Detected (Figure 6A). Western blotting to determine if the major eIF4G isoform, eIF4G1, associated with CNBP gave inconclusive results due to nonspecific background signals (data not shown). To determine if CNBP associates with translating ribosomes, we harvested HeLa cells in the presence of cycloheximide and subjected the resulting lysates to sucrose gradient sedimentation (Figure 6B). As observed for Gis2-GFP, most CNBP sedimented in the lightest Title Loaded From File fractions (fractions 1?, 74.6 ). Additionally, someCNBP sedimented in fractions containing ribosomal subunits and monosomes (fractions 4?, 23.7 ) and a small amount was detected in polysome-containing fractions (fractions 10?0, 1.6 ). Because omitting cycloheximide did not significantly alter the polyribosome profile as measured by UV absorbance (data not shown), we incubated the cells with puromycin, which causes premature termination of translation, prior to harvesting in cycloheximide. Puromycin was effective at reducing translation, as measured by decreased polysomes and increased 80S subunits (Figure 6C). Notably, following puromycin treatment, the fraction of CNBP in the lightest gradient fractions increased to 84.1 , while the amount of CNBP that sedimented with ribosomal subunits and 80S monosomes decreased (14.8 ), as did the fraction that sedimented with polyribosomes (0.6 ). We conclude that a small fraction of CNBP associates with translating ribosomes.CNBP Accumulates in Stress GranulesSince our experiments revealed that Gis2 was a component of P-bodies and stress granules, we determined if this localization was conserved for CNBP. In contrast to yeast, mammalian stress granules and P-bodies exhibit far less overlap in their protein components [39,40]. Using anti-CNBP antibodies in immunoflu-Figure 6. Some CNBP associates with PABPC1 and Eas green bars represent genes whose transcripts were detected at ,103 copies sediments with translating ribosomes. (A) HeLa cell lysates were subjected to immunoprecipitation with anti-CNBP antibodies. Proteins in immunoprecipitates were subjected to Western blotting to detect the poly(A) binding protein PABPC1 and eIF4G2. To assess the efficiency of immunoprecipitation, the level of CNBP in the immunoprecipitate was also determined. As a negative control, the blot was reprobed to detect GAPDH. (B and C) HeLa cells were either untreated (B) or incubated with puromycin for 20 minutes (C) prior to harvesting in cycloheximide. Lysates were sedimented in 15?0 sucrose gradients and fractions collected while monitoring OD254. Proteins were subjected to Western blotting to detect CNBP, PABP1C and ribosomal protein RPS6. doi:10.1371/journal.pone.0052824.gGis2 and CNBP Are Components of RNP Granulesorescence experiments, we found that CNBP was mostly cytoplasmic in HeLa cells (Figure 7A). In these unstressed cells, immunofluorescence with an antibody to the stress granule marker TIAR revealed that this protein was concentrated in nuclei (Figure 7A), as described [54]. To both have P-body markers and to induce formation of small P-bodies, we transfected the HeLa cells with plasmids in which RFP was fused to either the Dcp1 ortholog DCP1a (RFP-DCP1a) [55] or the Dhh1 ortholog RCK (RFP-RCK). Although transfection of either plasmid resulted in Pbody formation as described [55,56], CNBP was not detected in these foci (Figure 7B). To induce stress granules and increase P-body formation, we incubated the cells with arsenite, a strong inducer of oxidative stress. As expected [54,55], both P-bodies and stress granules became prominent (.Detected (Figure 6A). Western blotting to determine if the major eIF4G isoform, eIF4G1, associated with CNBP gave inconclusive results due to nonspecific background signals (data not shown). To determine if CNBP associates with translating ribosomes, we harvested HeLa cells in the presence of cycloheximide and subjected the resulting lysates to sucrose gradient sedimentation (Figure 6B). As observed for Gis2-GFP, most CNBP sedimented in the lightest fractions (fractions 1?, 74.6 ). Additionally, someCNBP sedimented in fractions containing ribosomal subunits and monosomes (fractions 4?, 23.7 ) and a small amount was detected in polysome-containing fractions (fractions 10?0, 1.6 ). Because omitting cycloheximide did not significantly alter the polyribosome profile as measured by UV absorbance (data not shown), we incubated the cells with puromycin, which causes premature termination of translation, prior to harvesting in cycloheximide. Puromycin was effective at reducing translation, as measured by decreased polysomes and increased 80S subunits (Figure 6C). Notably, following puromycin treatment, the fraction of CNBP in the lightest gradient fractions increased to 84.1 , while the amount of CNBP that sedimented with ribosomal subunits and 80S monosomes decreased (14.8 ), as did the fraction that sedimented with polyribosomes (0.6 ). We conclude that a small fraction of CNBP associates with translating ribosomes.CNBP Accumulates in Stress GranulesSince our experiments revealed that Gis2 was a component of P-bodies and stress granules, we determined if this localization was conserved for CNBP. In contrast to yeast, mammalian stress granules and P-bodies exhibit far less overlap in their protein components [39,40]. Using anti-CNBP antibodies in immunoflu-Figure 6. Some CNBP associates with PABPC1 and sediments with translating ribosomes. (A) HeLa cell lysates were subjected to immunoprecipitation with anti-CNBP antibodies. Proteins in immunoprecipitates were subjected to Western blotting to detect the poly(A) binding protein PABPC1 and eIF4G2. To assess the efficiency of immunoprecipitation, the level of CNBP in the immunoprecipitate was also determined. As a negative control, the blot was reprobed to detect GAPDH. (B and C) HeLa cells were either untreated (B) or incubated with puromycin for 20 minutes (C) prior to harvesting in cycloheximide. Lysates were sedimented in 15?0 sucrose gradients and fractions collected while monitoring OD254. Proteins were subjected to Western blotting to detect CNBP, PABP1C and ribosomal protein RPS6. doi:10.1371/journal.pone.0052824.gGis2 and CNBP Are Components of RNP Granulesorescence experiments, we found that CNBP was mostly cytoplasmic in HeLa cells (Figure 7A). In these unstressed cells, immunofluorescence with an antibody to the stress granule marker TIAR revealed that this protein was concentrated in nuclei (Figure 7A), as described [54]. To both have P-body markers and to induce formation of small P-bodies, we transfected the HeLa cells with plasmids in which RFP was fused to either the Dcp1 ortholog DCP1a (RFP-DCP1a) [55] or the Dhh1 ortholog RCK (RFP-RCK). Although transfection of either plasmid resulted in Pbody formation as described [55,56], CNBP was not detected in these foci (Figure 7B). To induce stress granules and increase P-body formation, we incubated the cells with arsenite, a strong inducer of oxidative stress. As expected [54,55], both P-bodies and stress granules became prominent (.
