A major pathway of eukaryotic mRNA turnover occurs by deadenylation-dependent decapping that exposes the transcript to 5'-->3' exonucleolytic degradation. A critical step in this pathway is decapping, since removal of the cap structure permits 5'-->3' exonucleolytic digestion. Based on alterations in mRNA decay rate from strains deficient in translation initiation, it has been proposed that the decapping rate is modulated by a competition between the cytoplasmic cap binding complex, which promotes translation initiation, and the decapping enzyme, Dcp1p. In order to test this model directly, we examined the functional interaction of Dcp1p and the cap binding protein, eukaryotic translation initiation factor 4E (eIF4E), in vitro. These experiments indicated that eIF4E is an inhibitor of Dcp1p in vitro due to its ability to bind the 5' cap structure. In addition, we demonstrate that in vivo a temperature-sensitive allele of eIF4E (cdc33-42) suppressed the decapping defect of a partial loss-of-function allele of DCP1. These results argue that dissociation of eIF4E from the cap structure is required before decapping. Interestingly, the temperature-sensitive allele of eIF4E does not suppress the decapping defect seen in strains lacking the decapping activators, Lsm1p and Pat1p. This indicates that these activators of decapping affect a step in mRNA turnover distinct from the competition between Dcp1 and eIF4E.
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