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.