Molecular Biology

Seven Is the Magic Number

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Science  21 Apr 2000:
Vol. 288, Issue 5465, pp. 401
DOI: 10.1126/science.288.5465.401b

The RNA transcript made from a DNA-encoded gene cannot be used immediately as a messenger RNA (mRNA) template for protein synthesis. Instead, non-protein coding portions of this pre-mRNA must be removed by splicing, and the 5′ end of the mRNA is capped to prevent degradation by exonucleases. A group of seven distinct Sm proteins (named for their reactivity with autoantibodies of the Sm serotype) participate in the biogenesis of snRNPs (small nuclear ribonucleoprotein particles), which are complexes of proteins and snRNAs that mediate splicing, and they bind as a heptameric ring to U4 snRNA. A second group of seven proteins (Lsm2-Lsm8, named Lsm for Sm-like) are known to associate with U6 snRNA.

Now, Tharun et al. and Bouveret et al. have found that the almost identical heptamer Lsm1-Lsm7 forms a cytoplasmic complex with several proteins involved in messenger RNA degradation - Pat1, an activator of decapping; Dcp1, decapping enzyme; and Xrn1, an exonuclease. The similarities in stoichiometry and structure amongst these heptameric rings suggest a common mode of binding to RNA substrates and possibly a common mode of function, perhaps in regulating RNA-RNA and RNA-protein interactions in the processing of pre-mRNA and the degradation of mRNA.—GJC

Nature404, 515 (2000); EMBO J.19, 1661 (2000).

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