FIG. 3.

Receptor activation of viral fusion proteins. Many viral fusion proteins are activated by host cell receptors, but there are many variations on how this happens (see text). Selected examples are cartooned: (A) Moloney MLV Env, (B) a paramyxovirus F according to the “association” model, (C) a paramyxovirus F according to the “dissociation” model. (D) A highly speculative model for HSV. Specific protein names are indicated under the respective proteins. The models show only the first step of fusion activation, formation of the prehairpin intermediate; subsequent steps (right arrow) lead in all cases to formation of a trimer-of-hairpins. Designations are: R, receptor; rb, receptor binding subunit; f, fusion subunit. The up arrows associated with the known fusion subunits (f) in the right hand images of all panels indicate the exposed fusion peptides, which insert into the target membrane. In (A, right panel) the receptor binding subunit is shown loosely attached (dotted line) to the fusion subunit (f). In a subsequent step, the rb dissociates (see text and Figure 4). In (D) gB is shown in a possible prehairpin conformation as is gH/gL (large upward arrow) consistent with the participation of both gB and gH/gL in fusion, but their exact roles at all stages of fusion remain to be clarified. Receptors are not shown in the right hand images, as it is not yet clear whether they remain bound at this and/or later stages.