HIV is an enveloped virus which causes acquired immune deficiency syndrome in humans. HIV infects the host T-cells and macrophages and deteriorates the whole immune system. The virus initiates the infection process by membrane fusion of the viral and host cell by the membrane protein gp160. Gp160 forms a trimer structure with each gp160 molecule comprising two non-covalently bonded subunit - gp120 and gp41. Gp120 binds to the host cell receptors and coreceptors, undergoes conformational change and moves away to expose gp41. The fusion between the viral and host cell membranes is catalyzed by the ectodomain of the gp41 protein. A large number of antibody binding sites are located in the gp120 and gp41 sequence. Also, gp120 and gp41 are capable of eliciting neutralizing antibodies in vivo. However, there are a large number of drawbacks for the neutralization of the HIV - (a) fast mutation of HIV strains to evade immune response, (b) non-exposure of most of the antibody epitope regions due to heavy glycosylation, and (c) non-neutralization of a broad range of HIV strains due to mutation. Due to these difficulties, there is no vaccine for HIV developed yet.My research work focused on production of large gp41 ectodomain constructs and biophysical and structural characterization. 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. One significant finding was observation of synergy between the N-terminal FP, gp41 ectodomain core region, and C-terminal MPER in vesicle fusion. The oligomeric state of gp41 ectodomain has been studied. 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 conformation. A new HIV fusion model has been proposed through this work, where: (1) hemifusion of membranes 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 -sheet arrangement. Another reason of interest was that MPER is a highly conserved region which can bind to few broadly neutralizing antibodies effective against various strains of HIV. Two of these antibodies bind to our designed and produced gp41 ectodomain constructs and therefore, adds a step forward towards investigation of the gp41 ectodomain as an immunogen in HIV vaccine development.
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