Physiological importance of RNA and protein mobility in the cell nucleus

Trafficking of proteins and RNAs is essential for cellular function and homeostasis. While it has long been appreciated that proteins and RNAs move within cells, only recently has it become possible to visualize trafficking events in vivo. Analysis of protein and RNA motion within the cell nucleus have been particularly intriguing as they have revealed an unanticipated degree of dynamics within the organelle. These methods have revealed that the intranuclear trafficking occurs largely by energy-independent mechanisms and is driven by diffusion. RNA molecules and non-DNA binding proteins undergo constrained diffusion, largely limited by the spatial constraint imposed by chromatin, and chromatin binding proteins move by a stop-and-go mechanism where their free diffusion is interrupted by random association with the chromatin fiber. The ability and mode of motion of proteins and RNAs has implications for how they find nuclear targets on chromatin and in nuclear subcompartments and how macromolecular complexes are assembled in vivo. Most importantly, the dynamic nature of proteins and RNAs is emerging as a means to control physiological cellular responses and pathways.