Spinal muscular atrophy (SMA) is a
neurodegenerative disease of motor neurons caused by reduced levels of functional survival of motor neurons (SMN)
protein. Cytoplasmic SMN directly interacts with spliceosomal Sm
proteins and facilitates their assembly onto U snRNAs. Nuclear SMN, in contrast, mediates recycling of
pre-mRNA splicing factors. In this study, we have addressed the function of SMN in the nucleus. We show that a
monoclonal antibody directed against SMN inhibits
pre-mRNA splicing. Interestingly, the mode of inhibition suggests a novel role for SMN in splicing that occurs prior to, or in addition to, its role in recycling. Using biochemical fractionation and anti-SMN immunoaffinity chromatography, we identified two distinct nuclear SMN complexes termed NSC1 and NSC2. The biochemical properties and
protein composition of NSC1 were determined in detail. NSC1 migrates in
sucrose gradients as a U
snRNA-free 20S complex containing at least 10
proteins. In addition to SMN, these include the SMN-interacting
protein 1 (SIP-1), the putative helicase dp103/Gemin3, the novel dp103/Gemin3-interacting
protein GIP1/Gemin4 and three additional
proteins with apparent masses of 43, 33 and 18 kDa, respectively. Most surprisingly, NSC1 also contains a specific subset of spliceosomal Sm
proteins. This shows that the SMN-Sm
protein interaction is not restricted to the cytoplasm. Our data imply that nuclear SMN affects splicing by modulating the Sm
protein composition of U snRNPs.