Figure 2

Paraspeckle dynamics. A model illustrating paraspeckle dynamics in the cell cycle. Three representative stages are shown: early G1; interphase; and telophase. The localization and behavior of paraspeckles throughout the cell cycle are highly dynamic. Early G1 (top): the nucleus of a human cell (large oval) contains two MENε/β loci (green circle), one on each chromosome 11q13 (blue territories). Paraspeckles (red circles or ovals) are generated at the transcriptionally active MENε/β loci, where paraspeckle proteins (smaller white, grey and black ovals in inset) associate with nascent MENε/β RNAs (black helices) to generate the paraspeckle. Interphase (lower right): the number of paraspeckles increases, typically to between 10 and 20 per nucleus. Newly generated paraspeckles are first localized to the MENε/β loci and then become distributed throughout the nucleus (indicated by arrows) by an unknown mechanism. Intact paraspeckles appear to be in a dynamic equilibrium, in which the flux of constituents between paraspeckles and nucleoplasm is balanced. The trajectories of redistribution of paraspeckles throughout the nucleus may be random as paraspeckles roam the interchromatin space by scanning specific target sites. Telophase (lower left): RNA polymerase II transcriptional activity is undetectable at this stage and, therefore, the levels of MENε/β decrease, which in turn causes paraspeckle disassembly. Paraspeckles are reassembled once MENε/β transcription restarts in the daughter cells.