The Methylosome, a 20S Complex Containing JBP1 and pICln, Produces Dimethylarginine-Modified Sm Proteins

Abstract

snRNPs, integral components of the pre-mRNA splicing machinery, consist of seven Sm proteins which assemble in the cytoplasm as a ring structure on the snRNAs U1, U2, U4, and U5. The survival motor neuron (SMN) protein, the spinal muscular atrophy disease gene product, is crucial for snRNP core particle assembly in vivo. SMN binds preferentially and directly to the symmetrical dimethylarginine (sDMA)-modified arginineand glycine-rich (RG-rich) domains of SmD1 and SmD3. We found that the unmodified, but not the sDMAmodified, RG domains of SmD1 and SmD3 associate with a 20S methyltransferase complex, termed the methylosome, that contains the methyltransferase JBP1 and a JBP1-interacting protein, pICln. JBP1 binds SmD1 and SmD3 via their RG domains, while pICln binds the Sm domains. JBP1 produces sDMAs in the RG domain-containing Sm proteins. We further demonstrate the existence of a 6S complex that contains pICln, SmD1, and SmD3 but not JBP1. SmD3 from the methylosome, but not that from the 6S complex, can be transferred to the SMN complex in vitro. Together with previous results, these data indicate that methylation of Sm proteins by the methylosome directs Sm proteins to the SMN complex for assembly into snRNP core particles and suggest that the methylosome can regulate snRNP assembly.

Interplay between chromatin remodelers and protein arginine methyltransferases

“…PRMT5 has been found in multiple complexes where it mediates diverse functions including RNA processing, transcriptional regulation, and muscle as well as germ line differentiation (Fabbrizio et al, 2002;Pal et al, 2003Pal et al, , 2004Ancelin et al, 2006;Guezennec et al, 2006;Dacwag et al, 2007). As a component of the 20S methylosome, PRMT5 mediates methylation of SmD1 and SmD3 proteins, which can interact with other components to promote assembly of the small nuclear riboprotein particles (UsnRNPs), which are involved in pre-mRNA splicing (Meister et al, 2001;Friesen et al, 2001). Another way PRMT5 participates in RNA processing is through its association with fibrillarin (FIB), the RNA methyltransferase found in the nucleolus and cajal bodies, which is involved in pre-rRNA processing and snRNP biogenesis (Yanagida et al, 2004).…”

Human Fibrillarin Forms a Sub-complex with Splicing Factor 2-associated p32, Protein Arginine Methyltransferases, and Tubulins α3 and β1 That Is Independent of Its Association with Preribosomal Ribonucleoprotein Complexes

“…Two types of arginine methyltransferases were identified as FIB-associated proteins. First, PRMT5 (originally identified as the Janus kinase binding protein 1, or JBP1) is a type II methyltransferase that catalyzes the formation of N G -monomethylarginine and symmetric N G ,NЈ G -dimethylarginine (35) and is the catalytic component of the methylosome that regulates snRNP assembly (36). The second enzyme, PRMT1, is the predominant type I methyltransferase in cells and catalyzes the formation of N G -monomethylarginine and asymmetric N G ,N G -dimethylarginine (35).…”

Arginine methylation mediated by the Arabidopsis homolog of PRMT5 is essential for proper pre-mRNA splicing

“…In the nucleus, PRMT5 usually functions in repressing target genes by methylating histone H4 Arginine 3 (H4R3), H3R8, as well as transcription factors/regulators (5,11,12). In the cytoplasm, PRMT5 mainly exists in a 20S methylosome complex, consisting of spliceosomal U snRNP (uridinerich small nuclear riboucleoprotein particles) Sm proteins (including Sm B/B′, D1, D2, D3, E, F, and G), PRMT5, pICln, and WD repeat protein/MEP50 (13)(14)(15). In this complex, U1, 2, 4, 5 snRNP Sm proteins Sm B/B′, D1, D3, and U6 snRNP-specific Sm-like protein LSm4 are symmetrically dimethylated by PRMT5 (13,16).…”

“…In the cytoplasm, PRMT5 mainly exists in a 20S methylosome complex, consisting of spliceosomal U snRNP (uridinerich small nuclear riboucleoprotein particles) Sm proteins (including Sm B/B′, D1, D2, D3, E, F, and G), PRMT5, pICln, and WD repeat protein/MEP50 (13)(14)(15). In this complex, U1, 2, 4, 5 snRNP Sm proteins Sm B/B′, D1, D3, and U6 snRNP-specific Sm-like protein LSm4 are symmetrically dimethylated by PRMT5 (13,16). Such methylation can increase the binding affinity of these Sm proteins for the downstream recipient, Survival Motor Neuron, the spinal muscular atrophy disease gene product (17,18).…”