Component of the virion core. Interacts with P4A/A10 and its cleaved form 4A. Localizes between the core and the membrane; might surround the outer core wall like a palisade (spikes). Expressed in the late phase of the viral replicative cycle. Its phosphorylation state is regulated by the F10 kinase and the H1 phosphatase. Belongs to the chordopoxvirinae A4 family.