The EFF-1A Cytoplasmic Domain Influences Hypodermal Cell Fusions in <i>C</i>. <i>elegans</i> But Is Not Dependent on 14-3-3 Proteins

<div><p>Background</p><p>Regulatory and biophysical mechanisms of cell-cell fusion are largely unknown despite the fundamental requirement for fused cells in eukaryotic development. Only two cellular fusogens that are not of clear recent viral origin have been identified to date, both in nematodes. One of these, EFF-1, is necessary for most cell fusions in <i>Caenorhabditis elegans</i>. Unregulated EFF-1 expression causes lethality due to ectopic fusion between cells not developmentally programmed to fuse, highlighting the necessity of tight fusogen regulation for proper development. Identifying factors that regulate EFF-1 and its paralog AFF-1 could lead to discovery of molecular mechanisms that control cell fusion upstream of the action of a membrane fusogen. Bioinformatic analysis of the EFF-1A isoform’s predicted cytoplasmic domain (endodomain) previously revealed two motifs that have high probabilities of interacting with 14-3-3 proteins when phosphorylated. Mutation of predicted phosphorylation sites within these motifs caused measurable loss of <i>eff-1</i> gene function in cell fusion <i>in vivo</i>. Moreover, a human 14-3-3 isoform bound to EFF-1::GFP <i>in vitro</i>. We hypothesized that the two 14-3-3 proteins in <i>C</i>. <i>elegans</i>, PAR-5 and FTT-2, may regulate either localization or fusion-inducing activity of EFF-1.</p><p>Methodology/Principal Findings</p><p>Timing of fusion events was slightly but significantly delayed in animals unable to produce full-length EFF-1A. Yet, mutagenesis and live imaging showed that phosphoserines in putative 14-3-3 binding sites are not essential for EFF-1::GFP accumulation at the membrane contact between fusion partner cells. Moreover, although the EFF-1A endodomain was required for normal rates of <i>eff-1</i>-dependent epidermal cell fusions, reduced levels of FTT-2 and PAR-5 did not visibly affect the function of wild-type EFF-1 in the hypodermis.</p><p>Conclusions/Significance</p><p>Deletion of the EFF-1A endodomain noticeably affects the timing of hypodermal cell fusions <i>in vivo</i>. However, prohibiting phosphorylation of candidate 14-3-3-binding sites does not impact localization of the fusogen. Hypodermal membrane fusion activity persists when 14-3-3 expression levels are reduced.</p>