Table 1. Examples of candidate fusogens.

Protein	Family	Organism	Suggested fusion event	Fusogenic scorea	Essential for fusion (0–2)	Structural similarity to fusogens (0–2)	Expressed at the time and place of fusion (0–2)	Sufficiency (0–4)b				References
								A	B	C	D
HA	Class I viral fusogens	Influenza virus	Viral infection	10	+	+	+	+	+	+	+	Reviewed in Kielian and Rey, 2006
p14	FAST proteins (Class IV viral fusogens)	Reptilian orthoreoviruses	Infected cells	10	+	+	+	+	+	+	+	Reviewed in Key and Duncan, 2014
E1	Fusexins (Class II viral fusogens)	Semliki Forest virus	Viral infection	9.5	+	+	+	+	+	+	±	Reviewed in Kielian and Rey, 2006
vSNARES, tSNARES	SNAREs (endoplasmic fusogens)	Eukaryotes and Archaea	Intracellular vesicles	9	+	+	+	+	+	ND	+	Reviewed in Jahn and Scheller, 2006
EFF-1	Fusexins (FF)	C. elegans; Nematodes	Epithelia, vulva, pharynx, axons	9	+	+	+	+	+	+	ND	Mohler et al., 2002; Shemer et al., 2004; Podbilewicz et al., 2006; Avinoam et al., 2011
Syncytins	Class I viral fusogens	Placental mammals	Placenta, osteoclasts, myoblasts, cancer cells	9	+	+	+	+	+	+	ND	Blond et al., 2000; Mi et al., 2000; Bjerregaard et al., 2006; Antony et al., 2007; Dupressoir et al., 2011; Søe et al., 2011
Atlastins	Dynamin (endoplasmic fusogen)	Eukaryotes	Endoplasmic reticulum	8	+	+	+	+	ND	ND	+	Reviewed in Hu and Rapoport, 2016
HAP2/GCS1	Fusexins (HAP2)	Arabidopsis; plants	Fertilization (sperm)	8	+	+	+	ND	+	+	ND	Mori et al., 2006; von Besser et al., 2006; Valansi et al., 2017
AFF-1	Fusexins (FF)	C. elegans; Nematodes	Epithelia, vulva, pharynx, dendrites	8	+	±	+	+	+	+	ND	Sapir et al., 2007; Avinoam et al., 2011
Myomaker	Multi-pass transmembrane protein	Mus musculus; Human	Myoblasts	6	+	ND	+	+	+	ND	ND	Millay et al., 2013; Bi et al., 2017; Gamage et al., 2017; Quinn et al., 2017; Zhang et al., 2017a; Leikina et al., 2018
Myomerger	Single-pass transmembrane protein
HAP2/GCS1	Fusexins (HAP2)	T. thermophila; Trypanosomes	Mating	6	+	+	+	ND	ND	ND	ND	Pinello et al., 2017
HAP2/GCS1	Fusexins (HAP2)	Chlamydomonas; Algae	Fertilization (mt− cell)	6	+	+	+	ND	ND	ND	ND	Liu et al., 2008; Fédry et al., 2017
Bouncer	Ly6/uPAR	Danio reri and Oryzias latipes	Fertilization (egg)	5	+	−	+	+	ND	ND	ND	Herberg et al., 2018
Izumo1	IgSF	M. musculus; Mammals	Fertilization (sperm)	4	+	−	+	−	−	ND	ND	Inoue et al., 2005, 2013, 2015; Chalbi et al., 2014
Juno	Folate receptors	M. musculus; Mammals	Fertilization (oocyte)	4	+	−	+	−	−	ND	ND	Bianchi et al., 2014; Kato et al., 2016
SPE-9	EGF repeats	C. elegans; Nematodes	Fertilization (sperm)	4	+	ND	+	ND	ND	ND	ND	Singson et al., 1998
Sns	IgSF	Drosophila	FCM	4	+	ND	+	−	−	ND	ND	Bour et al., 2000; Shilagardi et al., 2013
Duf/Kirre	IgSF	Drosophila	FC	4	+	ND	+	−	−	ND	ND	Ruiz-Gómez et al., 2000; Shilagardi et al., 2013
Bindin	Bindins	S. purpuratus; Sea urchins	Fertilization (sperm)	3	ND	ND	+	ND	ND	ND	+	Vacquier and Moy, 1977; Glabe, 1985; Vacquier, 2012
PRM1	Tetraspanin integral protein	S. cerevisiae; Yeast	Mating	3	±	ND	+	−	ND	ND	ND	Heiman and Walter, 2000; Olmo and Grote, 2010

The fusogenic score (0–10) was calculated using the following scoring system: + (requirement fulfilled) = max score, − (requirement not fulfilled) = 0 points, ND (not determined) = 0 points, and ± (requirement partially fulfilled) = half of max score.

^aThe fusogenic score (0–10).

^b(A) Sufficient in situ (0–1). (B) Fuse heterologous cells (0–1). (C) Fuse pseudo-typed virus (0–1). (D) In vitro liposome fusion (0–1).