Mutations in VPS16 and MRT1 stabilize mRNAs by activating an inhibitor of the decapping enzyme

Decapping is a rate-limiting step in the decay of many yeast mRNAs; the act ivity of the decapping enzyme therefore plays a significant role in determi ning RNA stability. Using an in vitro decapping assay, we have identified a factor, Vps16p, that regulates the activity of the yeast decapping enzyme, Dcp1p. Mutations in the VPS16 gene result in a reduction of decapping acti vity in vitro and in the stabilization of both wild-type and nonsense-codon -containing mRNAs in vivo. The mrt1-3 allele, previously shown to affect th e turnover of wild-type mRNAs, results in a similar in vitro phenotype. Ext racts from both vps16 and mrt1 mutant strains inhibit the activity of purif ied Flag-Dcp1p. We have identified a 70-kDa protein which copurifies with F lag-Dcp1p as the abundant Hsp70 family member Ssa1p/2p. Intriguingly, the i nteraction with Ssa1p/2p is enhanced in strains with mutations in vps16 or mn-tl. We propose that Hsp70s may be involved in the regulation of mRNA dec apping.