The release of Hand1 (green) from the nucleolus coincides with giant cell differentiation (top to bottom).

RILEY/MACMILLAN

Long viewed as merely a biofactory for ribosomes, the nucleolus has recently come to be seen as a multifunctional and dynamic subnuclear organelle. New support for this view comes from a study by David Martindill, Paul Riley (UCL Institute of Child Health, London, UK), and colleagues, who show that a cell fate regulator is held inactive in the nucleolus until phosphorylation releases it to trigger differentiation.

Differentiation of mouse trophoblast stem cells into a specialized cell type called giant cells requires Hand1, a bHLH transcription factor. Hand1 interacts with a wider variety of other bHLH partners than do others of its class. The authors thus wondered whether it might also interact with unrelated partners that help it time giant cell differentiation. Using a yeast two-hybrid approach, they found...

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