Cajal bodies (CBs) are evolutionarily conserved nuclear structures involved in the metabolism of spliceosomal small nuclear ribonucleoprotein particles (snRNPs). CBs are not present in all cell types, and the trigger for their formation is not yet known. Here, we depleted cells of factors required for the final steps of snRNP assembly and assayed for the presence of stalled intermediates in CBs. We show that depletion induces formation of CBs in cells that normally lack these nuclear compartments, suggesting that CB nucleation is triggered by an imbalance in snRNP assembly. Accumulation of stalled intermediates in CBs depends on the di-snRNP assembly factor SART3. SART3 is required for both the induction of CB formation as well as the tethering of incomplete snRNPs to coilin, the CB scaffolding protein. We propose a model wherein SART3 monitors tri-snRNP assembly and sequesters incomplete particles in CBs, thereby allowing cells to maintain a homeostatic balance of mature snRNPs in the nucleoplasm.
