JCB_201611108 2391..2408

The Sm class members of U-rich small nuclear ribonucleoprotein particles (UsnRNPs) constitute the central building blocks of major and minor spliceosomes. These macromolecular machines catalyze the removal of introns from pre-mRNAs, facilitating an essential step in the formation of a translatable mRNA (Matera and Wang, 2014). To cope with the task of splicing virtually all intron-containing pre-mRNAs, the steady-state level of cellular UsnRNPs is abundant in higher eukaryotes, reaching an estimated concentration of 10 μM per nucleus in HeLa cells (Montzka and Steitz, 1988). Therefore, efficient production of UsnRNPs and maintenance of their steady-state levels appear to be indispensable for eukaryotic cells. UsnRNPs are assembled in cells in a segmented pathway that starts with the transcription of UsnRNA precursor molecules by either polymerase II (for U1, U2, U4, U5, U11, U12, and U4atac snRNAs) or polymerase III (for U6 and U6atac snRNAs). The polymerase II–transcribed snRNAs are transiently exported from the nucleus to the cytoplasm, where they associate with the seven common (Sm) proteins B/B′, D1, D2, D3, E, F, and G. This leads to the formation of a toroidal ring on the UsnRNA, termed the Sm core (Raker et al., 1996, 1999; Kambach et al., 1999b; Stark et al., 2001; Urlaub et al., 2001; Chari et al., 2008; Wahl et al., 2009; Zhang et al., 2011). The Sm core is a common structural denominator of these UsnRNPs that enables several subsequent steps in the biogenesis pathway. This includes the conversion of the m7G-cap of the UsnRNAs to its hypermethylated form (m3G-cap) and the nuclear import of the assembled UsnRNP particle (Fischer and Lührmann, 1990; Hamm et al., 1990; Mouaikel et al., 2002, 2003). The stage at which specific snRNP proteins join the respective particles appears to be variable. Some can bind in the cytosol to their respective snRNA target, whereas others join the UsnRNA in nuclear Cajal bodies where UsnRNA modifications also occur (Achsel et al., 1999; Sleeman and Lamond, 1999; Jády et al., 2003; Stanĕk and Neugebauer, 2004). Unlike the Sm class of UsnRNPs, formation of U6 and U6atac snRNPs occurs exclusively in the nucleus. These particles lack the Sm proteins but instead contain seven Sm-like proteins (LSm2–8) that associate with the 3′ end of the snRNA to form an RNP that is structurally similar to the Sm core domain (Pannone et al., 1998, 2001; Achsel et al., 1999). The assembly of the Sm-class UsnRNPs involves several trans-acting factors. These are organized in two units, the protein arginine methyltransferase 5 (PRMT5) complex and the survival motor neuron (SMN) complex (Meister and Fischer, Specialized assembly factors facilitate the formation of many macromolecular complexes in vivo. The formation of Sm core structures of spliceosomal U-rich small nuclear ribonucleoprotein particles (UsnRNPs) requires assembly factors united in protein arginine methyltransferase 5 (PRMT5) and survival motor neuron (SMN) complexes. We demonstrate that perturbations of this assembly machinery trigger complex cellular responses that prevent aggregation of unassembled Sm proteins. Inactivation of the SMN complex results in the initial tailback of Sm proteins on the PRMT5 complex, followed by down-regulation of their encoding mRNAs. In contrast, reduction of pICln, a PRMT5 complex subunit, leads to the retention of newly synthesized Sm proteins on ribosomes and their subsequent lysosomal degradation. Overexpression of Sm proteins under these conditions results in a surplus of Sm proteins over pICln, promoting their aggregation. Our studies identify an elaborate safeguarding system that prevents individual Sm proteins from aggregating, contributing to cellular UsnRNP homeostasis. Impaired spliceosomal UsnRNP assembly leads to Sm mRNA down-regulation and Sm protein degradation