Spectraplakin Induces Positive Feedback between Fusogens and the Actin Cytoskeleton to Promote Cell-Cell Fusion.

Cell-cell fusion generally requires cellular fusogenic proteins and actin-propelled membrane protrusions. However, the molecular connections between fusogens and the actin cytoskeleton remain unclear. Here, we show that the Caenorhabditis elegans fusogen EFF-1 and F-actin are enriched at the cortex of the post-embryonic fusing cells, and conditional mutations of WASP and Arp2/3 delay cell-cell fusion by impairing EFF-1 localization. Our affinity purification and mass spectrometry analyses determined that an actin-binding protein, spectraplakin/VAB-10A, binds to EFF-1. VAB-10A promotes cell-cell fusion by linking EFF-1 to the actin cytoskeleton. Conversely, EFF-1 enhanced the F-actin bundling activity of VAB-10A in vitro, and actin dynamics in the cortex were reduced in eff-1 or vab-10a mutants. Thus, cell-cell fusion is promoted by a positive feedback loop in which actin filaments that are crosslinked by spectraplakin to recruit fusogens to fusion sites are reinforced via fusogens, thereby increasing the probability of further fusogen accumulation to form fusion synapses.