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. 2011 Sep 16;43(6):962-72.
doi: 10.1016/j.molcel.2011.08.008.

The DEAD-box Protein Ded1 Modulates Translation by the Formation and Resolution of an eIF4F-mRNA Complex

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Free PMC article

The DEAD-box Protein Ded1 Modulates Translation by the Formation and Resolution of an eIF4F-mRNA Complex

Angela Hilliker et al. Mol Cell. .
Free PMC article

Abstract

The translation, localization, and degradation of cytoplasmic mRNAs are controlled by the formation and rearrangement of their mRNPs. The conserved Ded1/DDX3 DEAD-box protein functions in an unknown manner to affect both translation initiation and repression. We demonstrate that Ded1 first functions by directly interacting with eIF4G to assemble a Ded1-mRNA-eIF4F complex, which accumulates in stress granules. After ATP hydrolysis by Ded1, the mRNP exits stress granules and completes translation initiation. Thus, Ded1 functions both as a repressor of translation, by assembling an mRNP stalled in translation initiation, and as an ATP-dependent activator of translation, by resolving the stalled mRNP. These results identify Ded1 as a translation initiation factor that assembles and remodels an intermediate complex in translation initiation.

Figures

Figure 1
Figure 1. Genetic approach to identify separation-of-function alleles of DED1
A. The conserved motifs of the DEAD-box protein family are shown in black (Fairman-Williams et al., 2010). Conservation among Ded1 orthologs is shown in dark grey (from yeast to humans) or light grey (among fungi). The numbers mark amino acid domains of Ded1 that are implicated in promoting translation (amino acids labeled above schematic) or in promoting assembly of a Ded1-eIF4F-mRNA complex (amino acids labeled below the schematic). B. Effects of over-expression of ded1 alleles from a galactose inducible promoter in the presence of wild-type, endogenous Ded1. C. Growth of yeast expressing single copies of wild type or mutant ded1 at low temperature. Strains containing a plasmid with wild type or mutant DED1 under the control of its endogenous promoter as the sole copy of DED1 (yRP2799) were spotted at the same concentration on rich media and grown at 16°C for 7 days. D. Cells were treated as in B. See also Table S4 and Figure S2.
Figure 2
Figure 2. Ded1 affects the accumulation of stress granules that contain a subset of translation initiation factors
A. Plasmids containing Ded1-GFP (pRP1556) or Pub1-mCh (pRP1661) were transformed into wild type yeast (yRP2065) and tested for co-localization with or without glucose. B. Strain yRP2065 was transformed with plasmids containing ded1Δ141-150-GFP (pRP2071) and Edc3-mCh (pRP1574). Foci formation was analyzed as in A. White traces show the outline of a cell. The arrows point to the foci in the inset. C. Strain yRP2065 was transformed with plasmids containing Pab1-GFP (pRP1657) and either an empty vector (pRP245) or wild type (pRP2086) or mutant DED1 (i.e. pRP2118) under a galactose-inducible promoter. Foci formation was assessed after 2 or 4 hours of induction in galactose. The same strains, when grown in sucrose where DED1 is not over-expressed, all resemble the empty vector control (data not shown). D. Yeast strains with GFP integrations in the chromosome (see Table S1) were transformed with either an empty vector (pRP1827) or wild type (pRP1559) or mutant DED1 (pRP1564) under a galactose-inducible promoter. Foci formation was assessed after 4 hours in galactose. E. Wild type DED1 (pRP1559) was over-expressed in yeast and immunoprecipitated under native conditions. RNA associated with Ded1 was extracted and analyzed by Northern blot with an oligo dT(36) probe. See also Figure S3.
Figure 3
Figure 3. Ded1 represses translation initiation via the assembly domains and prior to 48S accumulation
A. In vitro translation in yeast extract with capped, polyadenylated luciferase mRNA and increasing amounts of recombinant wild type or mutant Ded1. Translation in the absence of additional rDed1 was normalized to 100. Error bars represent the high and low values of duplicate reactions of a representative experiment. These mutants were tested in parallel at least three times with three independent yeast extract and recombinant protein preparations. B. Yeast extract, capped and polyadenylated radiolabeled MFA2 mRNA, and GMP•PNP were incubated with or without recombinant wild type rDed1 or rded1-E307A before separation by sucrose gradient. Fractions were measured for the presence of radiolabeled mRNA to assess 48S formation. See also Figure S4.
Figure 4
Figure 4. The C terminal assembly domain of Ded1 is critical for interaction with eIF4E/eIF4G
A. Yeast translation extract and rDed1 were incubated with m7G-sepharose and bound proteins identified by immunoblotting with antibodies against Ded1, eIF4E, or eIF4G. B. Interactions between recombinant Ded1 and GST or co-purified GST-eIF4G/eIF4E were tested by immunoprecipitation of eIF4G by glutathione in the presence of RNases. Proteins were detected by coomassie staining (top) and western blot (for Ded1, bottom). The input lanes show 5% of the reaction volume; pull-down lanes show the entire sample. C. As in Figure 4B, except that bound proteins were analyzed by western blot with anti-S antibody (for GST and GST-eIF4G1) or anti-Ded1 antibody. See also Figure S5, S6.
Figure 5
Figure 5. Ded1 interacts directly with eIF4G1 via its third RNA binding motif
A. Schematic of eIF4G1 and its protein binding motifs. B. Purified GST or full-length GST-eIF4G1 (in absence of eIF4E) in E. coli cell lysate was bound to gutathione resin prior to the addition of control buffer or 1 uM Ded1p. RNases were included in each reaction. Samples were resolved by PAGE and stained with coomassie. The input lanes showed 11.7% of the reactions in pull-down lanes. C. 1 uM of GST-eIF4G1 fragment was incubated overnight with 1 uM Ded1p and RNases before addition of glutathione sepharose. 11.1% of total input was shown for each reaction. Recombinant Ded1p was analyzed by western blot (bottom). eIF4G1 fragments were analyzed by coomassie (top); nearby ladder size in kDa is marked. The asterisk marks recombinant Ded1. See also Figure S6.
Figure 6
Figure 6. Ded1 ATPase and assembly domains promote translation
A. Extracts that were mock or Ded1 depleted were used for in vitro translation reactions as in Figure 3A. B. Sequential dilution of mock and Ded1 depleted extracts analyzed by western blot with α-Ded1 antibody. C. yRP2799 containing either wild type DED1 or ded1-tam as the sole copy of DED1 was assayed for translation by 35S incorporation during growth at 30°C, after a 2 hour shift to 16°C, and after recovery for 1 hour at 30°C(R). D. yRP2799 containing either wild type DED1, ded1-dam1, or ded1-tam as the sole copy of DED1 were transformed with plasmid pRP1657, which encodes Edc3-mCh (a P-body marker) and Pab1-GFP (a SG marker). Strains were grown to mid-log phase and then shifted to 16°C for 2 hours. Localization was assessed in the presence of 2% glucose, after 15 minutes of glucose deprivation, and after a subsequent 15-minute recovery in 2% glucose. Each image includes quantitation of P-body intensity, normalized to wild type Ded1 after glucose deprivation, and the average number of P-bodies per cell, shown in parentheses. See also Figure S4.
Figure 7
Figure 7. Working model of Ded1 function in translation
We propose that Ded1 first assembles a complex, minimally containing Ded1, mRNA, and eIF4F, via its assembly domains and through direct interactions with eIF4G. This complex is translationally inactive and can accumulate in SGs. Subsequently, Ded1 functions in a ATP-dependent step to remodel the Ded1-mRNA- eIF4F complex and return the mRNA to translation; this second step correlates with exit from SGs.

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