The E1 glycoprotein (p110) is a Rubella virus class II viral fusion protein. Viral fusion is inactive when bound to E2 in a mature virion. Viral attachment to target cell and subsequent endocytosis results in acidification of the endosome, dissociation of the E1/E2 heterodimer and trimerization of the E1 subunits. The E1 homotrimer is fusion active and promotes release of viral nucleocapsid in the cell cytoplasm after cell and viral membrane fusion. Tyrosine residues on the E1 cytoplasmic tail are critical for viral release.