the presence of structures near the 59 end is unlikely to inhibit decapping in vivo since even mRNAs with poly(G) tracts very near their 59 end undergo rapid decapping in vivo (Muhlrad et al. 1994, 1995), presumably because Dcp2 catalysis is not generally rate limiting for decapping in vivo (see below). Yeast cells contain additional decapping enzymes. The Dcs1 and Dcs2 proteins are members of the HIT family of pyrophosphatases and in vitro appear to cleave short RNA substrates (Liu et al. 2002). One function for Dcs1 in yeast is to cleave the m7GDP produced by decapping to m7GMP (Van Dijk et al. 2003), although how the m7GMP is further recycled is not known. Dcs2 can form heterodimers with Dcs1 and inhibit its activity, which occurs as cells enter dia- uxie (Malys and McCarthy 2006), although the signiﬁcance of this effect is not clear. The nuclear Rai1 protein is known to function as a endonuclease that can cleave near the 59 ends of mRNAs, and this has been suggested to function as a quality control mechanism for mRNA capping (Jiao et al. 2010). Following decapping, mRNAs are degraded in a 59 to 39 direction by the Xrn1 nuclease (Hsu and Stevens 1993; Muhlrad et al. 1994), which prefers mRNA substrates with a 59 monophosphate (Stevens 2001). Xrn1 has two highly conserved domains that fold into the active region of the enzyme, which is then stabilized by interactions with addi- tional domains (Chang et al. 2011; Jinek et al. 2011). The active site of Xrn1 couples unwinding of duplexes to the processivity of the enzyme, which explains how it can de- grade through structures without a helicase (Jinek et al. 2011). A paralog of Xrn1 is Rat1, which is typically localized to the nucleus and functions in nuclear RNA processing and/ or degradation pathways (see below). However, Rat1 can substitute for Xrn1 when it is localized to the cytoplasm due to mutation, indicating that no Xrn1-speciﬁc protein– protein interactions are required for mRNA degradation (Johnson 1997). Xrn1 is inhibited by the adenosine 39, 59 biphosphate (pAp), which is produced by sulfate assimila- tion (Dichtl et al. 1997), and cells can utilize this circuit to limit Xrn1 activity during various responses (Benard 2004; Todeschini et al. 2006). Model of mRNA decapping: A working model for mRNA decapping has three critical steps (Figure 2). First, the 59 cap structure must be exposed, and therefore the cytoplasmic cap-binding complex consisting of eIF4E and eIF4G needs to be lost from the mRNA. Second, the decapping enzyme must be recruited to the mRNA, which appears to be coor- dinated with the formation of a larger decapping complex, including the decapping enzyme, Xrn1, and several decapp- ing activators (see below). Third, catalysis by Dcp2 occurs, leading to rapid 59 to 39 degradation of the mRNA. Consis- tent with this model, proteins enhancing decapping can function by interfering with translation initiation factors, by binding RNA and forming scaffolds for assembly of the decapping machinery, or by promoting Dcp2 catalysis. Untranslating mRNPs complexed with the decapping ma- chinery can also aggregate into cytoplasmic RNP granules referred to as P-bodies although the speciﬁc role of these macromolecular complexes is not yet clear (see below). Decapping and translation initiation: Several observations argue that decapping is in competition with translation initiation and that decapping requires the loss of the cap- binding complex. This was ﬁrst suggested since the cap structure, which is recognized for decapping, also functions in promoting translation initiation by recruiting the eIF4E/ eIF4G translation initiation complex. Moreover, when trans- lation initiation is decreased by mutations in translation ini- tiation factors, a poor AUG context, or 59 UTR structures, there is a concomitant increase in decapping rates (Muhlrad et al. 1995; Lagrandeur and Parker 1999; Schwartz and Parker 1999). In addition, the eIF4E cap-binding protein can directly inhibit decapping in vitro (Schwartz and Parker 2000). These observations argue that decapping requires the mRNP to exchange the eIF4E/eIF4G cap-binding com- plex for the decapping enzyme. Stimulation of decapping by general activators: Several protein factors, referred to as either decapping enhancers or activators, are known to function to stimulate the rate of decapping in vivo (Table 2). The core set of proteins affect- ing decapping includes Dhh1, a DEAD-box helicase, Pat1, Edc1, Edc2, Edc3, Scd6, and the Lsm1–7 complex. Some of these decapping activators promote decapping by inhibit- ing translation initiation. For example, Dhh1, a member of the DEAD family of ATPases, represses translation in vitro, and its overexpression in cells inhibits translation and leads to the accumulation of cytoplasmic mRNP granules (Coller and Parker 2005; Swisher and Parker 2010; Carroll et al. 2011). Similarly, Pat1, Scd6, and Stm1, which affect the decapping of some mRNAs (Balagopal and Parker 2009), repress translation both in vivo and in vitro (Pilkington and Parker 2008; Nissan et al. 2010; Balagopal and Parker 2011; Rajyaguru et al. 2012). Decapping activators can inhibit translation at different steps. For example, the Pat1, Dhh1, and Scd6 proteins all appear to block translation before the formation of a 48S pre-initiation complex (Coller and Parker 2005; Nissan et al. 2010). For Scd6, this translation repression appears to occur by direct binding to eIF4G and inhibition of the joining of the 43S complex (Rajyaguru et al. 2012). In contrast, the Stm1 protein, which promotes decapping of a subset of yeast mRNAs (Balagopal and Parker 2009), inhibits translation after formation of an 80S complex, likely through direct interactions with the ribosome (Balagopal and Parker 2011). An unresolved issue is how inhibition of translation initiation by these factors leads to decapping. One possibility is that, by stalling initiation, it gives more time for dissoci- ation of the translation initiation factors to allow for decapp- ing complexes to associate with the mRNA. Alternatively, such a transition may involve an ordered exchange of factors on the mRNA, which is suggested by decapping activators, RNA Decay in Yeast 677