Ns implicate both Gis2 and CNBP in mRNA metabolism during stress and should facilitate future studies that define the precise molecular mechanisms by which these proteins act. Taken together, our results are consistent with a model in which Gis2 and CNBP function at the level of translation initiation to influence mRNA translation during stress. The finding that Gis2 displays RNA-dependent interactions with Pab1 and both eIF4G isoforms suggests that Gis2 may bind mRNAs that are undergoing circularization to form closed loop mRNPs. In support of an interaction at this step in initiation, the cap-binding protein eIF4E is also present in our Gis2-TAP purification (Table S1), and a recent study identified nearly 1000 Gis2-associated mRNAs [27]. Our discovery that Gis2 and CNBP are 548-04-9 web components of stress granules, which contain mRNPs stalled at a step prior to 60S subunit joining [39,40], both supports the idea that these proteins interact with mRNAs during initiation and suggests that Gis2 and CNBP could contribute to the translational repression of at least some mRNAs during stress. Although not definitive, our finding that polyribosome 4 IBP levels are slightly increased in gis2D dhh1D cells during glucose deprivation (Figure 5F) also supports the hypothesis that Gis2 contributes to translational repression during stress. Several recent studies have also suggested roles for Gis2 and CNBP in mRNA metabolism. In one set of studies, CNBP was isolated on an affinity column containing RNA derived from the internal ribosome entry sequence (IRES) of the ornithine decarboxylase (ODC) mRNA [23]. Because siRNAs against CNBP reduced internal initiation of an IRES-containing reporter in human cells, and overexpression of either CNBP or Gis2 increased translation of the reporter, both CNBP and Gis2 were proposed to function in internal initiation [9,15,23]. In another study, immunoprecipitation of Gis2-associated mRNAs resulted in the identification of hundreds of potential targets [27]. However, comparisons of mRNA levels in wild-type and gis2D cells, and of mRNA and protein levels in GIS2-overexpressing cells, did not reveal simple correlations between Gis2 levels and the fate of these mRNAs [27]. Although our data do not address whether Gis2 functions in internal initiation, our results that Gis2 shows RNAdependent interactions with translation initiation factors and is a component of P-bodies and stress granules are consistent with both the finding that Gis2 associates with mRNAs and the proposal that Gis2 binding may impact mRNA translation and stability [27]. Finally, our finding that human CNBP accumulates in stress granules is notable in light of in vitro studies demonstrating thatCNBP Depletion does not Affect Stress Granule FormationTo determine if CNBP is important for stress granule assembly or integrity, we used siRNAs to reduce CNBP levels. As a positive control, we also depleted heme regulated inhibitor (HRI), which is important for stress granule assembly [58]. Following incubation of the cells with arsenite, the fraction of HRI-depleted cells containing stress granules was strongly reduced, as measured byGis2 and CNBP Are Components of RNP GranulesFigure 7. CNBP accumulates in stress granules during arsenite treatment of HeLa cells. (A) HeLa cells were subjected to immunofluorencence to detect CNBP (green) and the stress granule marker TIAR (red). A merged image is shown. Bar, 20 mm. (B) HeLa cells transfected with plasmids expressing RFP-DCP.Ns implicate both Gis2 and CNBP in mRNA metabolism during stress and should facilitate future studies that define the precise molecular mechanisms by which these proteins act. Taken together, our results are consistent with a model in which Gis2 and CNBP function at the level of translation initiation to influence mRNA translation during stress. The finding that Gis2 displays RNA-dependent interactions with Pab1 and both eIF4G isoforms suggests that Gis2 may bind mRNAs that are undergoing circularization to form closed loop mRNPs. In support of an interaction at this step in initiation, the cap-binding protein eIF4E is also present in our Gis2-TAP purification (Table S1), and a recent study identified nearly 1000 Gis2-associated mRNAs [27]. Our discovery that Gis2 and CNBP are components of stress granules, which contain mRNPs stalled at a step prior to 60S subunit joining [39,40], both supports the idea that these proteins interact with mRNAs during initiation and suggests that Gis2 and CNBP could contribute to the translational repression of at least some mRNAs during stress. Although not definitive, our finding that polyribosome levels are slightly increased in gis2D dhh1D cells during glucose deprivation (Figure 5F) also supports the hypothesis that Gis2 contributes to translational repression during stress. Several recent studies have also suggested roles for Gis2 and CNBP in mRNA metabolism. In one set of studies, CNBP was isolated on an affinity column containing RNA derived from the internal ribosome entry sequence (IRES) of the ornithine decarboxylase (ODC) mRNA [23]. Because siRNAs against CNBP reduced internal initiation of an IRES-containing reporter in human cells, and overexpression of either CNBP or Gis2 increased translation of the reporter, both CNBP and Gis2 were proposed to function in internal initiation [9,15,23]. In another study, immunoprecipitation of Gis2-associated mRNAs resulted in the identification of hundreds of potential targets [27]. However, comparisons of mRNA levels in wild-type and gis2D cells, and of mRNA and protein levels in GIS2-overexpressing cells, did not reveal simple correlations between Gis2 levels and the fate of these mRNAs [27]. Although our data do not address whether Gis2 functions in internal initiation, our results that Gis2 shows RNAdependent interactions with translation initiation factors and is a component of P-bodies and stress granules are consistent with both the finding that Gis2 associates with mRNAs and the proposal that Gis2 binding may impact mRNA translation and stability [27]. Finally, our finding that human CNBP accumulates in stress granules is notable in light of in vitro studies demonstrating thatCNBP Depletion does not Affect Stress Granule FormationTo determine if CNBP is important for stress granule assembly or integrity, we used siRNAs to reduce CNBP levels. As a positive control, we also depleted heme regulated inhibitor (HRI), which is important for stress granule assembly [58]. Following incubation of the cells with arsenite, the fraction of HRI-depleted cells containing stress granules was strongly reduced, as measured byGis2 and CNBP Are Components of RNP GranulesFigure 7. CNBP accumulates in stress granules during arsenite treatment of HeLa cells. (A) HeLa cells were subjected to immunofluorencence to detect CNBP (green) and the stress granule marker TIAR (red). A merged image is shown. Bar, 20 mm. (B) HeLa cells transfected with plasmids expressing RFP-DCP.
