Cell fusion is essential for fertilization and formation of syncytia during organogenesis. About one-third of the somatic cells in C. elegans fuse to form an invariant pattern of syncytia during embryonic and postembryonic development. Most cell fusion in the embryo occurs in the epidermis during body elongation. Nearly all cell fusions require the integral membrane protein EFF-1: virtually all epidermal cells fail to fuse in
eff-1 mutants (Mohler, el al ., Dev. Cell 2: 355 (2002)). The proper spatiotemporal regulation of
eff-1 activity appears to ensure that cell fusion occurs between the appropriate cells during C. elegans development. We previously reported a zygotic embryonic lethal mutant,
fus-1 , in which the embryonic epidermal cells undergo hyperfusion (Kontani, el al ., IWM (2003)). In
fus-1 mutant embryos, formation of the epidermal syncytia is nearly normal during early embryogenesis; however, most or all epidermal cells, except the lateral seam cells, abnormally fuse into a single large syncytium in late stage embryos. This hyperfusion phenotype is suppressed by an
eff-1 mutation, indicating that
fus-1 represses
eff-1 -mediated cell fusion. We positionally cloned
fus-1 and found that it encodes the e subunit of the vacuolar H + -ATPase (V-ATPase). Immunoreactive FUS-1 is first detected in a punctate pattern in gut cells of comma-stage embryos. It is first detected in the epidermis of ~2-fold stage embryos and immunoreactivity increases in all epidermal cells except the lateral seam cells during late embryogenesis. Staining is particularly prominent around the surfaces of these epithelial cells. Together with the previous observation that hyperfusion is detected only in late stage
fus-1 mutant embryos, we speculate that FUS-1 acts to suppress
eff-1 -mediated cell fusion late in embryogenesis. V-ATPases are multi-subunit proton pumps that play important roles in various membrane transport, protein sorting, and degradation processes. The role of the e subunit of the V-ATPase is unknown; however, the yeast homolog of FUS-1 has been implicated in assembly of V-ATPase subunits. We performed RNAi to test the function of other V-ATPase subunits in proper cell fusion and found that interference of these genes induced hyperfusion of epidermal cells, similar to the phenotype seen in
fus-1 embryos. These results suggest that the V-ATPase may be required for proper localization of EFF-1 in epidermal cells to prevent inappropriate cell fusion. Alternatively, given the cell surface localization of the protein that we have observed, and implication of the V-ATPase in intracellular membrane fusion events (Bayer, el al ., J Cell Biol. 162: 211 (2003)), it may act more directly to regulate the membrane fusion-promoting activity of EFF-1.