Retrovirus-induced cell fusion is enhanced by protease treatment

The effect of proteases on retrovirus-induced cell fusion was studied. Both 'fusion from within' (fusion between infected and uninfected cells) and 'fusion from without' (fusion of uninfected cells by virus addition) was enhanced after treatment with proteases. An enhancement of up to 10-fold, giving 30% fusion of the cells, was observed. Protease treatment of infected cells or of virus particles enhanced fusion, indicating that cleavage of the virus surface proteins is important for cell fusion. Cell fusion is believed to reflect the virus-cell membrane fusion which occurs during the entry of enveloped viruses into cells. It is therefore possible that proteolytic cleavage of virus surface components during entry is involved in virus infection.