The SMN complex assembles Sm cores on snRNAs, a key step in the biogenesis of snRNPs, the spliceosome's major components. Here, using SMN complex inhibitors identified by high-throughput screening and a ribo-proteomic strategy on formaldehyde crosslinked RNPs, we dissected this pathway in cells. We show that protein synthesis inhibition impairs the SMN complex, revealing discrete SMN and Gemin subunits and accumulating an snRNA precursor (pre-snRNA)-Gemin5 intermediate. By high-throughput sequencing of this transient intermediate's RNAs, we discovered the previously undetectable precursors of all the snRNAs and identified their Gemin5-binding sites. We demonstrate that pre-snRNA 3′ sequences function to enhance snRNP biogenesis. The SMN complex is also inhibited by oxidation, and we show that it stalls an inventory-complete SMN complex containing pre-snRNAs. We propose a stepwise pathway of SMN complex formation and snRNP biogenesis, highlighting Gemin5's function in delivering pre-snRNAs as substrates for Sm core assembly and processing.
Bibliographical noteFunding Information:
We are grateful to the members of our laboratory for helpful discussions and comments on this manuscript. We thank Dr. Chao-Xing Yuan of the Proteomics Core facility for expert help with mass spectrometry experiments. The facility is supported by grants P30CA016520 (Abramson Cancer Center) and ES013508-04 (CEET). We thank Dr. Jonathan Schug of the Functional Genomics Core at the University of Pennsylvania School of Medicine for excellent help with high-throughput sequencing and data analysis. We also thank Dr. Larry N. Singh for help with processing and depositing the data. This work was supported by the Association Française Contre les Myopathies (AFM). G.D. is an Investigator of the Howard Hughes Medical Institute.