MBE Advance Access published online on August 7, 2008
Molecular Biology and Evolution, doi:10.1093/molbev/msn175
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Evolutionary diversification of the Sm family of RNA-associated proteins
1 Department of Biology, Indiana University, 1001 E. Third Street, Bloomington, Indiana 47405
2 Current address: Department of Ecology & Evolutionary Biology, University of California, Los Angeles, P.O. Box 951786, Los Angeles, California 90095-1786
* Author for correspondence: dgscofield{at}ucla.edu
Received for publication March 22, 2008. Revision received July 1, 2008. Accepted for publication July 12, 2008.
The Sm family of proteins are closely associated with RNA metabolism throughout all of life. These proteins form homomorphic and heteromorphic rings consisting of six or seven subunits with a characteristic central pore, the presence of which is critical for binding U-rich regions of single-stranded RNA. Eubacteria and Archaea typically carry one or two forms of Sm proteins, and assemble one homomorphic ring per Sm protein. Eukaryotes typically carry sixteen or more Sm proteins which assemble to form heteromorphic rings which lie at the center of a number of critical RNA-associated snRNPs. High Sm protein diversity and heteromorphic Sm rings are features stretching back to the origin of eukaryotes; very deep phylogenetic divisions among existing Sm proteins indicate simultaneous evolution across essentially all of existing eukaryotic life. Two basic forms of heteromorphic Sm rings are found in eukaryotes. Fixed Sm rings are highly stable and static, and are assembled around an RNA cofactor. Flexible Sm rings also stabilize and chaperone RNA, but assemble in the absence of an RNA substrate and, more significantly, associate with and dissociate from RNA substrates more freely than fixed rings. This suggests that the conformation of flexible Sm rings might be modified in some specific manner to facilitate association and dissociation with RNA. Diversification of eukaryotic Sm proteins may have been initiated by gene transfers and/or genome clashes that accompanied the origin of the eukaryotic cell itself, with further diversification driven by a greater need for steric specificity within increasingly complex snRNPs.