Late 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 (By similarity). Component of the intracellular mature virion (IMV) outer membrane. 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 IMV membranes (By similarity). Belongs to the poxviruses H2 family.