The DEAD box helicase, Dhh1p, functions in mRNA decapping and interacts with both the decapping and deadenylase complexes

A major pathway of mRNA turnover in eukaryotic cells initiates with deadenylation, leading to mRNA decapping and subsequent 5' to 3' exonuclease digestion. We show that a highly conserved member of the DEAD box family of helicases, Dhh1p, stimulates mRNA decapping in yeast. In dhh1 Delta mutants, mRNAs accumulate as deadenylated, capped species. Dhhlp's effects on decapping only occur on normal messages as nonsense-mediated decay still occurs in dhh1 Delta mutants. The role of Dhh1p in decapping appears to be direct, as Dhh1p physically interacts with several proteins involved in mRNA decapping including the decapping enzyme Dcp1p, as well as Lsm1p and Pat1p/ Mrt1p, which function to enhance the decapping rate. Additional observations suggest Dhh1p functions to coordinate distinct steps in mRNA function and decay. Dhh1p also associates with Pop2p, a subunit of the mRNA deadenylase. In addition, genetic phenotypes suggest that Dhh1p also has a second biological function. Interestingly, Dhh1p homologs in others species function in maternal mRNA storage. This provides a novel link between the mechanisms of decapping and maternal mRNA translational repression.