SUMMARYBackground-A 34-year-old woman presented with headache, feverish sensation and anxiety, rapidly followed by homicidal ideation, aggressive agitation, seizures, hypoventilation, hyperthermia and prominent autonomic instability requiring intubation and sedation. She developed episodes of hypotension and bradycardia with periods of asystole lasting up to 15 seconds. Upon weaning off sedation, her eyes opened but she was unresponsive to stimuli. There was muscle rigidity, frequent facial grimacing, rhythmic abdominal contractions, kicking motions of the legs, and intermittent dystonic postures of the right arm.

These findings identify fibrillarin and GAR1 as novel interactors of SMN and suggest a function for the SMN complex in the assembly and metabolism of snoRNPs. We propose that the SMN complex performs functions necessary for the biogenesis and function of diverse ribonucleoprotein complexes.

The survival of motor neurons (SMN) protein, the product of the gene responsible for the motor neuron degenerative disease spinal muscular atrophy (SMA), is part of a large macromolecular complex. The SMN complex is localized in both the cytoplasm and the nucleus and contains SMN, Gemin2, Gemin3, Gemin4, Gemin5, and a few not yet identified proteins. The SMN complex plays a key role in the biogenesis of spliceosomal small nuclear ribonucleoproteins (snRNPs) and other ribonucleoprotein particles. As a step toward the complete characterization of the components of the SMN complex, we generated stable cell lines that express FLAG-tagged SMN or Gemin2 under the control of a tetracycline-inducible promoter. Native SMN complexes of identical protein composition to those isolated by immunoprecipitation with anti-SMN antibodies were purified by affinity chromatography from extracts of both cell lines. Here we report the identification by mass spectrometry of a novel protein component of the SMN complex termed Gemin6. Co-immunoprecipitation, immunolocalization, and in vitro binding experiments demonstrate that Gemin6 is a component of the SMN complex that localizes to gems and interacts with several Sm proteins of the spliceosomal snRNPs. Spinal muscular atrophy (SMA)1 is an autosomal recessive disease characterized by degeneration of motor neurons of the spinal cord resulting in muscular atrophy (1). SMA is the most common genetic cause of infant mortality and the second most frequent lethal autosomal recessive disease after cystic fibrosis. Deletions or mutations in the survival of motor neurons (SMN1) gene, the SMA-causing gene, are detected in over 95% of SMA patients (2). SMN is essential for viability in all organisms and cell types tested, including humans, mice, chicken DT40 cells, Drosophila melanogaster, Caenorhabditis elegans, and Schizosaccharomyces pombe (3-9).SMN associates with Gemin2 (10), Gemin3 (11), Gemin4 (12), and Gemin5 (13) and a few not yet identified components to form a large multiprotein complex. The SMN complex is found both in the cytoplasm and in the nucleus, where it is concentrated in gems, nuclear bodies similar in size and number to Cajal (coiled) bodies and often associated with them (14). In addition to the integral components of the SMN complex described above, several proteins have been identified that directly interact with SMN and/or the Gemins. These include the spliceosomal snRNP Sm and Lsm proteins (10, 15-17), the snoRNP proteins fibrillarin and GAR1 (18, 19), RNA helicase A (20), hnRNP Q (21), and coilin (22). A strong correlation has emerged between a defect of SMN interaction with these proteins and the SMA phenotype, because SMN, but not SMN mutants found in SMA patients, interact directly with these proteins (15)(16)(17)(18)(19)(20)(21)(22). A common feature of these proteins is the presence of arginine-and glycine-rich (RG) domains that are necessary for their direct interaction with SMN (17-22). Specific arginine residues of these domains are di-methylated in vivo, and t...