InterPro domain: IPR034102

The eukaryotic Sm proteins (B/B', D1, D2, D3, E, F and G) assemble into a hetero-heptameric ring around the Sm site of the 2,2,7-trimethyl guanosine (m3G) capped U1, U2, U4 and U5 snRNAs (Sm snRNAs) forming the core of the snRNP particle. The snRNP particle, in turn, assembles with other components onto the pre-mRNA to form the spliceosome which is responsible for the excision of introns and the ligation of exons. Members of this family share a highly conserved Sm fold containing an N-terminal helix followed by a strongly bent five-stranded antiparallel beta-sheet [ 1 , 2 , 3 ].

Sm subunit D1 heterodimerizes with subunit D2 and three such heterodimers form a hexameric ring structure with alternating D1 and D2 subunits. The D1 - D2 heterodimer also assembles into a heptameric ring containing DB, D3, E, F, and G subunits [ 4 , 5 ].

1. Sm and Sm-like proteins assemble in two related complexes of deep evolutionary origin. EMBO J. 18, 3451-62
2. The Sm domain is an ancient RNA-binding motif with oligo(U) specificity. Proc. Natl. Acad. Sci. U.S.A. 98, 3685-9
3. Sm protein-Sm site RNA interactions within the inner ring of the spliceosomal snRNP core structure. EMBO J. 20, 187-96
4. An assembly chaperone collaborates with the SMN complex to generate spliceosomal SnRNPs. Cell 135, 497-509
5. Structural Basis of Assembly Chaperone- Mediated snRNP Formation. Mol. Cell 49, 692-703