Figure 4.

Binding and fusion of a Gamma-type Env protein with a host cell membrane. Env is shown as a single heterodimer rather than a trimer for simplicity. The receptor binding domain (RBD) is shown in blue, and the proline rich region (PRR) is shown in white. After binding of the SU subunit to a receptor, isomerization of the disulfide bond occurs, resulting in conversion to an intra-subunit bond within the CXXC motif of the SU subunit, releasing the SU subunit and ultimately resulting in fusion mediated by the TM subunit. The conformational change that TM undergoes is irreversible, and TM cannot revert to carry out fusion again.