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Decapping is a critical step in the conserved 5′-to-3′ mRNA decay pathway of eukaryotes. The hetero-octameric Lsm1-7–Pat1 complex is required for normal rates of decapping in this pathway. This complex also protects the mRNA 3′-ends from trimming in vivo. To elucidate the mechanism of decapping, we analyzed multiple  lsm1  mutants,  lsm1-6 ,  lsm1-8 ,  lsm1-9 , and  lsm1-14 , all of which are defective in decapping and 3′-end protection but unaffected in Lsm1-7–Pat1 complex integrity. The RNA binding ability of the mutant complex was found to be almost completely lost in the  lsm1-8  mutant but only partially impaired in the other mutants. Importantly, overproduction of the Lsm1-9p- or Lsm1-14p-containing (but not Lsm1-8p-containing) mutant complexes in wild-type cells led to a dominant inhibition of mRNA decay. Further, the mRNA 3′-end protection defect of  lsm1-9  and  lsm1-14  cells, but not the  lsm1-8  cells, could be partly suppressed by overproduction of the corresponding mutant complexes in those cells. These results suggest the following: (1) Decapping requires both binding of the Lsm1-7–Pat1 complex to the mRNA and facilitation of the post-binding events, while binding per se is sufficient for 3′-end protection. (2) A major block exists at the post-binding steps in the  lsm1-9  and  lsm1-14  mutants and at the binding step in the  lsm1-8  mutant. Consistent with these ideas, the  lsm1-9 ,  14  allele generated by combining the mutations of  lsm1-9  and  lsm1-14  alleles had almost fully lost the RNA binding activity of the complex and behaved like the  lsm1-8  mutant.

Activation of decapping involves binding of the mRNA and facilitation of the post-binding steps by the Lsm1-7–Pat1 complex