The nucleolar interface of RNA viruses.

In recent years, understanding of the nucleolus has undergone a renaissance. Once considered primarily as the sites of ribosome biogenesis, nucleoli are now understood to be highly dynamic, multifunctional structures that participate in a plethora of cellular functions including regulation of the cell cycle, signal recognition particle assembly, apoptosis and stress responses. Although the molecular/mechanistic details of many of these functions remain only partially resolved, it is becoming increasingly apparent that nucleoli are also common targets of almost all types of viruses, potentially allowing viruses to manipulate cellular responses and the intracellular environment to facilitate replication and propagation. Importantly, a number of recent studies have moved beyond early descriptive observations to identify key roles for nucleolar interactions in the viral life cycle and pathogenesis. While it is perhaps unsurprising that many viruses that replicate within the nucleus also form interactions with nucleoli, the roles of nucleoli in the biology of cytoplasmic viruses is less intuitive. Nevertheless, a number of positive-stranded RNA viruses that replicate exclusively in the cytoplasm are known to express proteins that enter the nucleus and target nucleoli, and recent data have indicated similar processes in several cytoplasmic negative-sense RNA viruses. Here, we review this emerging aspect of the virus-host interface with a focus on examples where virus-nucleolus interactions have been linked to specific functional outcomes/mechanistic processes in infection and on the nucleolar interfaces formed by viruses that replicate exclusively in the cytoplasm.