Figure 1.

A schematic of the kinetic steps involved in receptor and coreceptor engagement, which ultimately lead to membrane fusion or endocytosis. Receptors and coreceptors in the cell membrane are represented by black line segments and red zigzags, respectively. The projected contact area nucleated by the number of bound receptors is also shown. Only viral spikes that have a coreceptor bound can induce fusion. Endocytosis can occur only when the contact region grows to the surface area of the virus particle. (Left) The receptors and coreceptors both bind to the same viral spikes (blue circles). An example of such a virus is HIV-1, where spikes, likely composed of trimers of gp120/41, bind to both CD4 and CCR5. (Right) An example (such as herpes simplex virus) in which coreceptors and receptors bind to different spikes, with the ratio of receptor-binding spikes (blue circles) to coreceptor-binding spikes (yellow hexagons) defined by r.