Endosomes supporting fusion mediated by vesicular stomatitis virus glycoprotein have distinctive motion and acidification.

Most enveloped viruses infect cells by binding receptors at the cell surface and undergo trafficking through the endocytic pathway to a compartment with the requisite conditions to trigger fusion with a host endosomal membrane. Broad categories of compartments in the endocytic pathway include early and late endosomes, which can be further categorized into subpopulations with differing rates of maturation and motility characteristics. Endocytic compartments have varying protein and lipid components, luminal ionic conditions and pH that provide uniquely hospitable environments for specific viruses to fuse. In order to characterize compartments that permit fusion, we studied the trafficking and fusion of viral particles pseudotyped with the vesicular stomatitis virus glycoprotein (VSV-G) on their surface and equipped with a novel pH sensor and a fluorescent content marker to measure pH, motion and fusion at the single particle level in live cells. We found that the VSV-G particles fuse predominantly from more acidic and more motile endosomes, and that a significant fraction of particles is trafficked to more static and less acidic endosomes that do not support their fusion. Moreover, the fusion-supporting endosomes undergo directed motion.