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1 November 2008 Functional Characterization of the Human Placental Fusogenic Membrane Protein Syncytin 2
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Fusion of cytotrophoblasts into the multinucleated syncytiotrophoblast layer is essential for the development of a functional placenta. The envelope protein of a human endogenous retrovirus W (HERV-W) family member, syncytin 1, has been shown to mediate placental cell fusion. Recently, the envelope protein of another HERV family member (HERV-FRD), syncytin 2, has been identified and shown to be highly expressed in the placenta. To better understand the biology of syncytin 2, in this study we first investigated syncytin 2 gene expression in normal and preeclamptic placentas and then characterized the functions of syncytin 2. The expression of syncytin 2 gene was decreased in preeclamptic placentas and could be stimulated by the cAMP stimulant forskolin. The endoprotease furin was found to be involved in the posttranslational cleavage of syncytin 1 and 2 polypeptides into surface and transmembrane subunits. In addition, proper association of the subunits of syncytins 1 and 2 is probably required for the functional integrity of each protein, because subunit swapping of syncytins 1 and 2 failed to generate fusogenic chimeras. Finally, we demonstrated that the disulfide bridge-forming CX2C and CX7C motifs found in syncytins 1 and 2 are essential for their fusogenic activities, because mutations in the CX2C motif not only abolished fusogenesis but also functioned as dominant-negative mutants. Our results suggest that syncytin 2 may function as a second fusogenic protein for placental cell fusion..

Chie-Pein Chen, Liang-Fu Chen, Su-Ray Yang, Chia-Yu Chen, Chun-Chuan Ko, Geen-Dong Chang, and Hungwen Chen "Functional Characterization of the Human Placental Fusogenic Membrane Protein Syncytin 2," Biology of Reproduction 79(5), 815-823, (1 November 2008).
Received: 8 April 2008; Accepted: 1 June 2008; Published: 1 November 2008

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