Crystal structures of two Sm protein complexes and their implications for the assembly of the spliceosomal snRNPs

The U1, U2, U4/U6, and U5 small nuclear ribonucleoprotein particles (snRNPs ) involved in pre-mRNA splicing contain seven Sm proteins (B/B', D-1, D-2, D-3, E, F, and G) in common, which assemble around the Sm site present in f our of the major spliceosomal small nuclear RNAs (snRNAs). These proteins s hare a common sequence motif in two segments, Sm1 and Sm2, separated by a s hort variable linker. Crystal structures of two Sm protein complexes, D3B a nd D1D2, show that these proteins have a common fold containing an N-termin al helix followed by a strongly bent five-stranded antiparallel beta sheet, and the D1D2 and D3B dimers superpose closely in their core regions, inclu ding the dimer interfaces. The crystal structures suggest that the seven Sm proteins could form a closed ring and the snRNAs may be bound in the posit ively charged central hole.