Molecular Biology

At the Core of Splicing

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Science  18 May 2001:
Vol. 292, Issue 5520, pp. 1265
DOI: 10.1126/science.292.5520.1265c

In eukaryotes, the family of Sm proteins is known to participate in RNA processing events, such as splicing of pre-messenger RNAs. It is thought that seven distinct members of the family assemble into a doughnutlike shape and that this provides the binding surface for the uridine-rich region shared among the spliceosomal U RNAs. Related proteins have been identified in Archaea, and now two groups describe the crystal structures of these homoheptameric rings.

Törö et al. provide the structure of one of the two Sm proteins from Archaeoglobus fulgidus; the diameter of the doughnut is 65 angstroms, and the thickness is 30 angstroms. They also have been able to resolve three connected uridines in a complex of AF-Sm1 with a U5 oligoribonucleotide. The uridines are sandwiched between residues previously suggested to comprise the RNA binding site, and it appears that other bases would not fit nearly as snugly.

Mura et al. present the structure of the Sm representative from Pyrobaculum aerophilum. Their doughnut is of similar size, 65 by 38 angstroms, with a similar surface asymmetry of charge. On the other hand, their model of the complex with single-stranded RNA resembles a ring-on-a-string, in which the preference for the pyrimidine U over purine bases is explained by the narrowness of the central hole. — GJC

EMBO J.20, 2293 (2001); Proc. Natl. Acad. Sci. U.S.A. 98, 5532 (2001).

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