Figure 7. Summary of characteristics for fusion permissive and fusion non-permissive endosomes with a model for viral particle trafficking during cell entry.

A. Summary of characteristics of fusion permissive and non-permissive endosomes based on the single virus particle tracking and fusion data presented in Figures 4–6. B. Model of trafficking of VSV-G pseudotyped viral particles in fusion permissive and non-permissive endosomes. In this model, fusion is associated with the maturation of the subpopulation of fusion permissive endosomes, many of which also undergo directed motion, likely along elements of the cytoskeleton. The sorting of particles to fusion permissive or non-permissive endosomes does not appear to be preferential (see Discussion). Three fusion scenarios are displayed: In path (a) the virus fuses directly with the limiting endosomal membrane; in path (i) the virus fuses prior to directed motion of the endosome or in an endosome that does not undergo directed motion; in path (ii) the virus fuses after the endosome containing it undergoes directed motion. In path (b) the virus undergoes fusion with an intralumenal vesicle, which then undergoes retrofusion with the limiting membrane of the endosome in a more acidic compartment. The ILV-containing compartment also may or may not undergo directed motion prior to fusion. Components of the viral particle are as represented in Figure 1A.