Capped mRNA Degradation Intermediates Accumulate in the Yeast spb8-2 Mutant

Abstract

mRNA in the yeast Saccharomyces cerevisiae is primarily degraded through a pathway that is stimulated by removal of the mRNA cap structure. Here we report that a mutation in the SPB8 (YJL124c) gene, initially identified as a suppressor mutation of a poly(A)-binding protein (PAB1) gene deletion, stabilizes the mRNA cap structure. Specifically, we find that the spb8-2 mutation results in the accumulation of capped, poly(A)-deficient mRNAs. The presence of this mutation also allows for the detection of mRNA species trimmed from the 3 end. These data show that this Sm-like protein family member is involved in the process of mRNA decapping, and they provide an example of 3-5 mRNA degradation intermediates in yeast.The function of the poly(A) tail on mRNA in eucaryotes is the subject of much research. It has been shown that in the yeast Saccharomyces cerevisiae, the poly(A) tail acts to enhance the translation of the mRNA (reviewed in reference 36). This activity requires the poly(A)-binding protein Pab1p, which, through an interaction with a protein complex recognizing the cap structure, is thought to stimulate the binding of ribosomes to the 5Ј end of the mRNA (23,(39)(40)(41). Another potential function of the poly(A) tail is to stabilize mRNA. Several independent observations support this hypothesis. A series of detailed studies of both yeast and mammalian cells has documented that mRNA deadenylation usually precedes mRNA degradation (reviewed in reference 8). More specifically, it has been shown that in yeast, mRNA decapping, the initiating event in the degradation of the majority of mRNAs, occurs after deadenylation (9,26,27). Following decapping, these mRNAs are destroyed by the 5Ј-3Ј exoribonuclease Xrn1p (20,26). It has also been shown that decreasing the rate of poly(A) tail removal by mutagenizing mRNA (for example, see reference 28) results in lower rates of mRNA degradation.Degradation through the pathway of decapping and then digestion by the 5Ј-3Ј exonuclease Xrn1p is not the sole means by which mRNA is degraded in yeast. Three key observations form the basis for this conclusion. First, targeted disruption of either the decapping enzyme gene DCP1 or the exonuclease gene XRN1 does not lead to cell inviability or greater than four-to fivefold stabilization of mRNAs that are normally unstable (4,20). Second, the disruption of these genes does not change the stability of the most stable yeast mRNAs, such as PGK1 or ACT1, by more than a factor of 2 (4, 27). Finally, for yeast strains containing a disruption of the XRN1 gene or a mutation in the Dcp1 protein, the existence of mRNA species trimmed from the 3Ј end has been reported (27).A common feature of both the mRNA translation and degradation reactions is that the cap structure and the poly(A) tail appear to be involved in their regulation (reviewed in reference 42). The possibility that the roles of these two structures in the degradation reaction are functionally linked was supported by the recent report that mutations in the yeast decapping enzyme Dcp1p...

Identification and characterization of a host protein required for efficient template selection in viral RNA replication

“…Although the Lsm1p protein is related to core snRNP proteins, we found that it localized to the cytoplasm. As discussed below, these results and the ability of 3Ј poly(A) to suppress the LSM1-dependence of 1a-RNA3 interaction suggest that LSM1 may have even broader roles in cellular RNA metabolism than a previously identified contribution to mRNA decapping (26).…”

“…Lsm1p interacts with some other Sm-like proteins but not with snRNAs (24,25). Lsm1p mutation increases the accumulation of capped, poly(A)-deficient mRNAs, indicating a role for Lsm1p in mRNA decapping, which follows mRNA deadenylation and precedes mRNA degradation (26).…”

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“…Seven of these, Lsm2 -Lsm8, have been shown to interact with U6 snRNA and facilitate formation of U4/U6 RNA duplexes (Achsel et al, 1999). Recent reports demonstrated that yeast Lsm1 protein affects mRNA metabolism, particularly mRNA decapping and degradation (Boeck et al, 1998;Tharun et al, 2000) and it is possible that human Lsm1 regulates some specific genes related to tumour invasion or metastasis in prostate cancer cells. Further investigations are needed to clarify the molecular mechanisms of suppression of metastasis related to overexpression of the Lsm1 gene.…”

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“…This was suggested by the finding that Lsm6 and Lsm8 were required for nuclear mRNA decay (Kufel et al, 2004). A specific role for Lsm1-Lsm7 concerns activation of mRNA decay in P-bodies; depletion of individual yeast Lsm proteins results in the accumulation of capped, oligoadenylated mRNA transcripts (Boeck et al, 1998;Bonnerot et al, 2000;Bouveret et al, 2000;Tharun et al, 2000). This specific Lsm complex is recruited alongside other decay factors to U-rich tracts by the protein Pat1, after its displacement of cap-binding translation factors (Parker & Sheth, 2007).…”