Envelope protein part of the entry-fusion complex responsible for the virus membrane fusion with host cell membrane during virus entry. 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. Component of the mature virion (MV) membrane. The mature virion is located in the cytoplasm of infected cells and is probably released by cell lysis. 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 chordopoxvirinae A21 family.