heptad repeat 1-heptad repeat 2 region (ectodomain) of the transmembrane subunit of human T-cell leukemia virus type 1 (HTLV-1), and related domains
This domain subfamily spans both heptad repeats of the glycoprotein (gp)/transmembrane(TM) subunit of various endogenous retroviruses (ERVs) and infectious retroviruses, including HTLV-1, HTLV -2, primate Mason-Pfizer monkey virus, Moloney murine leukemia virus, simian T-cell lymphotropic virus, feline leukemia virus (FeLV), bovine leukemia virus, and various human endogenous retroviruses (HERVs), including, HERV-H1_c2q24.3, HERV-H2_3q26, HERV-F(c)1_cXq21.33, HERV-T_19q13.11, Syncytin-1 (HERV-W_c7q21.2/ ERVWE1), Syncytin-2 (HERV-FRD_6p24.1), and related domains. This domain includes an N-terminal heptad repeat, a CKS17-like immunosuppressive region, a CX6C motif that forms an intrasubunit 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 HR1s 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 modern ERVs, those that integrated into the host genome post-speciation, have a currently active exogenous counterpart, such as FeLV. 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. Syncytin-1 and Syncytin-2 are expressed in the placenta, and are fusogenic, although they have a different cell specificity for fusion. Syncytin-2, but not Syncytin-1, is immunosuppressive; its immunosuppressive domain may protect the fetus from the mother's immune system. Syncytin-1 may participate in the formation of the placental trophoblast; it is also implicated in cell fusions between cancer and host cells and between cancer cell, and in human osteclast fusion. This subfamily also contains a mouse envelope protein encoded by the Fv-4 env gene, that blocks infection by exogenous MuLV.
Jiyao W et al.(2023). "The conserved domain database in 2023.",