Retroviral envelope proteins mediate receptor recognition and membrane fusion during early infection. Endogenous envelope proteins may have kept, lost or modified their original function during evolution. SU mediates receptor recognition. TM anchors the envelope heterodimer to the viral membrane through one transmembrane domain. The other hydrophobic domain, called fusion peptide, mediates fusion of the viral membrane with the target cell membrane (By similarity). The surface (SU) and transmembrane (TM) proteins form a heterodimer. SU and TM are attached by noncovalent interactions or by a labile interchain disulfide bond (By similarity). Expressed in lung, placenta, testis and peripheral blood lymphocytes. Specific enzymatic cleavages in vivo yield the mature SU and TM proteins. This envelope protein is encoded by a human specific provirus. Has a type 1 genome. The HERV-K(HML-2) family contains type 1 and type 2 genomes depending on the absence or presence of 292 nucleotides at the 5'-end of the env gene resulting in Env proteins of distinct sizes. Despite their overall retroviral envelope structure HERV-K(HML-2) type 1 envelope proteins lack a predictable signal sequence. Subgenomic RNA transcripts coding for full-length envelope proteins have been detected for both type of genomes. Belongs to the beta type-B retroviral envelope protein family. HERV class-II K(HML-2) env subfamily. No predictable signal peptide.