Cell fusion plays key roles in various aspects of animal development. A gene required for epidermal cell fusions in C. elegans has recently been identified (1). However, in general, the molecular mechanisms underlying the fusion between cells are largely unknown. Also, it is unclear whether the well-characterized molecules mediating membrane fusion during vesicular transport within cells function in the fusion between cells as well. The anchor cell of the C. elegans hermaphrodite uterus induces the two cell types necessary for formation of the uterine-vulval connection. The anchor cell later fuses with the utse (uterine-seam cell) syncytium, which is formed by fusion of eight daughters of the uterine pi cells that are induced by the anchor cell. The utse syncytium forms a thin process dorsal to the vulva that can be broken, enabling eggs in the uterus to enter the vulval passageway to the outside. If the anchor cell fails to fuse with the utse, it obstructs this passageway, and eggs cannot be laid. In
ty10 mutants, the thin process of the utse appears normal, but the anchor cell fails to fuse with it. Subsequently, the unfused anchor cell either detaches from the adjacent tissue and floats free in the uterine lumen or remains dorsal to the vulva and degenerates. We observed that the
ty10 mutant defect was rescued by cosmid H15N14 in transgenic animals. We found that the
ty10 allele was a missense mutation in a conserved residue of H15N14.2, which encodes C. elegans N-ethylmaleimide sensitive factor (NSF). NSF has a conserved and essential role in intracellular membrane fusion. Specifically, NSF appears to be required to disassemble complexes of the membrane proteins (SNAREs) that mediate fusion so that they can be re-utilized. We found that elimination of H15N14.2 function by RNAi resulted in lethality at various embryonic and larval stages, depending on the time of administration. This is consistent with previous results (2, 3), and presumably reflects the basic cellular requirement for NSF. In addition, we found that animals containing
ty10 in trans to a deficiency died as embryos or larvae. Future experiments will be aimed at determining how the
ty10 mutation (which is in the D2 ATP binding domain of NSF) interferes specifically with anchor cell fusion without significantly affecting viability. The finding that NSF, a key mediator of intracellular fusion, is also required for the fusion between cells, provides an important link between these two processes. Future studies of the fusion between the anchor cell and the utse should further delineate similarities and differences in the genetic requirements for this event and for intracellular membrane fusion.