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. The neuromuscular disease spinal muscular atrophy (SMA) is characterized by degeneration of motor neurons of the spinal cord, leading to muscular weakness and atrophy (40). The survival-of-motor-neurons gene (SMN) is present as an inverted repeat on chromosome 5 at 5q13, and over 98% of SMA patients have mutations or deletions of the telomeric copy of the gene (SMN1), resulting in reduced levels of the survival motor neuron (SMN) protein (31; reviewed in reference 8). snRNP core particles assemble in the cytoplasm from newly exported snRNAs and the core Sm proteins (SmB, SmD1, SmD2, SmD3, SmE, SmF, and SmG). Cap hypermethylation of the U snRNAs requires that the core Sm proteins assemble on the Sm sites of the U1, U2, U4, and U5 snRNAs. snRNP Sm core particles are believed to be constructed of a sevenmember ring containing each of the Sm proteins with a single U snRNA bound in the center of the ring (28). The presence of the properly assembled Sm core as well as the 2,2,7-trimethylguasnosine (m 3 G) cap is required for snRNP import to the nucleus (13, 16, 17, 27, 38, 39, 41). Regions conserved in all of the Sm proteins (Sm motifs 1 and 2) (51) are most likely required for proper folding of these proteins and their reciprocal interactions (28). In the cytoplasm, SMN is associated with the Sm proteins (9, 36). In Xenopus oocytes, microinjection of anti-SMN complex antibodies inhibits or stimulates snRNP core particle formation, and transient expression of a dominant-negative mutant of SMN in mammalian cells seques