EFF-AFF was first identified when EFF1 mutants were found to block cell fusion in all epidermal and vulval epithelia in the worm [1]. However, fusion between the anchor cell and the utse syncytium that establishes a continuous uterine-vulval tube proceeds normally in eff-1 mutants and thus Aff1 was established as necessary for this and the fusion of heterologous cells in C. elegans [2]. The transmembrane forms of FF proteins, like most viral fusogens, possess an N-terminal signal sequence followed by a long extracellular portion, a predicted transmembrane domain, and a short intracellular tail. A striking conservation in the position and number of all 16 cysteines in the extracellular portion of FF proteins from different nematode species suggests that these proteins are folded in a similar 3D structure that is essential for their fusogenic activity [3]. C. elegans AFF-1 and EFF-1 proteins are essential for developmental cell-to-cell fusion and can merge insect cells. Thus FFs comprise an ancient family of cellular fusogens that can promote fusion when expressed on a viral particle [4].

1: The type I membrane protein EFF-1 is essential for developmental cell fusion. Mohler WA, Shemer G, del Campo JJ, Valansi C, Opoku-Serebuoh E, Scranton V, Assaf N, White JG, Podbilewicz B; Dev Cell. 2002;2:355-362. PMID:11879640

2: AFF-1, a FOS-1-regulated fusogen, mediates fusion of the anchor cell in C. elegans. Sapir A, Choi J, Leikina E, Avinoam O, Valansi C, Chernomordik LV, Newman AP, Podbilewicz B; Dev Cell. 2007;12:683-698. PMID:17488621

3: Viral and developmental cell fusion mechanisms: conservation and divergence. Sapir A, Avinoam O, Podbilewicz B, Chernomordik LV; Dev Cell. 2008;14:11-21. PMID:18194649

4: Conserved eukaryotic fusogens can fuse viral envelopes to cells. Avinoam O, Fridman K, Valansi C, Abutbul I, Zeev-Ben-Mordehai T, Maurer UE, Sapir A, Danino D, Grunewald K, White JM, Podbilewicz B; Science. 2011;332:589-592. PMID:21436398