Cell cycle dependent nucleolar localization of the coronavirus nucleocapsid protein.

The nucleolus is a dynamic sub-nuclear structure which is involved in ribosome subunit biogenesis, modulation of cell growth and response to cell stress. The nucleolar proteome varies particularly with regard to the cell cycle. Viral proteins can localize to the nucleolus and using the coronavirus nucleocapsid (N) protein as a model, the cell cycle dependent trafficking of viral proteins to the nucleolus was investigated. Cell synchronization studies coupled to live cell confocal microscopy indicated that nucleolar localization of N protein was greater in the G(2)/M phase of the cell cycle than at other stages. This result was reinforced when FRAP and FLIP analysis indicated that N protein was more mobile within the nucleoplasm and nucleolus in the G(2)/M phase of the cell cycle. The data suggested that viral nucleolar proteins can also localize to the nucleolus in a cell cycle dependent manner and this may be related to dynamic trafficking.