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Abstract
The human T-cell leukemia virus transmembrane glycoprotein (TM) is a typical class 1 membrane fusion protein and a subunit of the viral envelope glycoprotein complex. Following activation, the TM undergoes conformational transitions from a native nonfusogenic state to a fusion-active pre-hairpin intermediate that subsequently resolves to a compact trimer-of-hairpins or six-helix bundle. Disruption of these structural transitions inhibits membrane fusion and viral entry and validates TM as an anti-viral and vaccine target. To investigate the immunological properties of fusion-active TM, we have generated a panel of monoclonal antibodies that recognize the coiled-coil domain of the pre-hairpin intermediate. Antibody reactivity is highly sensitive to the conformation of the coiled coil as binding is dramatically reduced or lost on denatured antigen. Moreover, a unique group of antibodies are 100-1000-fold more reactive with the coiled coil than the trimer-of-hairpins form of TM. The antibodies recognize virally expressed envelope, and significantly, some selectively bind to envelope only under conditions that promote membrane fusion. Most importantly, many of the antibodies potently block six-helix bundle formation in vitro. Nevertheless, viral envelope was remarkably resistant to neutralization by antibodies directed to the coiled coil. The data imply that the coiled coil of viral envelope is poorly exposed to antibody during membrane fusion. We suggest that resistance to neutralization by antibodies directed to fusion-associated structures is a common property of retroviral TM and perhaps of other viral class I fusion proteins. These observations have significant implications for vaccine design.
Highlights
N-terminal ␣-helices (N-helices) of adjacent fusion protein monomers
Production of Anti-transmembrane glycoprotein (TM) monoclonal antibodies (MAbs)—We have expressed a fragment of human T-cell leukemia virus type 1 (HTLV-1) TM that spans the N-terminal ␣-helical leucine/isoleucine heptad repeat as a fusion to maltose-binding protein (MBP) [12, 23] (Fig. 1)
This recombinant TM-derived fusion protein, referred to as MBP-fishhook, is trimeric as revealed by size exclusion chromatography and by native PAGE [12], and binds to synthetic peptides that mimic the C-helical region of the HTLV-1 trimer-of-hairpins [12, 24]
Summary
N-terminal ␣-helices (N-helices) of adjacent fusion protein monomers. Subsequently, a C-terminal ␣-helical (C-helix) segment of each monomer docks into grooves on the surface of the coiled coil to yield a six-helix bundle or trimer-of-hairpins structure. The accumulating data indicate that both group I and II MAbs recognize the coiled coil and that the epitopes recognized by the antibodies are highly conformation-specific and are available for binding only on trimeric forms of TM.
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