Semliki Forest Virus Induced Cell-Cell Fusion and Pore Formation

Abstract

Membrane fusion is an important and ubiquitous process in cell biology: In a living cell membrane-bound compartments are continuously either separated or united through fusion reactions. The fusion of cell membranes represents a crucial step in myogenesis, osteogenesis and fertilization (White and Blobel, 1989). Many enveloped animal viruses enter host cells by endocytosis and subsequent membrane fusion of the virion envelope and the endosomal membrane. In most cases, this fusion is triggered by the low intracompartmental pH (for reviews see White et al., 1983; Stegmann et al., 1989; White 1992). One of the best studied viruses with respect to entry is Semliki Forest Virus (SFV) (reviewed in Kielian and Helenius, 1986). The SFV envelope contains 80 copies of a fusogenic protein spike (von Bonsdorff and Harrison, 1978). The spike protein is homotrimeric, each monomer consisting of three subunits El, E2 and E3 (Mr 51, 52 and 11 kDa, respectively). In mildly acidic pH (< 6.2), the SFV spike proteins are irreversibly changed to their fusion-active conformation (Kielian and Helenius, 1985). El is considered to be the fusion protein, since virus particles devoid of E2 and E3 are still infectious (Omar and Koblet, 1988) and E2/E3 expressed in the absence of El is fusion negative (Kondor-Koch et al., 1983). The notion is further supported by the fact that the El protein contains near the N-terminus a hydrophobic sequence highly conserved among alphaviruses (Garoff et al., 1980). This sequence could represent the “fusion peptide” that may insert into the target membrane and thereby initiate fusion. Site directed mutagenesis in the fusion peptide results in altered pH thresholds and efficiency of fusion (Levy-Mintz and Kielian, 1991) suggesting a crucial role for this peptide in membrane fusion.