Gemin4: A novel component of the SMN complex that is found in both gems and nucleoli

The survival of motor neurons (SMN) protein, the product of the neurodegene rative disease spinal muscular atrophy (SMA) gene, is localized both in the cytoplasm and in discrete nuclear bodies called gems. In both compartments SMN is part of a large complex that contains several proteins including Ge min2 (formerly SIP1) and the DEAD box protein Gemin3. In the cytoplasm, the SMN complex is associated with snRNP Sm core proteins and plays a critical role in spliceosomal snRNP assembly. In the nucleus, SMN is required for p re-mRNA splicing by serving in the regeneration of spliceosomes. These func tions are likely impaired in cells of SMA patients because they have reduce d levels of functional SMN, Here, we report the identification by nanoelect rospray mass spectrometry of a novel component of the SMN complex that we n ame Gemin4, Gemin4 is associated in vivo with the SMN complex through a dir ect interaction with Gemin3, The tight interaction of Gemin4 with Gemin3 su ggests that it could serve as a cofactor of this DEAD box protein. Gemin4 a lso interacts directly with several of the Sm core proteins. Monoclonal ant ibodies against Gemin4 efficiently immunoprecipitate the spliceosomal U snR NAs U1 and U5 from Xenopus oocytes cytoplasm, Immunolocalization experiment s show that Gemin4 is colocalized with SMN in the cytoplasm and in gems. In terestingly, Gemin4 is also detected in the nucleoli, suggesting that the S MN complex may also function in preribosomal RNA processing or ribosome ass embly.