Identification of PCBP2, a facilitator of IRES-mediated translation, as a novel constituent of stress granules and processing bodies

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Abstract
Recent advances in microscopic techniques have shed light on the roles of specific subcellular structures in the regulation of gene expression. One such structure is the stress granule (SG), which is engaged in stress-triggered translational arrest by sequestering pre-initiation complexes of translation. Recent studies revealed the spatial, compositional, and functional linkage of the SG to the processing body (P-body), another cytoplasmic structure that has been implicated in mRNA degradation and siRNA- or miRNA-mediated gene silencing. In this study, we report that PCBP2, a facilitator of IRES (Internal Ribosomal Entry Site)-mediated translation, is a novel constituent of the SG and P-body. Immunofluorescence studies revealed that while PCBP2 is diffusely distributed throughout the nucleoplasm and the cytoplasm, the protein is enriched in a subset of P-bodies under normal conditions. Upon exposure to heat and arsenic stress, PCBP2 became predominantly accumulated at the SG, but was still present in Dcp1a-positive P-bodies. Live-cell imaging revealed the dynamic association of PCBP2-enriched P-bodies and the SG, and FRAP experiments demonstrated that PCBP2 actively moves in and out of the SG and P-body. Taken together, these results suggest that PCBP2 shuttles between the cytoplasm and the two structures under stress. We propose that PCBP2 may be involved in stress-induced remodeling of mRNP complexes and that it may also play a role in the rapid transition of certain silenced mRNAs into a translationally active state. Additionally, given the property of PCBP2 as a nuclear–cytoplasmic shuttling protein, PCBP2 may play a role in directly targeting nascent mRNPs to specific P-bodies for storage.