Multinucleate cells are widespread in nature, yet the mechanism by which cells fuse their plasma membranes is poorly understood. To identify animal fusogens, we performed new screens for mutations that abolish cell fusion within tissues of C. elegans throughout development. We identified the gene eff-1, which is expressed as cells acquire fusion competence and encodes a novel integral membrane protein. EFF-1 sequence motifs suggest physicochemical actions that could cause adjacent bilayers to fuse. Mutations in the extracellular domain of EFF-1 completely block epithelial cell membrane fusion without affecting other perfusion events such as cell generation, patterning, differentiation, and adhesion. Thus, EFF-1 is a key component in the mechanism of cell fusion, a process essential to normal animal development.

In addition to large domains, many short motifs mediate functional post-translational modification of proteins as well as protein-protein interactions and protein trafficking functions. We have constructed a motif database comprising 312 unique motifs and a web-based tool for identifying motifs in proteins. Functional motifs predicted by MnM can be ranked by several approaches, and we validated these scores by analyzing thousands of confirmed examples and by confirming prediction of previously unidentified 14-3-3 motifs in EFF-1.

EFF-1 can confer potent fusogenic activity to nonfusing cell types. However, it is normally targeted only to fusion-fated cell borders via mutual interaction between EFF-1-expressing cells and relocalization to the plasma membrane. Because EFF-1 appears evolutionarily unique to nematodes, multiple mechanisms may have evolved for controlled plasma-membrane fusion in development.

BackgroundRegulatory 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 Caenorhabditis elegans. 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 eff-1 gene function in cell fusion in vivo. Moreover, a human 14-3-3 isoform bound to EFF-1::GFP in vitro. We hypothesized that the two 14-3-3 proteins in C. elegans, PAR-5 and FTT-2, may regulate either localization or fusion-inducing activity of EFF-1.Methodology/Principal FindingsTiming 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 eff-1-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.Conclusions/SignificanceDeletion of the EFF-1A endodomain noticeably affects the timing of hypodermal cell fusions in vivo. 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.

Retroviral insertion at the Evi1 locus causes high level of expression of the gene in myeloid neoplasms in mice and in nonmalignant expansions of myeloid cells in humans and monkeys. In these settings, it is suggested that EVI1 confers a survival advantage on myeloid cells. Here, we investigate the survival phenotype in DA-1 cells, a leukemic cell line with provirally activated Evi1. We report that short hairpin-mediated suppression of Evi1 in DA-1 cells induces apoptosis via the intrinsic/mitochondrial pathway: DNA fragmentation and histone release are both induced, as is reduction in mitochondrial membrane potential. In addition, procasapses 3 and 9, but not caspase 8 or Bid, are cleaved following Evi1 knockdown; phosphoAKT remains unchanged. Furthermore, mRNA expression profiling following Evi1 suppression show transcriptional changes in several apoptotic regulators, including a 3.5-fold decrease in Bcl2a1 (bfl/A1), a prosurvival member of the Bcl-2 family. To assess whether EVI1 regulates Bcl2a1 expression in a cell type other than DA-1 cells, we transduced primary murine Lin-/Sca-1+/c-Kit+ cells with EVI1 via retroviral vector, and then assessed the expression of Bcl2a1. This revealed that EVI1 induced a more than five-fold increase in Bcl2a1 mRNA expression, as measured by quantitative PCR. Furthermore, transduction of primary murine bone marrow cells with retroviruses bearing either Bcl2a1 or Evi1 resulted in a significant decrease in spontaneous apoptosis, as assessed by the activity of caspases 3 and 7. To further test if Bcl2a1 is necessary for leukemic transformation by Evi1, we assessed the ability of Evi1 to confer serial replating ability on primary bone marrow cells from Bcl2a1−/− mice. Bone marrow was harvested from Bcl2a1−/− and C57BL6 mice and transduced with retrovirus containing either no gene or Evi1. While in C57BL6 mice, Evi1 induced a significant increase in colonies, bone marrow from Bcl2a1−/− mice were resistant to transformation by Evi1. To show that this was due to the lack of Bcl2a1, the gene was added back via retrovirus. While Bcl2a1 by itself did not induce significant number of colonies over vector, when introduced into Bcl2a1−/− cells together with Evi1, there was a significant increase in colony formation. These data indicate that transformation of bone marrow cells by Evi1 depends on the presence of the Bcl2a1 gene. We further show EVI1 can transcriptionally upregulate a BCL2A1::luc reporter that harbors 1.37 kb of the human BCL2A1 upstream sequence. Our analysis of the Bcl2a1 promoter indicates that the effect of EVI1 is likely indirect. Based on our findings, we propose that EVI1 acts to block apoptosis in DA-1 cells by transcriptionally activating Bcl2a1.