A Sm-like protein complex that participates in mRNA degradation

In eukaryotes, seven Sm proteins bind to the U1, U2, U4 and U5 spliceosomal snRNAs while seven Sm-like proteins (Lsm2p-Lsm8p) are associated with U6 s nRNA. Another yeast Sm-like protein, Lsm1p, does not interact with U6 snRNA , Surprisingly, using the tandem affinity purification (TAP) method, we ide ntified Lsm1p among the subunits associated with Lsm3p, Coprecipitation exp eriments demonstrated that Lsm1p, together with Lsm2p-Lsm7p, forms a new se ven-subunit complex. We purified the two related Sm-like protein complexes and identified the proteins recovered in the purified preparations by mass spectrometry, This confirmed the association of the Lsm2p-Lsm8p complex wit h U6 snRNA, In contrast, the Lsm1p-Lsm7p complex is associated with Pat1p a nd Xrn1p exoribonuclease, suggesting a role in mRNA degradation. Deletions of LSM1, 6, 7 and PAT1 genes increased the half-life of reporter mRNAs, Int erestingly, accumulating mRNAs were capped, suggesting a block in mRNA deca y at the decapping step. These results indicate the involvement of a new co nserved Sm-like protein complex and a new factor, Pat1p, in mRNA degradatio n and suggest a physical connection between decapping and exonuclease trimm ing.