Alters the structure of the nuclear envelope by interacting with host CBX5 and disrupting CBX5 association with LBR. Involved in the perinuclear-nuclear localization of the capsid protein VP1 during virion assembly and maturation. Plays an important role in the release of progeny virions from infected cells and in viral propagation, probably by acting as a viral ionic channel in the host plasma membrane. Allows influx of extracellular calcium ions in the host cell. May contribute to viral genome transcription and translation of viral late proteins (By similarity). Homooligomer. Interacts with VP1 (By similarity). Interacts with large T antigen; this interaction may impact upon the activity of T-antigen on the control of viral gene transcription and replication. Interacts with small t antigen. Interacts with host CBX5; this interaction induces the dissociation of CBX5 from LBR, resulting in destabilization of the nuclear envelope (By similarity). Mostly perinuclear. Phosphorylated by host kinase. Phosphorylation segregates agnoprotein in cytoplasm, whereas unphosphorylated agnoprotein migrate to the nucleus (By similarity). Produced by alternative initiation of the late mRNA. Belongs to the polyomavirus agnoprotein family.