Semliki Forest virus particles containing only the E1 envelope glycoprotein are infectious and can induce cell-cell fusion.

Hydrophobic interaction chromatography (phenyl- and octyl-Sepharose) was performed with Semliki Forest virus to investigate the effect of low pH on its hydrophobicity. At neutral pH, the virus could be bound to the column and completely eluted by the detergent NP-40. Low pH treatment of virus prior to application to the column resulted in stronger binding as reflected by the increased amount of detergent necessary to totally elute the virus. If, however, the low pH treatment was done after binding of the virus to the column, only 15% of the input virus could be eluted by the detergent, indicating a drastic increase in hydrophobicity. Thus binding of the virus to a hydrophobic environment potentiates the effect of low pH on viral hydrophobicity. Trypsin digestion of column-bound virus after low pH treatment resulted in complete digestion of E2 and E3; however, E1 was totally resistant. From this result, we conclude that E1 alone is responsible for the hydrophobic interaction. We have made use of these observations to produce viral particles which were devoid of E2 and E3 by trypsin digestion in the presence of octyl glucoside. These E1 viral particles were infectious and could induce membrane fusion. We conclude that only E1 is necessary and sufficient to mediate membrane fusion. Acid pH induces a drastic increase in the hydrophobicity of E1 which probably facilitates its interaction with the lipid bilayers during the fusion event in endosomes.