Figure 2 Model for mRNA decapping. by promoting translation initiation, and deadenylated mRNAs are enhanced for decapping by the Pat1/Lsm1–7 complex binding and promoting translation repression and decapping (Tharun 2009). Control decapping on speciﬁc mRNAs: Differences in mRNA decapping rates appear to arise due to two features of individual mRNAs. First, since decapping is inversely related to translation initiation, features that inherently decrease translation initiation are expected to increase decapping rates. For example, the faster decapping rate of the MFA2 mRNA as compared to the PGK1 mRNA is due to a poor AUG context on the MFA2 mRNA (Lagrandeur and Parker 1999). In addition, mRNAs contain sites for sequence-speciﬁc mRNA-binding proteins that enhance decapping. For exam- ple, the binding of Puf3 to the Cox17 mRNA enhances decapping of this mRNA following deadenylation (Olivas and Parker 2000). An important correlation is that many features that increase the decapping rate—including 39 UTR or coding region elements, AUG context, or 59 stem-loop structures— 680 R. Parker also increase deadenylation rate (Muhlrad and Parker 1992; Caponigro et al. 1993; Muhlrad et al. 1995; Caponigro and Parker 1996; Lagrandeur and Parker 1999; Olivas and Parker 2000). This suggests that a critical aspect of controlling cyto- plasmic mRNA function is an exchange of mRNAs between mRNPs complexed with translation factors and protected from decay and an mRNP associated with decay factors and having reduced translation rate and enhanced deadenylation and decapping. A key issue for future work is determining the speciﬁcs of these mRNPs and how transitions between both states occur. Decapping of speciﬁc mRNAs can also be triggered in- dependently of deadenylation. For example, the Edc1 mRNA is decapped without prior deadenylation, and this is due to a poly(U) tract in the 39 UTR that sequesters the poly(A) tail and renders it nonfunctional (Muhlrad and Parker 2005). Alternatively, in an autoregulatory loop, the Rps28b mRNA assembles a decapping complex through Rps28 binding to a speciﬁc stem loop in its 39 UTR that allows decapping to proceed independently of deadenylation (Badis et al. 2004).