Yeast Sm-like proteins function in mRNA decapping and decay

One of the main mechanisms of messenger RNA degradation in eukaryotes occur s by deadenylation-dependent decapping which leads to 5'-to-3' decay(1,2). A family of Sm-like (Lsm) proteins has been identified, members of which co ntain the 'Sm' sequence motif, form a complex with U6 small nuclear RNA and are required for pre-mRNA splicing(3-9). Here we show that mutations in se ven yeast Lsm proteins (Lsm1-Lsm7) also lead to inhibition of mRNA decappin g. In addition, the Lsm1-Lsm7 proteins coimmunoprecipitate with the mRNA de capping enzyme (Dcp1), a decapping activator (Pat1/Mrt1) and with mRNA. Thi s indicates that the Lsm proteins may promote decapping by interactions wit h the mRNA and the decapping machinery. In addition, the Lsm complex that f unctions in mRNA decay appears to be distinct from the U6-associated Lsm co mplex, indicating that Lsm proteins form specific complexes that affect dif ferent aspects of mRNA metabolism.