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 (By similarity). 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). Primary culture of CD34+, lung, thymus, ileal mucosa, small intestine, and testis. Specific enzymatic cleavages in vivo yield the mature SU and TM proteins. Belongs to the beta type-B retroviral envelope protein family. HERV class-II K(HML-4) env subfamily. No predictable signal peptide.