CryoEM Density maps and atomic models that support the findings of this study have been deposited in the Electron Microscopy Database and the Protein Databank with the accession codes EMD-0000 (Whole particle reconstruction), EMD-0000 (Vertex Complex), EMD-00000 (Localized reconstruction of the penton region), EMD-00000 (Focused Classification of the penton region) and PDB ID 0Q0U (Atomic model of the asymmetric unit).  