Plays a role in virus cell-to-cell and plant-to-plant transmission. Interacts with virion icosahedral capsid and movement protein, thereby facilitating virion cell-to-cell transmission through plasmodesmata opened by viral movement protein. Also interacts with aphid transmission factor, attaching the virion to aphid stylet when the animal feeds on an virus infected plant. Aphid saliva may later detach the virion, inducing release of infectious particles when the animal feeds on a new plant. Homotetramer, through coiled-coil domain. Homotrimer when bound on icosehadral capsid. Interacts with capsid protein, and with movement protein. Belongs to the caulimovirus ORF III family.