Intranuclear crystal formation in picornavirus-infected cells.

Cells infected with echovirus or poliovirus were incubated at a suboptimal temperature (28 degrees C). When the cells were examined by electron microscopy, not only were crystalline arrays of mature virus particles observed in the cytoplasm but crystals composed of immature virus-like particles were also evident in the less electron-dense central region of the nucleus. Immunofluorescent and autoradiographic studies revealed the presence of viral proteins and RNA in the nucleus. These findings strongly suggest that the intranuclear crystals are composed of incomplete virus particles and that crystal formation at such a low temperature may be a remarkable feature common to picornaviruses. Possible mechanisms for the intranuclear synthesis of viral constituents are discussed.