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

• The fusion protein EFF-1 is targeted to early endosomes
• Dynamin and RAB-5 downregulate EFF-1 in C. elegans embryos
• Transient and dynamic localization of EFF-1 to apical cell membranes mediates fusion
• Prevention of EFF-1 endocytosis induces excessive cell fusion

SummaryCell-cell fusion plays essential roles during fertilization and organogenesis. Previous studies in C. elegans led to the identification of the eukaryotic fusion protein (EFF-1 fusogen), which has structural homology to class II viral fusogens. Transcriptional repression of EFF-1 ensures correct fusion fates, and overexpression of EFF-1 results in embryonic lethality. EFF-1 must be expressed on the surface of both fusing cells; however, little is known regarding how cells regulate EFF-1 surface exposure. Here, we report that EFF-1 is actively removed from the plasma membrane of epidermal cells by dynamin- and RAB-5-dependent endocytosis and accumulates in early endosomes. EFF-1 was transiently localized to apical domains of fusion-competent cells. Effective cell-cell fusion occurred only between pairs of cell membranes in which EFF-1 localized. Downregulation of dynamin or RAB-5 caused EFF-1 mislocalization to all apical membrane domains and excessive fusion. Thus, internalization of EFF-1 is a safety mechanism preventing excessive cell fusion.

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