Envelope protein required for virus entry into host cell and for cell-cell fusion (syncytium formation). Part of a stable entry-fusion complex (EFC) which is at least composed of proteins A16, A21, A28, G3, G9, H2, J5, and L5. Formation of the viral membrane is necessary for the assembly of the complex. Interacts directly with protein H2. Component of the mature virion (MV) membrane. Expressed in the late phase of the viral replicative cycle. Contains two intramolecular disulfide bonds. They are created by the viral disulfide bond formation pathway, a poxvirus-specific pathway that operates on the cytoplasmic side of the MV membranes. Belongs to the poxviridae A28 protein family.