FIG. 6.

Schematic diagram to illustrate the proposed mechanism of soluble-RBD-dependent activation of fusion. The viral membrane containing the Env protein is at the top. Env is depicted with the RBD (oval) connected by the proline-rich region (curved line) to the C-terminal segment (light shaded rectangle). The transmembrane domain (dark shaded rectangle) is connected to the C-terminal segment by a disulfide bond. The cell membrane containing the virus receptor is at the bottom. (A) Proposed steps leading to the activation of fusion, assuming a mechanism in which the viral RBD and the C-terminal segment of SU interact in cis. The viral RBD is depicted as an oval that, on receptor contact, undergoes a conformational change (depicted as a rectangle). In this situation, fusion is triggered by a specific interaction between the bound conformation of the RBD and the C-terminal segment and is dependent upon the conserved histidine residue. (B) Proposed steps for RBD-dependent Fr-MLV (env ΔH8) infection in trans. Receptor binding to viral RBD (ΔH) results in exposure of the C-terminal segment of SU, enabling a productive interaction with soluble RBD bound to a distinct receptor. As in cis, the interaction between soluble RBD and the C-terminal segment in trans is strongly dependent on the conserved histidine residue.