Nuclear bodies are membraneless organelles that play important roles in genome functioning. A specific type of nuclear bodies known as interphase prenucleolar bodies (iPNBs) are formed in the nucleoplasm after hypotonic stress from partially disassembled nucleoli. iPNBs are then disassembled, and the nucleoli are reformed simultaneously. Here, we show that diffusion of B23 molecules (also known as nucleophosmin, NPM1) from iPNBs, but not fusion of iPNBs with the nucleoli, contributes to the transfer of B23 from iPNBs to the nucleoli. Maturation of pre-ribosomal RNAs (rRNAs) and the subsequent outflow of mature rRNAs from iPNBs led to the disassembly of iPNBs. We found that B23 transfer was dependent on the synthesis of pre-rRNA molecules in nucleoli; these pre-rRNA molecules interacted with B23 and led to its accumulation within nucleoli. The transfer of B23 between iPNBs and nucleoli was accomplished through a nucleoplasmic pool of B23, and increased nucleoplasmic B23 content retarded disassembly, whereas B23 depletion accelerated disassembly. Our results suggest that iPNB disassembly and nucleolus assembly might be coupled through RNA-dependent exchange of nucleolar proteins, creating a highly dynamic system with long-distance correlations between spatially distinct processes.

A N Belozersky Institute of Physico Chemical Biology M 5 Lomonosov Moscow State University Moscow 119992 Russia

A N Belozersky Institute of Physico Chemical Biology M 5 Lomonosov Moscow State University Moscow 119992 Russia LIA1066 French Russian Joint Cancer Research Laboratory Villejuif 94805 France

A N Belozersky Institute of Physico Chemical Biology M 5 Lomonosov Moscow State University Moscow 119992 Russia LIA1066 French Russian Joint Cancer Research Laboratory Villejuif 94805 France UMR8126 Université Paris Sud CNRS Institut de cancérologie Gustave Roussy Villejuif 94805 France

Centre for Biomedical Image Analysis Faculty of Informatics Masaryk University Botanická 68a Brno 602 00 Czech Republic Laboratory of Mathematical Methods of Image Processing Faculty of Computational Mathematics and Cybernetics M 5 Lomonosov Moscow State University Moscow 119992 Russia

Department of Cell Biology and Histology Faculty of Biology M 5 Lomonosov Moscow State University Moscow 119992 Russia

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