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

Spliceosomal small nuclear ribonucleoprotein particles (snRNPs) undergo a complex maturation pathway containing multiple steps in the nucleus and in the cytoplasm. snRNP biogenesis is strictly proofread and several quality control checkpoints are placed along the pathway. Here, we analyzed the fate of small nuclear RNAs (snRNAs) that are unable to acquire a ring of Sm proteins. We showed that snRNAs lacking the Sm ring are unstable and accumulate in P-bodies in an LSm1-dependent manner. We further provide evidence that defective snRNAs without the Sm binding site are uridylated at the 3' end and associate with DIS3L2 3'→5' exoribonuclease and LSm proteins. Finally, inhibition of 5'→3' exoribonuclease XRN1 increases association of ΔSm snRNAs with DIS3L2, which indicates competition and compensation between these two degradation enzymes. Together, we provide evidence that defective snRNAs without the Sm ring are uridylated and degraded by alternative pathways involving either DIS3L2 or LSm proteins and XRN1.

• MeSH
• Exoribonucleases metabolism   MeSH
• HeLa Cells MeSH
• Nucleic Acid Conformation * MeSH
• Humans MeSH
• Organelles metabolism   MeSH
• SMN Complex Proteins metabolism   MeSH
• RNA-Binding Proteins metabolism   MeSH
• Proto-Oncogene Proteins metabolism   MeSH
• RNA, Small Nuclear chemistry   metabolism   MeSH
• Base Sequence MeSH
• RNA Stability MeSH
• RNA Transport * MeSH
• Protein Binding MeSH
• Binding Sites MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• DIS3L2 protein, human MeSH Browser
• Exoribonucleases MeSH
• GEMIN5 protein, human MeSH Browser
• LSM1 protein, human MeSH Browser
• SMN Complex Proteins MeSH
• RNA-Binding Proteins MeSH
• Proto-Oncogene Proteins MeSH
• RNA, Small Nuclear MeSH

Cajal bodies (CBs) are nuclear non-membrane bound organelles where small nuclear ribonucleoprotein particles (snRNPs) undergo their final maturation and quality control before they are released to the nucleoplasm. However, the molecular mechanism how immature snRNPs are targeted and retained in CBs has yet to be described. Here, we microinjected and expressed various snRNA deletion mutants as well as chimeric 7SK, Alu or bacterial SRP non-coding RNAs and provide evidence that Sm and SMN binding sites are necessary and sufficient for CB localization of snRNAs. We further show that Sm proteins, and specifically their GR-rich domains, are important for accumulating snRNPs in CBs. Accordingly, core snRNPs containing the Sm proteins, but not naked snRNAs, restore the formation of CBs after their depletion. Finally, we show that immature but not fully assembled snRNPs are able to induce CB formation and that microinjection of an excess of U2 snRNP-specific proteins, which promotes U2 snRNP maturation, chases U2 snRNA from CBs. We propose that the accessibility of the Sm ring represents the molecular basis for the quality control of the final maturation of snRNPs and the sequestration of immature particles in CBs.

• MeSH
• Cell Nucleus genetics   MeSH
• Coiled Bodies genetics   metabolism   MeSH
• HeLa Cells MeSH
• Humans MeSH
• Ribonucleoprotein, U2 Small Nuclear genetics   MeSH
• Gene Expression Regulation genetics   MeSH
• RNA, Small Nuclear genetics   MeSH
• Spliceosomes genetics   MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Ribonucleoprotein, U2 Small Nuclear MeSH
• RNA, Small Nuclear MeSH
• U2 small nuclear RNA MeSH Browser