Assessment of membrane fusion efficiency and its use for distinguishing epitopes on the fusion (F) protein of Sendai virus (HVJ).

Upon incubation of radioiodinated Sendai virus with Ehrlich ascites tumor cells to induce viral envelope-cell membrane fusion, TCA-soluble radioactivity was found to be rapidly released from the virus into the medium. The amount of the TCA-soluble 125I species released into the medium was correlated with the extent of envelope-membrane fusion. Using a new method based on this phenomenon, we evaluated the fusion-inhibiting activity of monoclonal anti-F protein antibodies and could distinguish epitopes within partially overlapping regions of the F protein by their effects on fusion activity. We concluded this method is a reliable and sensitive one for measuring envelope-membrane fusion.