The Cajal body (CB) is a conserved non-membrane nuclear structure where several steps of small nuclear RNP particle (snRNP) biogenesis take place. It has been proposed that CB formation follows a liquid-liquid phase separation model, but this hypothesis has never been rigorously tested. Here, we applied live-cell imaging to show that the key CB assembly factor coilin is mobile within the CB, and we revealed a diffusion barrier that limits the coilin exchange between CBs and the nucleoplasm. We generated single aa mutations and demonstrated that RNA-dependent coilin oligomerization and coilin interaction with snRNP are essential for CB formation and maintenance. We applied these data to formulate a mathematical model that links the movement of coilin within the nucleoplasm, CB, and across the boundary with its oligomerization and snRNP binding. Our results illustrate CB as a structure dynamically responding to snRNP assembly and recycling.

• MeSH
• Coiled Bodies * metabolism   genetics   MeSH
• HeLa Cells MeSH
• Nuclear Proteins * metabolism   genetics   MeSH
• Humans MeSH
• Protein Multimerization MeSH
• Mutation MeSH
• Ribonucleoproteins, Small Nuclear * metabolism   genetics   MeSH
• Spliceosomes * metabolism   genetics   MeSH
• Protein Binding MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Nuclear Proteins * MeSH
• p80-coilin MeSH Browser
• Ribonucleoproteins, Small Nuclear * MeSH

Spliceosomal snRNPs are multicomponent particles that undergo a complex maturation pathway. Human Sm-class snRNAs are generated as 3'-end extended precursors, which are exported to the cytoplasm and assembled together with Sm proteins into core RNPs by the SMN complex. Here, we provide evidence that these pre-snRNA substrates contain compact, evolutionarily conserved secondary structures that overlap with the Sm binding site. These structural motifs in pre-snRNAs are predicted to interfere with Sm core assembly. We model structural rearrangements that lead to an open pre-snRNA conformation compatible with Sm protein interaction. The predicted rearrangement pathway is conserved in Metazoa and requires an external factor that initiates snRNA remodeling. We show that the essential helicase Gemin3, which is a component of the SMN complex, is crucial for snRNA structural rearrangements during snRNP maturation. The SMN complex thus facilitates ATP-driven structural changes in snRNAs that expose the Sm site and enable Sm protein binding.

• MeSH
• HeLa Cells MeSH
• snRNP Core Proteins genetics   MeSH
• Humans MeSH
• RNA Precursors * metabolism   MeSH
• SMN Complex Proteins metabolism   MeSH
• Ribonucleoproteins, Small Nuclear metabolism   MeSH
• RNA, Small Nuclear * metabolism   MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Research Support, N.I.H., Extramural MeSH
• Names of Substances
• snRNP Core Proteins MeSH
• RNA Precursors * MeSH
• SMN Complex Proteins MeSH
• Ribonucleoproteins, Small Nuclear MeSH
• RNA, Small Nuclear * MeSH