Structural Basis of HIV-1 Tethering to Membranes by the BST-2/Tetherin Ectodomain

Abstract

SUMMARY The restriction factor Bst2/tetherin contains two membrane anchors which are employed to retain some enveloped viruses including HIV-1 tethered to the plasma membrane in the absence of virus encoded antagonists. The 2.77 Å crystal structure of the extracellular core presented here reveals a parallel 90 Å long disulfide linked coiled-coil domain while the complete extracellular domain forms an extended 170 Å long rod-like structure based on small angle X-ray scattering data. Mutagenesis analyses indicate that both the coiled-coil and the N-terminal region are required for retention of HIV-1, suggesting that the elongated structure can function as a molecular ruler to bridge long distances. The structure reveals substantial irregularities and instabilities throughout the coiled-coil, which contribute to its low stability in the absence of disulfide bonds. We propose that the irregular coiled-coil provides conformational flexibility and ensures that Bst2/tetherin anchoring in the plasma and the newly formed virus membrane do not interfere with budding.

The Size and Conservation of a Coiled-coil Structure in the Ectodomain of Human BST-2/Tetherin Is Dispensable for Inhibition of HIV-1 Virion Release

“…One of the structural prerequisites for virus tethering is the formation of cysteine-linked dimers (29,30). Indeed, structural models of the BST-2 ectodomain suggest that BST-2 forms a parallel dimer that forms a 16 -17-nm long rod-like coiled-coil (25)(26)(27)(28), which must be anchored at both ends in a membrane. Exactly how a cell-virus tether is formed or how many BST-2 dimers are required to effectively tether a virion to the plasma membrane is currently unclear.…”

“…1A). Structural analyses of BST-2 indicate that the BST-2 ectodomain forms a continuous ␣-helix that forms a parallel dimeric coiled-coil in its C-terminal region (25)(26)(27)(28). However, the precise role of the coiled-coil and, in particular, possible constraints regarding the size of the coiled-coil remain unclear.…”

“…In classical coiled-coils, the 7-amino acid heptad motif places hydrophobic residues in the a and d positions and charged residues in the e and g positions to form stabilizing salt bridges resulting in "knobs into holes" packing of the helices (53). Heptad repeats in the BST-2 ectodomain were reported for the C-terminal half of the BST-2 ectodomain (25,26) and are indicated as green and yellow boxes in Fig. 5A.…”

“…The Structure of the Ectodomain of BST-2 at Its N Terminus Is Flexible-Recent studies revealed the crystallographic structure of the human and mouse BST-2 ectodomain (25)(26)(27)(28). We compiled the full-length structures of human BST-2 deposited into the RCSB PDB protein database using YASARA (Fig.…”

“…However, we recently found evidence to suggest that the C-terminal membrane anchor could be a second transmembrane domain rather than a glycosylphosphatidylinositol anchor (24). The two TM domains are separated by ϳ120 residues that constitute the ectodomain of the protein, which is predicted to form a coiled-coil structure (25)(26)(27)(28). The N-terminal half of the BST-2 ectodomain contains three cysteine residues, which are involved in the formation of covalent cysteinelinked dimers (20,21,29,30).…”