Fig 1

(A) Genomic localization of ERV-E5.1.38-DanRer element in zebrafish. The locus of ERV-E5.1.38-DanRer element on zebrafish chromosome 5 is indicated in red arrow. The ERV-E5.1.38-DanRer is distributed inside si:zfos-375h5.1. The upstream hspb1, ywhag1, tspoap1, supt4h1, hsf5 and downstream rpain, c1qbp, znhit3, myo19 genes of ERV-E5.1.38-DanRer are presented, and att means attachment site. (B) Schematic diagram showing the transcription and splicing of env38 gene, as well as the structural characterization of Env38 protein, in which a signal peptide (1–18 amino acid), a furin cleavage site (R/K-X-R/K-R, 227–230 amino acid), an immunosuppressive domain (ISD, 309–325 amino acid), a transmembrane (TM) domain (383–405 amino acid) and conserved C-X-X-C and C-X5/6/7-C motifs are presented. The LTR consists of U3, R, and U5. The transcription of env38 begins at 5’-LTR-R and ends at 3’-LTR-R. (C) The five predicted N-glycosylation sites in SU subunit and two in TM subunit are marked with ASN residues. SU: surface subunit; TM: transmembrane subunit. (D) Hydrophobicity analysis of Env38 protein showed that the fusion peptide next to the furin-cleavage site was partial hydrophobic. FP: fusion peptide; NHR: N-terminal heptad repeats; CHR: C-terminal heptad repeats; TM: transmembrane domain. (E) Tertiary structure of Env38 monomer modeled by I-TASSER. Prominent central α-helical coiled-coil (CC) structures were found in the TM subunit of Env38 monomer protein. The top five threading templates were 6vkm, 6rx3A, 6vkm, 6fgzA, and 5yfpH. TM: transmembrane subunit; SU: surface subunit. (F) Tertiary structure of the NHR-CHR trimeric domain of Env38 TM subunit predicted by SWISS-MODEL program with crystal structures of human Syncytin 1 in post-fusion conformation (SMTL ID: 6rx1.1) as a model. A homotrimer with a fusion core structure was formed by three NHR-CHR domains in which three helical NHR peptides formed a central core and three helical CHR peptides packed into the grooves on the surface of the central core.