An endogenous retroviral envelope syncytin and its cognate receptor identified in the viviparous placental Mabuya lizard

Proc Natl Acad Sci U S A. 2017 Dec 19;114(51):E10991-E11000. doi: 10.1073/pnas.1714590114. Epub 2017 Nov 21.


Syncytins are envelope genes from endogenous retroviruses that have been captured during evolution for a function in placentation. They have been found in all placental mammals in which they have been searched, including marsupials. Placental structures are not restricted to mammals but also emerged in some other vertebrates, most frequently in lizards, such as the viviparous Mabuya Scincidae. Here, we performed high-throughput RNA sequencing of a Mabuya placenta transcriptome and screened for the presence of retroviral env genes with a full-length ORF. We identified one such gene, which we named "syncytin-Mab1," that has all the characteristics expected for a syncytin gene. It encodes a membrane-bound envelope protein with fusogenic activity ex vivo, is expressed at the placental level as revealed by in situ hybridization and immunohistochemistry, and is conserved in all Mabuya species tested, spanning over 25 My of evolution. Its cognate receptor, required for its fusogenic activity, was searched for by a screening assay using the GeneBridge4 human/Chinese hamster radiation hybrid panel and found to be the MPZL1 gene, previously identified in mammals as a signal-transducing transmembrane protein involved in cell migration. Together, these results show that syncytin capture is not restricted to placental mammals, but can also take place in the rare nonmammalian vertebrates in which a viviparous placentotrophic mode of reproduction emerged. It suggests that similar molecular tools have been used for the convergent evolution of placentation in independently evolved and highly distant vertebrates.

Keywords: endogenous retrovirus; envelope protein; placenta; receptor; syncytin.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Carrier Proteins / genetics*
  • Carrier Proteins / metabolism
  • Cell Line
  • Endogenous Retroviruses / genetics*
  • Endogenous Retroviruses / metabolism
  • Evolution, Molecular
  • Female
  • Gene Expression
  • Gene Expression Profiling
  • Gene Products, env / genetics*
  • Gene Products, env / metabolism
  • High-Throughput Nucleotide Sequencing
  • Immunohistochemistry
  • Lizards / genetics*
  • Lizards / metabolism
  • Phylogeny
  • Placenta / metabolism*
  • Pregnancy
  • Pregnancy Proteins / genetics*
  • Pregnancy Proteins / metabolism
  • Viral Envelope Proteins / genetics


  • Carrier Proteins
  • Gene Products, env
  • Pregnancy Proteins
  • Viral Envelope Proteins
  • syncytin

Associated data

  • GENBANK/MG254887
  • GENBANK/MG254891