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Molecular and Cellular Biology, December 2001, p. 8289-8300, Vol. 21, No. 24
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.24.8289-8300.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

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

Westley J. Friesen,1 Sergey Paushkin,1 Anastasia Wyce,1 Severine Massenet,1 G. Scott Pesiridis,1 Gregory Van Duyne,1 Juri Rappsilber,2 Matthias Mann,2 and Gideon Dreyfuss1,*

Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6148,1 and Protein Interaction Laboratory, CEBI, and Department of Biochemistry and Molecular Biology, University of Southern Denmark, DK-5230 Odense M, Denmark2

Received 24 July 2001/Returned for modification 10 September 2001/Accepted 13 September 2001

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 arginine- and glycine-rich (RG-rich) domains of SmD1 and SmD3. We found that the unmodified, but not the sDMA-modified, 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.

* Corresponding author. Mailing address: Howard Hughes Medical Institute and Department of Biochemistry & Biophysics, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-6148. Phone: (215) 898-0398. Fax: (215) 573-2000. E-mail: gdreyfuss{at}hhmi.upenn.edu.

Molecular and Cellular Biology, December 2001, p. 8289-8300, Vol. 21, No. 24
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.24.8289-8300.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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