This protein appears to play an important role in virus penetration at the level of cell fusion. The N-terminal proximal region is essential for fusion ability. Essential in fusing the outermost of the two Golgi-derived membranes enveloping the virus with the plasma membrane, and in its subsequent release extracellularly. Homotrimer, covalently linked. Envelope fraction of virions. Belongs to the poxviruses fusion protein family.