Scaffold protein which forms a transitory spherical honeycomb lattice providing curvature and rigidity to the convex membrane of crescent and immature virions (IV). This association occurs concomitantly with viral membrane formation. Targeted by the drug rifampicin, which prevents the formation of this lattice, and hence virus morphogenesis. In the presence of rifampicin, irregularly shaped membranes that lack the honeycomb layer accumulate around areas of electron-dense viroplasm. This layer is lost from virions during maturation from IV to mature virion (MV), through the proteolysis of A17 N-terminus (By similarity). Homotrimer (By similarity). Self-assembles to form a layer. Interacts with A17 (via N-terminus); this interaction is necessary for D13 association with membranes (By similarity). Associates transitorily with crescent and IV membranes. Displays structure similarities to capsid proteins. Belongs to the poxviridae protein D13 family.