Scission of the protruding bud leaves local membrane instability at the rear end of the virus particle ( Figure 5E ), which may be exaggerated during sample preparation. In contrast, the front end of the filamentous particle is a well-defined hemisphere. The difference between the two ends could reflect a destabilization at the rear end of the particle induced by scission. Alternatively, the hemispherical front end of the particle may be stabilized by a structure involved in initiating envelopment, similar to the front end of bullet-shaped rhabdovirus particles [41, 42] .