RAB-5- and DYNAMIN-1-Mediated Endocytosis of EFF-Fusogen Controls Cell-Cell Fusion

Cell-cell fusion plays essential roles during fertilization and organogenesis. Previous studies in C. elegans led to the identification of the eukaryotic fusion protein (EFF-fusogen), which has structural homology to class II viral fusogens. Transcriptional repression of EFF-ensures correct fusion fates, and overexpression of EFF-results in embryonic lethality. EFF-must be expressed on the surface of both fusing cells; however, little is known regarding how cells regulate EFF-surface exposure. Here, we report that EFF-is actively removed from the plasma membrane of epidermal cells by dynamin- and RAB-5-dependent endocytosis and accumulates in early endosomes. EFF-was transiently localized to apical domains of fusion-competent cells. Effective cell-cell fusion occurred only between pairs of cell membranes in which EFF-localized. Downregulation of dynamin or RAB-caused EFF-mislocalization to all apical membrane domains and excessive fusion. Thus, internalization of EFF-is a safety mechanism preventing excessive cell fusion.