SMN-independent Subunits of the SMN Complex IDENTIFICATIONOF A SMALL NUCLEAR RIBONUCLEOPROTEIN ASSEMBLY INTERMEDIATE*

The survival of motor neurons (SMN) complex is essential for the biogenesis of small nuclear ribonucleoprotein (snRNP) com-plexes in eukaryotic cells. Reduced levels of SMN cause themotor neurondegenerativedisease, spinalmuscular atrophy.We identify here stable subunitsof theSMNcomplex thatdonot containSMN. Sedimentation and immunoprecipitation experiments using cell extracts reveal at least three complexes composed of Gemin3,-4, and-5; Gemin6,-7, and unrip; and SMN with Gemin2, as well as free Gemin5. Complexes containing Gemin3-Gemin4-Gemin5 and Gemin6-Gemin7-unrip persist at similar levels when SMN is reduced. Incells, immunofluorescencemicroscopyshowsdifferen-tial localization of Gemin5 after cell stress. We further show that the Gemin5-containing subunits bind small nuclear RNA inde-pendently of the SMN complex and without a requirement for exogenousATP.ATPhydrolysis is, however, required fordisplace