Mediates ssDNA packaging into virion, it locates to the internal surface of the capsid, thereby displacing the internal scaffolding protein B during virion formation. Protein J binds to and is packaged with the viral ssDNA. Additionally, protein J plays a role in viral attachment efficiency to the host cell. Interacts with F protein. situated at the interface between the internal surface of the capsid and the nucleic acid. Belongs to the microviridae J protein family. Icosahedral capsid structure