Redox Potentials in the Regulation of Viral Fusion

Abstract

The acidic interior of endosomes is the trigger for fusion of viruses within this organelle. Whether conditions of endosomal interiors, in addition to low pH, regulate the extent of fusion has not been well explored. The redox potential (RP) within an endosome is regulated by NADPH oxidase and the RP varies according to endosome type. We have investigated whether RP can affect fusion by fusing cells expressing a viral fusion protein to target cells, varying the RP of the external solution. The RP was varied, and clamped, from −150 mV to 0 mV by altering the ratio of Cys/Cyss; cytochrome c was used to establish positive RPs in the external solution. For the class II and class III fusion proteins tested, the more reducing (negative RP) the solution, the greater was the extent of fusion. That is, the extent of fusion varied greatly with RP. The pH-dependence of fusion was, however, independent of RP. For the tested class I fusion proteins, including influenza HA which utilizes the low pH within endosomes, fusion was independent of external redox potentials. To identify the stage(s) of fusion at which redox potentials are consequential, we created a hemifusion intermediate, varying the RP upstream and downstream of the intermediate. The creation of hemifusion was strongly dependent on the RP; the transition from hemifusion to full fusion was less dependent on the RP. Therefore, RPs have strong control on early steps in the fusion process, but only weak control of late steps.