Accumulation of secretory protein precursors in Escherichia coli induces the heat shock response

Involvement of the DnaK-DnaJ-GrpE chaperone team in protein secretion in Escherichia coli

We used depletion studies designed to further investigate the role of the DnaK, DnaJ, and GrpE heat shock proteins in the SecB-dependent and SecB-independent secretion pathways. Our previous finding that SecB-deficient strains containing the grpE280 mutation were still secretion proficient raised the possibility that GrpE was not involved in this secretory pathway. Using depletion studies, we now demonstrate a requirement for GrpE in this pathway. In addition, depletion studies demonstrate that while DnaK, DnaJ, and GrpE are involved in the secretion of the SecB-independent proteins (alkaline phosphatase, ribose-binding protein, and beta-lactamase), they are not the primary chaperones in this process.

Folding of a mutant maltose-binding protein of Escherichia coli which forms inclusion bodies

The maltose-binding protein (MalE) of Escherichia coli is the periplasmic component of the transport system for malto-oligosaccharides. We have examined the characteristics of a Mal- mutant of malE corresponding to the double substitution Gly32 --> Asp/Ile33 --> Pro, MalE31, previously obtained by random mutagenesis. In vivo, the MalE31 precursor is efficiently processed, but the mature protein forms inclusion bodies in the periplasm. Furthermore, the accumulation of insoluble MalE31 is independent of its cellular localization; MalE31 lacking its signal sequence forms inclusion bodies in the cytoplasm. The native MalE31 protein can be purified by affinity chromatography from inclusion bodies after denaturation by 8 M urea. The renatured protein exhibits full maltose binding affinity (Kd= 9 x 10(-7) M), suggesting that its folded structure is similar to that of the wild-type protein. Unfolding/refolding experiments show that MalE31 is less stable (-5. 5 kcal/mol) than the wild-type protein (-9.5 kcal/mol) and that folding intermediates have a high tendency to form aggregates. In conclusion, the observed phenotype of cells expressing malE31 can be explained by a defective folding pathway of the protein.

Transcriptional regulation of stress-inducible genes in procaryotes

In procaryotes such as Escherichia coli, transcriptional activation of heat shock genes in response to elevated temperature is caused primarily by transient increase in the amount of sigma 32 (rpoH gene product) specifically required for transcription from the heat shock promoters. The increase in sigma 32 level results from increased translation of rpoH mRNA and from stabilization of sigma 32 which is ordinarily very unstable. Some of the factors and cis-acting elements that constitute the complex regulatory circuits have been identified and characterized, but detailed mechanisms as well as nature of sensors and signals remain to be elucidated. Whereas this "classical" heat shock regulon (sigma 32 regulon) provides major protective functions against thermal stress, a second heat shock regulon mediated by sigma E (sigma 24) encodes functions apparently required under more extreme conditions, and is activated by responding to extracytoplasmic signals. These regulons mediated by minor sigma factors (sigma 32 in particular) appear to be conserved in most gram-negative bacteria, but not in gram-positive bacteria.

Effects of temperature and of heat shock on the expression and action of the colicin A lysis protein

At low temperature, the synthesis of the colicin A lysis protein in Escherichia coli was slowed down, and consequently its functioning was retarded. The rates were restored when the bacteria were shifted for 10 min to 42 degrees C, except in an rpoH mutant, suggesting that one or more proteins regulated by sigma 32 is necessary for expression of colicin A lysis protein.

Beta-galactosidase is inactivated by intermolecular disulfide bonds and is toxic when secreted to the periplasm of Escherichia coli

The wild-type LamB-LacZ hybrid protein inhibits the export machinery upon induction when assayed by biochemical and genetic techniques, a phenotype referred to as hybrid protein jamming. This hybrid protein also renders cells sensitive to growth in the presence of the inducer maltose, presumably because of the jamming. We constructed a new version of this fusion by adding alkaline phosphatase, encoded by phoA, to the C terminus of the LamB-LacZ hybrid protein. This tripartite protein, LamB-LacZ-PhoA, is as toxic to cells as the hybrid LamB-LacZ; however, it does not jam at temperatures greater than 33 degrees C. Extreme C-terminal sequences of LacZ function as a critical folding domain and are therefore responsible for stabilizing the LacZ structure. To determine if this region of LacZ is important for jamming, we recombined a late nonsense mutation (X90) onto the hybrid construct. We found the toxicity of this new hybrid, LamB-LacZX90, to be nearly identical to that of the full-length protein, but it also does not jam the secretion machinery. This suggests that jamming is caused by LacZ folding. We found no inhibition of secretion in the tripartite and X90 fusion strains at 37 degrees C, suggesting that the toxicity of the new fusions is novel. Under these conditions, the tripartite and X90 fusion proteins form disulfide-bonded aggregates with high molecular weights in the periplasm. Accordingly, we believe that LacZ disrupts some essential function(s) in the periplasm.

Heterologous protein secretion and the versatile Escherichia coli haemolysin translocator

Heterologous proteins synthesized in the Gram-negative bacterium Escherichia coli in bioreactor culture may accumulate in one of three 'compartments':the cytoplasm, the periplasm, or the extracellular medium. Many overexpressed proteins from various origins have been purified from each of these locations. However, to date, each system has required specific tailoring to meet the stringent requirements for each protein product to ensure correct folding, activity and appropriate yield. The E. coli haemolysin secretion system appears to provide a flexible mechanism with which to secrete a wide variety of heterologous fusion proteins into the extracellular medium.

Protein secretion pathway in Escherichia coli

The export of proteins to the Escherichia coli periplasm is a well established system for heterologous protein production. With a better understanding of the protein export (SecA, Y-dependent) process and a greater awareness of the conditions necessary for correct folding of proteins in the periplasm, serious efforts are now being made to manipulate this system to achieve substantial increases in the yield of authentically folded proteins. Further advances in the development of methods for the recovery of recombinant proteins from the culture medium have made the use of fusion proteins secreted by the protein A or haemolysin pathways a more attractive option. Recent studies of the haemolysin system indicate its ability to secrete a wide range of polypeptides, including normally cytoplasmic proteins. As their features and potential applications become much clearer, a rapidly expanding number of protein-secretion mechanisms in Gram-negative bacteria are becoming available for heterologous protein expression. Most, if not all, of these systems can be successfully transplanted into E. coli, providing a wider choice of systems for the future.

The cellular response to unfolded proteins: intercompartmental signaling

Both prokaryotic and eukaryotic cells respond to the accumulation of unfolded proteins by increasing the transcription of genes encoding molecular chaperones and other stress-responsive proteins. Different sets of genes are activated when particular cellular compartments are burdened with unfolded proteins. Cells thus maintain mechanisms to monitor changes in the concentration of unfolded proteins not only in the cytosol, but also in membrane-bound extracytoplasmic compartments. During the past year, work in yeast has identified a transmembrane receptor that appears to play a pivotal role in the regulation of protein folding. This receptor monitors the concentration of available chaperone molecules in the endoplasmic reticulum and transmits a signal to the cytosol to activate the transcription of nuclear genes encoding chaperones that are localized in the endoplasmic reticulum. Work using Escherichia coli suggests that prokaryotes also contain an intercompartmental 'unfolded protein' signaling pathway, in this case from the periplasmic space or outer membrane to the cytoplasm.

Induction of heat shock proteins by abnormal proteins results from stabilization and not increased synthesis of sigma 32 in Escherichia coli

Accumulation of abnormal proteins in cells of bacteria or eukaryotes can induce synthesis of a set of heat shock proteins. We examined such induction following addition of azetidine (a proline analog) or synthesis of a heterologous protein (human prourokinase) in Escherichia coli. Synthesis of heat shock proteins under these conditions increased almost immediately and continued with increasing rates until it reached a maximum after 30 to 60 min at 30 degrees C. The induction was closely accompanied by an increase in the cellular level of sigma 32 specifically required for transcription of heat shock genes. The increase in sigma 32 initially coincided with increased synthesis of heat shock proteins but then exceeded the latter, particularly following accumulation of prourokinase. The sigma 32 level increase upon either treatment was found to result solely from stabilization of sigma 32, which is ordinarily very unstable, and not from increased synthesis of sigma 32. This is in contrast to what had been found when cells were exposed to a higher temperature, at which both increased synthesis and stabilization of sigma 32 contributed to the increased sigma 32 level. On the basis of these and other findings, we propose that abnormal proteins stabilize sigma 32 by a pathway or a mechanism distinct from that used for the induction of sigma 32 synthesis known to occur at the level of translation. Evidence further suggests that the DnaK chaperone plays a crucial regulatory role in induction of the heat shock response by abnormal proteins.

Effects of reduced levels of GroE chaperones on protein metabolism: enhanced synthesis of heat shock proteins during steady-state growth of Escherichia coli

The GroE heat shock proteins (GroEL and GroES) of Escherichia coli represent major molecular chaperones that participate in folding (and assembly) of a variety of proteins and are essential for cell growth at all temperatures. We have examined the effects of reducing the cellular content of GroE on the synthesis and stability of proteins during steady-state growth with near-normal rates. The GroE protein level was manipulated by placing groE under the control of lacUV5 promoter on a multicopy plasmid in a strain lacking the chromosomal groE operon. When this strain was grown with a limited concentration (40 microM) of inducer (IPTG [isopropyl-beta-D-thiogalactopyranoside]) at 37 degrees C, the GroE level and growth rate were comparable to those of the wild type. When cells were depleted of IPTG, they continued to grow at or below 37 degrees C albeit at reduced rates, despite the much-reduced GroE level (ca. 25% of that of wild type). Under these conditions, the cellular contents of at least 13 polypeptides were affected. Among the most striking effects was the enhanced synthesis of a set of heat shock proteins which resulted from the increased level of sigma 32 which is required for transcription of heat shock genes. This increase in the sigma 32 level was brought about by both stabilization and increased synthesis of sigma 32. Other proteins affected by the reduced GroE level included two proteins (enzymes of the Entner-Doudoroff pathway) encoded by the edd-eda operon and the ribosomal protein S6, suggesting that the GroE chaperones are involved in regulating expression of genes for carbohydrate metabolism and in modulating biogenesis or function of the ribosome.

Truncated mutants of the colicin A lysis protein are acylated and processed when overproduced

The lipid modification and processing of truncated mutants of the colicin A lysis protein were observed after overproduction by Escherichia coli bacteria, but at a rate far slower than that of the wild-type. The unmodified precursor form of the mutants was stable over hour(s). The truncated mutants provoked lethality, but neither caused protein release nor quasi-lysis.

The DnaK homologue of the marine Vibrio sp. strain S14 binds to the unprocessed form of a carbon starvation-specific periplasmic protein

The Escherichia coli DnaK homologue in Vibrio sp. strain S14 was shown to possess chaperone function for translocation during carbon starvation. This was demonstrated by using the method of co-immunoprecipitation. DnaK co-precipitated with the carbon starvation-specific periplasmic space protein Csp5 three hours after the onset of carbon starvation. Pulse-chasing of the protein with radiolabelled methionine followed by the addition of an excess of unlabelled methionine demonstrated that the Csp5 protein was translocated across the inner membrane. Only the cytoplasmic unprocessed precursor form of Csp5 co-precipitated with DnaK. The non-covalent binding between the two proteins was found to be ATP-dependent, as the addition of ATP released the interaction between DnaK and the precursor form of Csp5, as was shown on silver-stained SDS-polyacrylamide gels and by Western blot analysis. We suggest that DnaK maintains the carbon starvation-inducible protein Csp5 in a translocation-competent form in the cytoplasm.

Adaptation of Escherichia coli to the uncoupler of oxidative phosphorylation 2,4-dinitrophenol

Escherichia coli was found to adapt to the uncoupler of oxidative phosphorylation 2,4-dinitrophenol. The rates of synthesis of 53 proteins were increased following exposure to 2,4-dinitrophenol. Adaptation was accelerated when the cofactor pyrroloquinoline quinone was provided in the growth medium.