| CD9 |
1999 |
CD9 is expressed on the surface of the oocyte and accumulates during the attachment event; it may modulate the integrity of the oocyte membrane; its precise role in sperm–egg fusion remains unclear |
CD9 is a tetraspanin with four transmembrane domains and two extracellular loops (short and long) |
Miyado et al., 2000; Le Naour et al., 2000; Kaji et al., 2000; Chen et al., 1999; Umeda et al., 2020; Zimmerman et al., 2016; Zhang and Huang, 2012; Dahmane et al., 2019; Runge et al., 2007; Zhu et al., 2002; Chalbi et al., 2014; Rubinstein et al., 2006; Ziyyat et al., 2006
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| IZUMO1 |
2005 |
IZUMO1 relocates to the equatorial region of the sperm head after the acrosome reaction; high-affinity binding of IZUMO1 to JUNO results in initial attachment of sperm and egg in the PVS |
The protein has an N-terminal 4HB, followed by a β-hinge and an IgSF domain; the structure is stabilized by five disulfide bonds |
Inoue et al., 2005; Ellerman et al., 2009; Young et al., 2015; Satouh et al., 2012; Aydin et al., 2016; Ohto et al., 2016; Nishimura et al., 2016; Kato et al., 2016
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| JUNO |
2014 |
JUNO is expressed on the surface of the oocyte membrane and serves as the receptor of IZUMO1 |
JUNO has structural similarity to folate receptors; it is a globular α/β protein composed of five α helices, three 310 helices, and four short β strands stabilized by eight disulfide bonds |
Bianchi et al., 2014; Kato et al., 2016; Han et al., 2016; Jean et al., 2019; Yamaguchi et al., 2007; Aydin et al., 2016; Ohto et al., 2016
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| SPACA6 |
2014 |
SPACA6 is expressed in sperm and localized to the equatorial segment after the acrosome reaction, but its specific role in sperm–egg fusion remains unknown |
The three-dimensional structure of SPACA6 is currently unknown; SPACA6 is similar in organization to IZUMO1 with a signal peptide, followed by an α-helical domain, an IgSF domain, a transmembrane helix, and a cytoplasmic tail |
Lorenzetti et al., 2014; Noda et al., 2020; Barbaux et al., 2020
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| TMEM95 |
2014 |
TMEM95 is localized to the equatorial segment of sperm and is essential for sperm–egg fusion and male fertility in mice, but its specific role in sperm–egg fusion is currently unknown |
The structure of TMEM95 is currently unknown; TMEM95 consists of a signal peptide, an N-terminal helix-rich region, a transmembrane helix, and a leucine-rich cytoplasmic domain |
Pausch et al., 2014; Zhang et al., 2016; Noda et al., 2020; Fernandez-Fuertes et al., 2017; Lamas-Toranzo et al., 2020
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| SOF1 |
2020 |
SOF1 is predicted to be a secreted factor essential for fusion; its role is still not fully understood |
No structural information to date; primary sequence analysis revealed the presence of conserved LLLL and CFNLAS motifs |
Noda et al., 2020
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| FIMP |
2020 |
FIMP is involved in sperm–egg fusion; only the transmembrane form is important in fertilization, but its role is still not fully determined |
No structural information to date |
Fujihara et al., 2020
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| DCST1/DCST2 |
2021 |
DCST1 and DCST2 are involved in sperm–egg fusion; stability of SPACA6 is regulated by DCST1/2; DCST1/DCST2 are evolutionary conserved in vertebrates and invertebrates |
No structural information to date; contains six putative transmembrane helices |
Inoue et al., 2021
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