Nopp140 Functions as a Molecular Link Between the Nucleolus and the Coiled Bodies

Coiled bodies are small nuclear organelles that are highly enriched in small nuclear RNAs, and that have long been thought to be associated with the nucleolus. Here we use mutational analysis, transient transfections, and the yeast two-hybrid system to show that the nucleolar phosphoprotein Nopp140 functions as a molecular link between the two prominent nuclear organelles. Exogenous Nopp140 accumulated in the nucleolus rapidly, but only after a lag phase in coiled bodies, suggesting a pathway between the two organelles. The expression of partial Nopp140 constructs exerted dominant negative effects on the endogenous Nopp140 by chasing it and other antigens that were common to both organelles out of the nucleolus. The alternating positively and negatively charged repeat domain of Nopp140 was required for targeting to both organelles. In addition, partial Nopp140 constructs caused formation of novel structures in the nucleoplasm and, in the case of the conserved carboxy terminus, led to the dispersal of coiled bodies. As a final link, we identified the coiled body–specific protein p80 coilin in a yeast two-hybrid screen with Nopp140. The interaction of the two proteins was confirmed by coimmunoprecipitation. Taken together, Nopp140 appeared to shuttle between the nucleolus and the coiled bodies, and to chaperone the transport of other molecules.