In keeping with the physiological nucleolar dynamics in early embryogenesis, the nucleoli of transplanted nuclei disassemble in eggs and early embryos and reassemble at the blastula stage. Now, in Nature Cell Biology, Nobuaki Kikyo and colleagues have begun to analyse the molecular mechanisms underlying nucleolar disassembly.

Using dispersal of nucleolar phosphoprotein B23 in the nucleoplasm of embryonic XL2 nuclei as a marker of disassembly, a nucleolar-disassembly activity from Xenopus laevis egg extract was purified. The activity contained two germ-cell proteins — FRGY2a and FRGY2b (FRGY2a/b) — that share 83% amino-acid identity and are known to function as transcription factors and maternal messenger RNA-masking proteins in messenger ribonucleoprotein (mRNP) particles in oocytes. The authors then tested the ability of FRGY2a/b, recombinant (r)FRGY2a and rFRGY2b to disassemble nucleoli — in each case, nucleoli were disassembled in a similar and dose-dependent manner, showing that FRGY2a or FRGY2b alone is sufficient for nucleolar disassembly.

By creating FRGY2a mutants, Kikyo and colleagues went on to show that the disassembly property of FRGY2a is restricted to the carboxy-terminal domain (rFRGY2a-C), and that four BA islands (basic/aromatic amino acids) — which provide extensive, non-selective charge interactions with RNA — contribute cumulatively to disassembly.

To examine its in vivo effects, XL2 cells were transfected with rFRGY2a or rFRGY2a-C. rFRGY2a-C localized preferentially to the nucleus and induced both dispersal of B23 and nucleolar disappearance. By contrast, rFRGY2a localized mainly in the cytoplasm and didn't disperse B23 or reduce nucleolar size. However, by mutating the RNA-binding cold-shock domain in the amino terminus some rFRGY2a entered the nuclei and triggered disassembly.

Because transcription of rRNA is crucial for maintaining nucleolar integrity, the authors monitored rRNA synthesis during disassembly. Importantly, FRGY2a/b didn't disrupt rRNA synthesis and is, to the authors' knowledge, “... the first protein to be identified that can disassemble nucleoli without disrupting rRNA synthesis”. Finally, Kikyo and colleagues showed that disassembly by FRGY2a/b could be reversed, indicating that the essential core structure of these nucleoli is retained.

Nucleolar disassembly by FRGY2a/b will have a considerable impact on the inefficient nuclear-cloning process, because proper nucleolar disassembly and reassembly is presumed to be crucial for the normal development of cloned embryos. In addition, this type of nuclear-remodelling study will “...provide key insights into the structural and functional organization of the nucleus”.