Structural Basis of Assembly Chaperone-Mediated snRNP Formation

Small nuclear ribonucleoproteins (snRNPs) represent key constituents of major and minor spliceosonnes. snRNPs contain a common core, composed of seven Sm proteins bound to snRNA, which forms in a step-wise and factor-mediated reaction. The assembly chaperone pICIn initially mediates the formation of an otherwise unstable pentanneric Sm protein unit. This so-called 6S complex docks subsequently onto the SMN complex, which removes pICIn and enables the transfer of pre-assembled Sm proteins onto snRNA. X-ray crystallography and electron microscopy was used to investigate the structural basis of snRNP assembly. The 6S complex structure identifies pICIn as an Sm protein mimic, which enables the topological organization of the Sm pentamer in a closed ring. A second structure of 6S bound to the SMN complex components SMN and Gemin2 uncovers a plausible mechanism of pICIn elimination and Sm protein activation for snRNA binding. Our studies reveal how assembly factors facilitate formation of RNA-protein complexes in vivo.