A multiprotein complex mediates the ATP-dependent assembly of spliceosomalU snRNPs

The spliceosomal snRNPs U1, U2, U4 and U5 contain a common RNP structure te rmed the Sm core that is formed by the binding of Sm proteins onto the U sn RNA. Although isolated Sm proteins assemble spontaneously onto U snRNAs in vitro, there is increasing evidence that SMN and its interactor Gemin2 are involved in this process in vivo. Here, we describe a cell-free assay syste m for the assembly of U snRNPs that closely reproduces in vivo conditions. Using this system, we show that assembly of U1 snRNP depends on ATP. Immuno depletion of SMN-Gemin2 from the extract abolished assembly even though the extract contained high levels of Sm proteins. An affinity-purified macromo lecular SMN complex consisting of 16 components including all Sm proteins r estored assembly in the immunodepleted extract. These data provide the firs t direct evidence that a complex containing SMN and Gemin2 mediates the act ive assembly of spliceosomal U snRNPs.