Fusogenic activity of EFF-1 is regulated via dynamic localization in fusing somatic cells of C-elegans

Background: Many animal tissues form via fusion of cells. Yet in all instances of developmental cell fusion, the mechanism underlying fusion of plasma membranes remains poorly understood. EFF-1 is required for most somatic cell fusions in C. elegans, and misexpressed EFF-1 alters the normal pattern of fusing hypodermal cells. However, the autonomous activity of EFF-1, the rules governing its specificity, and the mechanism of its action have not been examined. Results: We show that EFF-1 acts as a cellular fusogen, capable of inducing fusion of virtually any somatic cells in C. elegans, yet targeted precisely to fusion-fated contacts during normal development. Misexpression of EFF-1 in early embryos causes fusion among groups of cells composed entirely of nonfusion-fated members. Measurements of cytoplasm diffusion in induced fusion events show that ectopic EFF-1 expression produces fusion pores similar to those in normal fusion events. GFP-labeled EFF-1 is specifically targeted to fusion-competent cell contacts via reciprocal localization to the touching membranes of EFF-1-expressing cells. EFF-1 function is also governed by intercellular barriers that prohibit cell fusion between distinct tissues. Analysis of mutant versions of EFF-1 indicates a novel mode of fusogenicity, employing neither a phospholipase active site nor hydrophobic fusion-peptide acting solely in pore formation. Conclusions: EFF-1 can confer potent fusogenic activity to nonfusing cell types. However, it is normally targeted only to fusion-fated cell borders via mutual interaction between EFF-1-expressing cells and relocalization to the plasma membrane. Because EFF-1 appears evolutionarily unique to nematodes, multiple mechanisms may have evolved for controlled plasma-membrane fusion in development.