Membrane fusion plays a crucial role in intracellular vesicular transport, intercellular fusion and viral infectivity. The fusion reaction has been supposed to underlie the following three steps ; (i) specific docking of two membranes, (ii) perturbation of lipid bilayer at fusion point(s) and forming fusion pore(s), (iii) dilation of fusion pore(s). For the intracellular membrane fusion in membrane trafficking pathways, the recent convergence of genetic and biochemical approaches has led to the

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... identification of a number of proteins that involved in specific docking of two membranes (step (i)). However, the details in step(ii) have not been clarified. In this review, I will focus on putative "fusion peptide" of viral fusogenic proteins as an activator of the step (ii). Through the works on the fusion of liposomes mediated by several synthetic peptides that are derived from the fusion peptide of influenza virus hemagglutinin, we found that the fusion is triggered when the peptides become more hydrophobic under fusion-competent conditions, and also found that the common structural features of the fusogenic peptides is not "amphiphilic helical structure". The amphiphilic peptides appear to require other factor(s) to cause fusion. I propose that uneven distribution of the side-chain bulkiness of peptide in ordered structure is important factor for fusion active peptide.