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TAP is a general mRNA export receptor and is highly conserved among eukaryotes. The nematode Caenorhabditis elegans has another TAP‐like protein, NXF‐2, but little is known about its function. In this study, we show that NXF‐2 is specifically expressed in germ cells and forms a novel granular structure that is different from that of P granules and that NXF‐2 granules are anchored to the nuclear periphery in the mitotic region of the hermaphrodite gonad. In contrast, NXF‐2 granules are released within the whole cytoplasm in the meiotic region, where the feminization gene tra‐2 starts to function. Both inhibition of XPO‐1 (an ortholog of the export receptor CRM1) and mutation of the nuclear export signal of NXF‐2 caused the release of NXF‐2 granules from the nuclear periphery, indicating that anchoring of NXF‐2 granules depends on XPO‐1 function. Moreover, inhibition of NXF‐2 resulted in a substantial nuclear accumulation of the reporter mRNA carrying the tra‐2 3′UTR. These results suggest that, together with XPO‐1, NXF‐2 exports and anchors tra‐2 mRNA to the nuclear periphery to avoid precocious translation until the germ cells reach the meiotic region, thereby contributing to the regulation of tra‐2 mRNA expression. [ABSTRACT FROM AUTHOR]

In Caenorhabditis elegans, germline cells remain transcriptionally silenced during embryogenesis. The transcriptional silencing is achieved by two different mechanisms: One is the inhibition of RNA polymerase II in P2‐P4 cells at the establishment stage, and another is chromatin‐based silencing in two primordial germ cells (PGCs) at the maintenance stage; however, the molecular mechanism underlying chromatin‐based silencing is less understood. We investigated the role of the chromodomain protein MRG‐1, which is an essential maternal factor for germline development, in transcriptional silencing in PGCs. PGCs lacking maternal MRG‐1 showed increased levels of two histone modifications (H3K4me2 and H4K16ac), which are epigenetic markers for active transcription, and precocious activation of germline promoters. Loss of MES‐4, a H3K36 methyltransferase, also caused similar derepression of the germline genes in PGCs, suggesting that both MRG‐1 and MES‐4 function in chromatin‐based silencing in PGCs. In addition, the mrg‐1 null mutant showed abnormal chromosome structures and a decrease in homologous recombinase RAD‐51 foci in PGCs, but the mes‐4 null mutant did not show such phenotypes. Taken together, we propose that MRG‐1 has two distinct functions: chromatin‐based transcriptional silencing and preserving genomic integrity at the maintenance stage of PGCs. [ABSTRACT FROM AUTHOR]

HERMES, also called RBPMS, is a conserved RNA binding protein with a single RNA recognition motif ( RRM) that is abundantly expressed in retinal ganglion cells ( RGCs) and in the heart in vertebrates. Here, we identified NonO and PSF as the interacting proteins of HERMES only when the neuronal differentiation of the retinal cell line RGC-5 was induced. Although NonO and PSF are nuclear paraspeckle components, these proteins formed cytoplasmic granules with HERMES in the neurites. G3 BP1, a component of stress granules, was also colocalized to the granules, interacting with NonO and HERMES even in the absence of cellular stress. Consistent with a previous report that KIF5 interacts with neuronal granules, the localization of KIF5A overlapped with the cytoplasmic granules in differentiated RGC-5 cells. Thus, our study strongly suggests that the cytoplasmic granule containing HERMES, NonO, PSF, and G3 BP1 is a neuronal RNA-protein granule that is transported in neurites during retinal differentiation. [ABSTRACT FROM AUTHOR]