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Molecular and Cellular Biology, September 2002, p. 6533-6541, Vol. 22, No. 18
0270-7306/02/$04.00+0     DOI: 10.1128/MCB.22.18.6533-6541.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

The SMN Complex Is Associated with snRNPs throughout Their Cytoplasmic Assembly Pathway

Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6148,¹ European Molecular Biology Laboratory, D-69117 Heidelberg, Germany²

Received 20 March 2002/ Returned for modification 2 May 2002/ Accepted 14 June 2002

The common neurodegenerative disease spinal muscular atrophy is caused by reduced levels of the survival of motor neurons (SMN) protein. SMN associates with several proteins (Gemin2 to Gemin6) to form a large complex which is found both in the cytoplasm and in the nucleus. The SMN complex functions in the assembly and metabolism of several RNPs, including spliceosomal snRNPs. The snRNP core assembly takes place in the cytoplasm from Sm proteins and newly exported snRNAs. Here, we identify three distinct cytoplasmic SMN complexes, each representing a defined intermediate in the snRNP biogenesis pathway. We show that the SMN complex associates with newly exported snRNAs containing the nonphosphorylated form of the snRNA export factor PHAX. The second SMN complex identified contains assembled Sm cores and m₃G-capped snRNAs. Finally, the SMN complex is associated with a preimport complex containing m₃G-capped snRNP cores bound to the snRNP nuclear import mediator snurportin1. Thus, the SMN complex is associated with snRNPs during the entire process of their biogenesis in the cytoplasm and may have multiple functions throughout this process.

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