Structural protein that forms a matrix surrounding the mature virion (MV) through interaction with protein A26. Presence of protein A25 in the virion structurally prevents direct virus-cell fusion mechanism (By similarity). Interacts (via N-terminus) with protein A26. Present above the membrane of mature virions (MV). Some orthopoxviruses such as cowpox, ectromelia, and raccoonpox viruses, form large cytoplasmic inclusions within which mature virions are embedded by a process called occlusion. In those viruses, A26 bridges mature virion with inclusion bodies through interactions with proteins A25 and A27 on the mature virion membrane. In variola virus, the protein A25 is deleted in its C-terminal region and no inclusion body is observed. Belongs to the poxviridae A25 protein family.