Vesicular stomatitis virus (VSV; the prototype rhabdovirus) fusion is triggered at low pH and mediated by glycoprotein G, which undergoes a low-pH-induced structural transition. A unique feature of rhabdovirus G is that its conformational change is reversible. This allows G to recover its native prefusion state at the viral surface after its transport through the acidic Golgi compartments. The crystal structures of G pre- and postfusion states have been elucidated, leading to the identification of several acidic amino acid residues, clustered in the postfusion trimer, as potential pH-sensitive switches controlling the transition back toward the prefusion state. We mutated these residues and produced a panel of single and double mutants whose fusion properties, conformational change characteristics, and ability to pseudotype a virus lacking the glycoprotein gene were assayed. Some of these mutations were also introduced in the genome of recombinant viruses which were further characterized. We show that D268, located in the segment consisting of residues 264 to 273, which refolds into postfusion helix F during G structural transition, is the major pH sensor while D274, D395, and D393 have additional contributions. Furthermore, a single passage of recombinant virus bearing the mutation D268L (which was demonstrated to stabilize the G postfusion state) resulted in a pseudorevertant with a compensatory second mutation, L271P. This revealed that the propensity of the segment of residues 264 to 273 to refold into helix F has to be finely tuned since either an increase (mutation D268L alone) or a decrease (mutation L271P alone) of this propensity is detrimental to the virus. [ABSTRACT FROM AUTHOR]