive loss of viability. We observed two mechanisms of cell death induced by lethal temperatures: with and without lysis. The number of cells lysed by heat decreased at later stages of the growth curve, when cells were pre-treated at lower temperatures for 10 minutes and when cells were pre-treated with novobiocin, nalidixic acid and cadmium chloride. Cell lysis was similar in wild type, rpoH, groE and dnaK mutant cells as well as in cells which overproduce heat shock proteins GroE or DnaK. Results using cells aligned for cell division and cells growing at 42 degrees C, 45 degrees C and 47 degrees C suggest that cells near division are more sensitive to lysis and that a high concentration of heat-shock proteins increases their resistance to lysis.

Selective in vivo rescue by GroEL/ES of thermolabile folding intermediates to phage P22 structural proteins

The in vivo conformational substrates of the GroE chaperonins have been difficult to identify, in part because of limited information on in vivo polypeptide chain folding pathways. Temperature-sensitive folding (tsf) mutants have been characterized for the coat protein and tailspike protein of phage P22. These mutations block intracellular folding at restrictive temperature by increasing the lability of folding intermediates without impairing the stability or function of the native state. Overexpression of GroEL/ES suppressed the defects of tsf mutants at 17 sites in the coat protein, by improving folding efficiency rather than assembly efficiency or protein stability. Immunoprecipitation experiments demonstrated that GroEL interacted transiently with newly synthesized wild-type coat protein and that this interaction was prolonged by the tsf mutations. Folding defects of the tailspike polypeptide chains were not suppressed. A fraction of the tsf mutant tailspike chains bound to GroEL but were inefficiently discharged. The results suggest that 1) thermolabile folding intermediates are natural substrates of GroEL/ES; 2) although GroEL may bind such intermediates for many proteins, the chaperoning function is limited to a subset of substrate proteins; and 3) a key reason for the heat-shock response may be to stabilize thermolabile folding intermediates at elevated temperatures.