such as Pat1 and Scd6, that can directly interact with trans- lation factors and the decapping enzyme (Nissan et al. 2010; Rajyaguru et al. 2012). An important area for future re- search is determining how mRNPs are remodeled to allow decapping complexes to form and degrade the mRNA. A second role of decapping activators is to promote the assembly of a larger decapping complex. The core set of decapping components shows an extensive network of direct interactions as determined by protein-binding experiments with recombinant proteins and supported by co-ip and two- hybrid analyses (Decker et al. 2007; Nissan et al. 2010). On the basis of coimmunoprecipitation (co-ip) experiments and the dependence of interactions on RNA, there appear to be two complexes that assemble on mRNAs targeted for decapping. One complex consists of the Pat1 protein, the Lsm1–7 complex, and Xrn1 (Bouveret et al. 2000; Tharun et al. 2000; Tharun and Parker 2001). This complex is thought to assemble on the 39 end of deadenylated mRNAs on the basis of its binding speciﬁcity in vitro (Chowdhury et al. 2007) and the exonuclease trimming of the 39 end of deadenylated mRNAs in pat1Δ or lsm1Δ strains (Boeck et al. 1998; Tharun et al. 2000; He and Parker 2001). A second set of interacting proteins consists of the Dcp1, Dcp2, Edc3, or Scd6 and Dhh1, although whether all these factors can as- sociate at the same time remains to be determined. Within and between these complexes, Pat1 and Edc3 appear to play important scaffolding roles and interact with many com- ponents of the decapping machinery (Decker et al. 2007; Nissan et al. 2010). A third role of decapping activators is to directly stimulate decapping by Dcp2. For example, the Edc3 and Pat1 proteins directly bind Dcp2 and enhance its activity in puriﬁed systems (Harigaya et al. 2010; Nissan et al. 2010). Similarly, the para- logs Edc1 and Edc2, which are high-copy suppressors of tem- perature-sensitive alleles in Dcp1 or Dcp2 (Dunckley et al. 2001), bind RNA and stimulate Dcp2 either in extracts or in reconstituted systems (Schwartz et al. 2003; Steiger et al. 2003). Edc1, and presumably Edc2 as well, directly bind Dcp1 to stimulate the decapping enzyme by enhancing both Km and kcat of Dcp2 (Borja et al. 2011). Several observations suggest that Dcp2 catalysis is not generally rate limiting for decapping in vivo. First, conditional mutations in Dcp1 or Dcp2 that compromise decapping ac- tivity in vitro do not signiﬁcantly affect mRNA decapping rates in vivo (Tharun and Parker 1999; Dunckley et al. 2001; Steiger et al. 2003). Second, strains lacking Edc1, Edc2, and/or Edc3 do not show defects in mRNA decay rates in vivo unless the decapping ability of Dcp2 is reduced by mutation (Dunckley et al. 2001; Kshirsagar and Parker 2004). Third, deletion of the C-terminal domain of Pat1, which stimulated Dcp2 in vitro, has only a marginal effect on mRNA decapping rates in vivo (Pilkington and Parker 2008; Nissan et al. 2010). Taken together, these observations suggest that, at least in mid-log growth, the rate-limiting step in decapping is the translation repression and assembly of a decapping complex on the mRNP. Several observations suggest that Pat1 and the Lsm1–7 complex function in an mRNP rearrangement that enhances decapping activity. First, lsm1Δ strains accumulate mRNA and the decapping machinery in P-bodies, arguing that the Lsm1–7 complex functions at a late stage in decapping after the mRNA has exited translation (Sheth and Parker 2003; Teixeira and Parker 2007). Second, the middle and carboxy- terminal domains of Pat1 interact with themselves, and, while the C-terminal domain is sufﬁcient to bind Dcp2 and activate decapping in vitro, its interaction with Dcp2 is blocked by the middle domain of Pat1 (Nissan et al. 2010). This raises the possibility that, for Pat1 to interact with Dcp2 and activate decapping catalysis, a conformational change in Pat1 between the middle and C-terminal domains is required. Strikingly, the Lsm1–7 complex appears to interact with both the middle and the C-terminal domain of Pat1 (Pilkington and Parker 2008; Nissan et al. 2010). Thus, a working model is that the binding of Pat1 to the mRNA with the Lsm1–7 complex allows for the formation of a binding site for Dcp2 in Pat1’s C-terminal domain that is sufﬁcient to activate catalysis. Several other factors have been identiﬁed as promoting decapping (Zuk et al. 1999; Table 2). Most notably, muta- tions in one of the genes encoding eIF5a lead to slower decapping of mRNAs, perhaps because of alteration to trans- lation initiation and/or translation elongation (Zuk and Jacobson 1998; Saini et al. 2009). The Stm1 and Sbp1 pro- teins interact with the ribosome and eIF4G, respectively, and thereby appear to promote the ability of Dhh1 to promote decapping (Segal et al. 2006; Balagopal and Parker 2009, 2011; Rajyaguru et al. 2012), although the mechanism by which these proteins stimulate Dhh1 function is unknown. Control of decapping: Poly(A) tails as inhibitors of decapp- ing: Several observations argue that the ability of the poly (A) tail to inhibit decapping is partially mediated through the poly(A)-binding protein (Pab1). First, it has been dem- onstrated in yeast that decapping occurs when the poly(A) tail length has been shortened to an oligo(A) length of 12 residues (Decker and Parker 1993). This is approximately the minimum length required for Pab1 binding (Sachs et al. 1987). Second, in pab1 mutant strains, decapping is un- coupled from deadenylation (Caponigro and Parker 1995; Morrissey et al. 1999). In this case, intermediates in mRNA decay, trapped by inhibiting 59 to 39 degradation in cis with strong secondary structures, are produced as decapped mRNA fragments with long poly(A) species (Caponigro and Parker 1995). This indicates that, in the absence of Pab1, the absolute requirement for prior deadenylation be- fore decapping is not necessary. A second reason for decapping occurring after dead- enylation is that the Pat1/Lsm1–7 complex prefers to bind oligoadenylated mRNAs in vitro (Chowdhury et al. 2007) and associates with mRNAs after deadenylation in vivo (Tharun and Parker 2001). Thus, a working model is that polyadenylated mRNAs are protected by Pab1 binding and RNA Decay in Yeast 679