Conditional dependence of fusion from within and other cell membrane alterations by Newcastle disease virus.

Fusion from within (FFWI) by Newcastle disease virus occurs optimally in medium maintained at pH 8.2, whereas fusion from without is relatively insensitive to the pH of the medium in the range of 7.0 to 8.3. The pH-sensitive events in FFWI take place in the synthesis of the hypothetical fusion factor rather than in the response to it. pH pulse and pH shift experiments have localized the pH-sensitive events between 4 and 6.5 h postinfection (a period of synthesis of proteins required for FFWI), but before the fusion process. The pH sensitivity is not due to a pH-sensitive interference phenomenon. Virus production and the appearance of hemadsorbing cell surfaces are also pH sensitive, but for these functions the pH optima depend upon the virus strains tested. The independence of FFWI, hemadsorption, and virus production is discussed. Also discussed are the possible roles of virus-specific proteins in the fusion process.