heptad repeat 1-heptad repeat 2 region (ectodomain) of the transmembrane subunit of the human endogenous retrovirus ENVV1, and related domains
This domain subfamily spans both heptad repeats of the glycoprotein (gp)/transmembrane subunit of various endogenous retroviruses (ERVs), including chicken FET-1 (Female Expressed Transcript 1) protein, and the envelope proteins of the human ERVs (HERVs): ENVV1 (also known as HERV-V2_c19q13.41) and ENVV2 (also known as HERV-V1_c19q13.41 ). This domain belongs to a larger superfamily containing the HR1-HR2 domain of endogenous retroviruses (ERVs) and infectious retroviruses, such as Ebola virus, Rous sarcoma virus and human immunodeficiency virus type 1. This domain includes an N-terminal heptad repeat, a CKS17-like immunosuppressive region, a CX6C motif that forms an intra-subunit disulfide bond, and a C-terminal heptad repeat. N-terminal to HR1-HR2 region is a fusion peptide (FP), and C-terminal, is a membrane-spanning region (MSR). Viral infection involves the formation of a trimer-of-hairpins structure (three HR1 helices, buttressed by three HR2 helices lying in antiparallel orientation). In this structure, the FP (inserted in the host cell membrane) and MSR (inserted in the viral membrane) are in close proximity. ERVs are likely to originate from ancient germ-line infections by active retroviruses. Some ERVs play specific roles in the host, including placental development, protection of the host from infection by related pathogenic and exogenous retroviruses, and genome plasticity. FET-1 may have an ovary-determining role. The FET-1 gene is located on the female specific W chromosome in chickens. During the sex-determining period, the FET-1 transcript is up-regulated in the cortex of the left gonad (the only gonad which develops in female chickens); it is also expressed at a lower level, in neural tissue and waste collection ducts. The genes encoding ENVV1 and ENVV2 proteins are located in tandem on chromosome 19q13.41, and show placenta-specific expression in human and baboon.
Comment: The conformation of Ebola virus HR1 changes significantly in the pre-fusion (three-helix bundle) and post-fusion (six-helix bundle) conformations of the HR1-HR2 region.
Jiyao W et al.(2023). "The conserved domain database in 2023.",