An efficient production and characterization of HIV-1 gp41 ectodomain with fusion peptide in Escherichia coli system

Folded monomers and hexamers of the ectodomain of the HIV gp41 membrane fusion protein: potential roles in fusion and synergy between the fusion peptide, hairpin, and membrane-proximal external region

HIV is an enveloped virus and fusion between the HIV and host cell membranes is catalyzed by the ectodomain of the HIV gp41 membrane protein. Both the N-terminal fusion peptide (FP) and C-terminal membrane-proximal external region (MPER) are critical for fusion and are postulated to bind to the host cell and HIV membranes, respectively. Prior to fusion, the gp41 on the virion is a trimer in noncovalent complex with larger gp120 subunits. The gp120 bind host cell receptors and move away or dissociate from gp41 which subsequently catalyzes fusion. In the present work, large gp41 ectodomain constructs were produced and biophysically and structurally characterized. One significant finding is observation of synergy between the FP, hairpin, and MPER in vesicle fusion. The ectodomain-induced fusion can be very efficient with only ∼15 gp41 per vesicle, which is comparable to the number of gp41 on a virion. Conditions are found with predominant monomer or hexamer but not trimer and these may be oligomeric states during fusion. Monomer gp41 ectodomain is hyperthermostable and has helical hairpin structure. A new HIV fusion model is presented where (1) hemifusion is catalyzed by folding of gp41 ectodomain monomers into hairpins and (2) subsequent fusion steps are catalyzed by assembly into a hexamer with FPs in an antiparallel β sheet. There is also significant interest in the gp41 MPER because it is the epitope of several broadly neutralizing antibodies. Two of these antibodies bind our gp41 ectodomain constructs and support investigation of the gp41 ectodomain as an immunogen in HIV vaccine development.

Escherichia coli-expressed near full length HIV-1 envelope glycoprotein is a highly sensitive and specific diagnostic antigen

Background

The Human Immunodeficiency Virus type 1 (HIV-1) envelope glycoprotein gp160, useful in detecting anti-HIV-1 antibodies, is difficult to express in heterologous hosts. The major hurdles are its signal sequence, strong hydrophobic regions and heavy glycosylation. While it has not been possible to express full length recombinant (r)-gp160 in E. coli, it can be expressed in insect and mammalian cells, but at relatively higher cost. In this work, we report E. coli-based over-expression of r-gp160 variant and evaluate its performance in diagnostic immunoassays for the detection of anti-HIV-1 antibodies.

Methods

A deletion variant of r-gp160 lacking hydrophobic regions of the parent full length molecule was expressed in E. coli and purified to near homogeneity using single-step Ni(II)-affinity chromatography. Biotinylated and europium(III) chelate-labeled versions of this antigen were used to set up one- and two-step time-resolved fluorometric double antigen sandwich assays. The performance of these assays was evaluated against a collection of well-characterized human sera (n=131), that included an in-house panel and four commercially procured panels.

Results

In-frame deletion of three hydrophobic regions, spanning amino acid residues 1–43, 519–538 and 676–706, of full length HIV-1 gp160 resulted in its expression in E. coli. Both the one- and two-step assays manifested high sensitivity unambiguously identifying 75/77 and 77/77 HIV-1 positive sera, respectively. Both assays also identified all 52 HIV-seronegative sera correctly. Between the two assays, the mean signal-to-cutoff value of the two-step assay was an order of magnitude greater than that of the one-step assay. Both assays were highly specific manifesting no cross-reactivity towards antibodies specific to other viruses like hepatitis B, C, and human T cell leukemia viruses.

Conclusions

This study has demonstrated the expression of r-gp160 variant in E. coli, by deletion of hydrophobic regions, and its purification in reasonable yields. This underscores the potential for cost saving in antigen production. Evaluation of this antigen in a double antigen sandwich two-step assay showed it to be a highly sensitive and specific HIV-1 diagnostic reagent. The amenability of this assay to the one-step format suggests its potential utility in developing a rapid point-of-care HIV-1 diagnostic test.

Solid-state nuclear magnetic resonance (NMR) spectroscopy of human immunodeficiency virus gp41 protein that includes the fusion peptide: NMR detection of recombinant Fgp41 in inclusion bodies in whole bacterial cells and structural characterization of purified and membrane-associated Fgp41

Human immunodeficiency virus (HIV) infection of a host cell begins with fusion of the HIV and host cell membranes and is mediated by the gp41 protein, a single-pass integral membrane protein of HIV. The 175 N-terminal residues make up the ectodomain that lies outside the virus. This work describes the production and characterization of an ectodomain construct containing the 154 N-terminal gp41 residues, including the fusion peptide (FP) that binds to target cell membranes. The Fgp41 sequence was derived from one of the African clade A strains of HIV-1 that have been less studied than European/North American clade B strains. Fgp41 expression at a level of ~100 mg/L of culture was evidenced by an approach that included amino acid type (13)CO and (15)N labeling of recombinant protein and solid-state NMR (SSNMR) spectroscopy of lyophilized whole cells. The approach did not require any protein solubilization or purification and may be a general approach for detection of recombinant protein. The purified Fgp41 yield was ~5 mg/L of culture. SSNMR spectra of membrane-associated Fgp41 showed high helicity for the residues C-terminal of the FP. This was consistent with a "six-helix bundle" (SHB) structure that is the final gp41 state during membrane fusion. This observation and negligible Fgp41-induced vesicle fusion supported a function for SHB gp41 of membrane stabilization and fusion arrest. SSNMR spectra of residues in the membrane-associated FP provided evidence of a mixture of molecular populations with either helical or β-sheet FP conformation. These and earlier SSNMR data strongly support the existence of these populations in the SHB state of membrane-associated gp41.