Class II viral fusion protein. Fusion activity is inactive as long as E1 is bound to E2 in mature virion. After virus attachment to target cell and clathrin-mediated endocytosis, acidification of the endosome would induce dissociation of E1/E2 heterodimer and concomitant trimerization of the E1 subunits. This E1 homotrimer is fusion active, and promotes release of viral nucleocapsid in cytoplasm after endosome and viral membrane fusion. The cytoplasmic tail of spike glycoprotein E1 modulates virus release. The surface glycoproteins display an irregular helical organization and a pseudo-tetrameric inner nucleocapsid arrangement 