Deletion of hydrophobic domains of viral glycoproteins increases the level of their production in Escherichia coli

Expression of membrane proteins at the Escherichia coli membrane for structural studies

Structural biology of membrane proteins is often limited by the first steps in obtaining sufficient yields of proteins because native sources are seldom. Heterologous systems like bacteria are then commonly employed for membrane protein over-expression. Escherichia coli is the main bacterial host used. However, overproduction of a foreign membrane protein at a non-physiological level is usually toxic for cells or leads to inclusion body formation. Those effects can be reduced by optimizing the cell growth conditions, choosing the suitable bacterial strain and expression vector, and finally co-expressing the target protein and the b-subunit of E. coli adenosine triphosphate (ATP)-synthase, which triggers the proliferation of intracytoplasmic membranes. This chapter is devoted to help the experimenter in choosing the appropriate plasmid/bacterial host combination for optimizing the amount of the target membrane protein produced in its correct folded state.

Expression and characterization of N-terminal domain of Epstein-Barr virus latent membrane protein 2A in Escherichia coli

Latency of Epstein-Barr virus (EBV) is maintained by the transmembrane protein latent membrane protein (LMP) 2A, which mimics the B-cell receptor (BCR) and perturbs BCR signaling. LMP2A contains a cytoplasmic N-terminal domain composed of 119 amino acids, which provides signals that are responsible for the association with various signal molecules, resulting in negative regulation of B-cell signaling and the EBV lytic cycle. In the present study, to obtain N-terminal domain of LMP2A (LMP2A NTD, 13 kDa) in Escherichia coli for structural analysis, a strategy for obtaining the unfused form of LMP2A NTD without any fusion partners was proposed. Recombinant LMP2A NTD has previously been expressed using the GST fusion system in E. coli [Virology 268 (2000) 178, J. Virol. 71 (1997) 4752, Mol. Cell. Biol. 20 (2000) 8526]. However, we were unable to obtain untagged LMP2A NTD from this construct because of rapid proteolysis by thrombin. To overcome the proteolysis by thrombin, C-terminal His-tagged LMP2A NTD and intein-fused LMP2A NTD were prepared. As a result, LMP2A NTD without a fusion partner could be successfully obtained using non-enzymatic cleavage. The secondary structure of the recombinant LMP2A NTD was analyzed using circular dichroism. In aqueous solution, LMP2A NTD adopts an unordered structure, which was not affected by varying pH and salt concentration. In addition, any secondary structural components of LMP2A NTD were not induced in the membrane-mimicking environments, suggesting that LMP2A NTD may intrinsically have a random coil-like structure. The biological activity of recombinant LMP2A NTD was monitored by chemical shift perturbation in HSQC spectra of LMP2A NTD with or without WW domains, which result supports that the structural change induced by WW domains is restricted within narrow region.

Amino acid content of recombinant proteins influences the metabolic burden response

Recombinant protein production in Escherichia coli often results in a dramatic cellular stress response best characterized by a decrease in overall cell fitness. We determined that the primary sequence (the amino acid sequence) of the recombinant protein alone plays an important role in mitigating this response. To do so, we created two polypeptides, modeled after the 39-40 amino acid Defensin class of proteins, which contained exclusively the five least (PepAA; His, Trp, Tyr, Phe, Met), or most (PepCO: Ala, Glu, Gln, Asp, Asn) abundant amino acids in E. coli. We determined that overexpression of PepAA resulted in a drastic decrease in growth rate compared to overexpression of PepCO, our model Defensin protein MGD-1, or the 26 amino acid polypeptide contained within the pET-3d vector backbone. We further determined, using Affymetrix E. coli gene chips, that differences among the whole-genome transcriptional responses of these model systems were best characterized by altered expression of genes whose products are involved in translation, transport, or metabolic functions as opposed to stress response genes. Based on these results, we confirmed that translation efficiency was significantly reduced in cells overexpressing PepAA compared with the other model polypeptides evaluated.

Mammalian apoptosis-inducing protein, HAP, induces bacterial cell death

In attempting to produce the HAP, endoplasmic reticulum (ER) targeted apoptosis-inducing protein, as a GST-fusion protein we found that the expression of HAP, but not GST alone, induced bacterial cell death. The HAP protein inhibited the bacterial growth within 30 min after inducting HAP expression. The transmission electron microscopic examination revealed that the morphology of the bacterial cells expressing hap was changed dramatically: unusually elongated phenotype compared with those of controls and finally leading to cell death. The lethality of HAP was relieved by the addition of vitamin E as a reducing agent and under anaerobic growth conditions. These results suggest that a trace amount of HAP induces bacterial cell death and the death is related with reactive oxygen species (ROS).

Interplay of SOS induction, recombinant gene expression, and multimerization of plasmid vectors in Escherichia coli

Using pBR322- and pUC-derived plasmid vectors, a homologous (Escherichia coli native esterase) and three heterologous proteins (human interleukin-2, human interleukin-6, and Zymomonas levansucrase) were synthesized in E. coli IC2015(recA::lacZ) and GY4786 (sfiA::lacZ) strains. Via time-course measurement of beta-galactosidase activity in each recombinant culture, the SOS induction was estimated in detail and the results were systematically compared. In recombinant E. coli, the SOS response did not happen either with the recombinant insert-negative plasmid backbone alone or the expression vectors containing the homologous gene. Irrespective of gene expression level and toxic activity of synthesized foreign proteins, the SOS response was induced only when the heterologous genes were expressed using a particular plasmid vector, indicating strong dependence on the recombinant gene clone and the selection of a plasmid vector system. It is suggested that in recombinant E. coli the SOS response (i.e., activation of recA expression and initial sfiA expression) may be related neither to metabolic burden nor toxic cellular event(s) by synthesized heterologous protein, but may be provoked by foreign gene-specific interaction between a foreign gene and a plasmid vector. Unlike in E. coli XL1-blue(recA(-)) strains used, all expression vectors encoding each of the three heterologous proteins were multimerized in E. coli IC2015 strains in the course of cultivation, whereas the expression vectors containing the homologous gene never formed the plasmid multimers. The extent of multimerization was also dependent on a foreign gene insert in the expression vector. As a dominant effect of the SOS induction, recombinant plasmid vectors used for heterologous protein expression appear to significantly form various multimers in the recA(+) E. coli host.

Effect of tandem rare codon substitution and vector-host combinations on the expression of the EBV gp110 C-terminal domain in Escherichia coli

Gp110 of Epstein-Barr virus (EBV) is a glycoprotein that functions exclusively during the assembly of EBV nucleocapsid and the release of infectious EBV. Its C-terminal tail domain (gp110 CTD) is essential for gp110's function and may provide signals that are responsible for the assembly and release of EBV. In the present study, to get large amounts of gp110 CTD for structural analysis, the effects of vector system, codon usage, and host strain on expression levels of gp110 CTD in Escherichia coli have been investigated. The coding region of gp110 CTD (11 kDa) was subcloned into the expression vectors pSE 280, pET-15b, pET-29a, pMAL-c2x, and pGEX-4T-1. Except the pMAL-c2x construct, all the others failed to express detectable amounts of recombinant gp110 CTD. Substituting a tandem rare AGA (Arg) codon with a synonymous CGC (Arg) codon facilitated expression of the recombinant protein, while a protease-deficient host E. coli strain helped in the accumulation of a soluble form of gp110 CTD fusion. The secondary structures of the obtained recombinant gp110 CTD purified from soluble extracts and inclusion bodies were compared using circular dichroism analysis. In aqueous solutions, both samples equally adopt a mixed alpha-helix and beta-sheet conformation as well as a partly unordered structure. Notably, in the membrane-mimicking environments the helical propensity of gp110 CTD increased up to the previously predicted level based on its sequence, suggesting that gp110 CTD may fold into a more stable conformation through interactions with the cell membrane.

High-level expression of a truncated wall-associated protein A from the dental cariogenic Streptococcus mutans

Streptococcus mutans plays a primary role in the formation of dental caries. Previously, in our laboratory, an S. mutans genomic library was prepared, and the wapA gene was cloned into the shuttle vector, pSA4/4B2. To generate overexpression of wapA and to facilitate efficient purification of the WapA protein for use as an immunogen, an expression vector with the strong tac promoter was used. In order to answer questions regarding the optimization of solubility and expression based on gene size or the hydrophobicity of the protein product, 12 truncated constructs of the wapA gene were prepared using PCR. The truncated products were subcloned into the pGEX-6P-1 glutathione S-transferase (GST) fusion vector and expressed in E. coli BL21. The fusion proteins were analyzed by SDS-PAGE and confirmed by analysis with anti-GST and anti-WapA antibodies. Our study suggests that abrogation of the wapA promoter is necessary for expression of this gene in this expression system. Deletion of the signal peptide and the hydrophobic C terminus of WapA increased expression compared with the full-length construct, and truncation at the protease cleavage site of the C-terminal region greatly increased the stability of the protein without a loss in reactivity with the anti-WapA antibody. Western immunoblot analysis with anti-WapA antiserum clearly showed that the majority of the epitopes of the GST-WapA fusions are located in the N-terminal region of WapA. The immunogenicity of the various WapA fusion products is being examined in mice and rats to further map the immunologically dominant regions of the protein. This method effectively increased the expression of WapA and should contribute to the further understanding of gene expression of E. coli, as well as aid in the characterization of this protein for future immunologic evaluation.

Heat-inactivation of plasmid-encoded CI857 repressor induces gene expression from Ind- lambda prophage in recombinant Escherichia coli

We have observed significant cell lysis upon temperature up-shift of recombinant Escherichia coli cultures harboring CI857-repressed lambda-based expression vectors. This event, that becomes evident about 30-40 min after the heat shock, takes place when using the lambda promoter system in Ind- lysogenic strains, but not in others commonly employed for recombinant gene expression. These results strongly suggest that the thermosensitive CI857 repressor, encoded by the expression vector, competes with CI Ind- molecules for binding to the prophage operator region, allowing for expression of lytic genes from the integrated Ind- viral genome upon temperature up-shift. Transcription of viral lytic genes does not include unspecific expression of a reporter sulA::lacZ gene fusion carried in the prophage genome. These results prompt, however, to carefully evaluate the limitations of expression systems based on pL/pR-CI857 in bacterial strains modified through lambda Ind- gene transfer vehicles.

The expression of recombinant genes from bacteriophage lambda strong promoters triggers the SOS response in Escherichia coli

The production of several non-related heterologous proteins in recombinant Escherichia coli cells promotes a significant transcription of recA and sfiA SOS DNA repair genes. The activation of the SOS system occurs when the expression of plasmid-encoded genes is directed by the strong lambda lytic promoters, but not by IPTG-controlled promoters either at 37 or at 42 degrees C, and it is linked to an extensive degradation of the proteins after their synthesis. The triggering signal for the SOS response could be an important arrest of cell DNA replication observed within the first hour after the induction of recombinant gene expression. The stimulation of this DNA repair system can partially account for the toxicity exhibited by recombinant proteins on actively producing E. coli cells.

Expression in Escherichia coli and purification of soluble forms of the F protein of bovine respiratory syncytial virus

Six fragments of the F gene from bovine respiratory syncytial virus (BRSV) were engineered into the pMAL-c2 Escherichia coli expression vector and expressed as C-terminal maltose-binding protein (MBP) fusion products. The resulting polypeptides were partially soluble and single-step purified by affinity chromatography. These fusion proteins were recognized in Western blots by several MAbs directed against human respiratory syncytial virus F protein. In addition, rabbit polyclonal antisera raised against two purified MBP-derived proteins reacted with the BRSV-F protein.

Mapping, cloning and sequencing of a glycoprotein-encoding gene from bovine herpesvirus type 1 homologous to the gE gene from HSV-1

In order to map and identify the glycoprotein-encoding gene from bovine herpesvirus type 1 (BHV-1), homologous to the gE glycoprotein from herpes simplex virus type 1 (HSV-1), a region of the unique short sequence from the BHV-1 genome has been sequenced. The sequenced region contains an ORF coding for a polypeptide of 575 amino acids (aa). The aa sequence presents substantial similarity to that of the glycoprotein gE from HSV-1 and to homologous proteins of related viruses such as pseudorabies virus, equine herpesvirus type 1 and varicella zoster virus. The aa sequence presents additional characteristics compatible with the structure of a viral glycoprotein: signal peptide, putative glycosylation sites and a long C-terminal transmembrane alpha-helix.

Cytoplasmic and periplasmic production of human apolipoprotein E in Escherichia coli using natural and bacterial signal peptides

To understand the toxicity of high levels of heterologous human serum apolipoprotein E (ApoE) in Escherichia coli, as well as to prepare a system for producing the structural domains of this protein, plasmids were constructed in which the coding sequence of the N-terminal domain or all of ApoE followed E. coli or human apolipoprotein signal peptides (SP) or the N-terminal eleven amino acids (f10) of the gene 10-encoded protein of phage T7. High levels of production of the 22-kDa N-terminal domain (22K) of ApoE were obtained either as an f10::22K fusion protein, or using the natural SP, or SP derived from the periplasmic protein, alkaline phosphatase (PhoA), or from the outer membrane protein A (OmpA). Microsequencing showed that the SP of sPhoA::22K and sOmpA::22K, but not sApoE::22K, were correctly processed and, in the former cases, the protein could be released from the cells by osmotic shock. The extent of maturation of sPhoA::22K depended upon the host strain; with JM109, about 50% of the protein was not processed. Microsequencing of the f10::22K fusion protein, which could easily be purified following lysis of the cells, showed that the N-terminal methionine had been removed in agreement with the length parameter rule. Although considerable levels of the f10::ApoE fusion protein could be produced in the cytoplasm, production was markedly less using the PhoA signal peptide and the protein was not easily isolated following osmotic shock. The recombinant protein was biologically active after reconstitution with lipids in spite of the N-terminal modifications introduced.(ABSTRACT TRUNCATED AT 250 WORDS)