Recent work has demonstrated that some actively transcribed genes closely associate with nuclear pore complexes (NPC) at the nuclear periphery. The Saccharomyces cerevisiae Mlp1 and Mlp2 proteins are components of the inner nuclear basket of the nuclear pore that mediate interactions with these active genes. To investigate the physical link between the NPC and active loci, we identified proteins that interact with the carboxyl-terminal globular domain of Mlp1 by tandem affinity purification coupled with mass spectrometry. This analysis led to the identification of several components of the Spt-Ada-Gcn5-acetyltransferase (SAGA) histone acetyltransferase complex, Gcn5, Ada2, and Spt7. We utilized co-immunoprecipitation and in vitro binding assays to confirm the interaction between the Mlp proteins and SAGA components. Chromatin immunoprecipitation experiments revealed that Mlp1 and SAGA components associate with the same region of the GAL promoters. Critically, this Mlp-promoter interaction depends on the integrity of the SAGA complex. These results identify a physical association between SAGA and the NPC, and support previous results that relied upon visualization of GAL loci at the nuclear periphery by microscopy (Cabal, G. G. Genovesio, A., Rodriguez-Navarro, S., Zimmer, C., Gadal, O., Lesne, A., Buc, H., Feuerbach-Fournier, F., Olivo-Marin, J.-C., Hurt, E. C., and Nehrbass, U. (2006) Nature 441, 770 -773). We propose that a physical interaction between nuclear pore components and the SAGA complex can link the actively transcribed GAL genes to the nuclear pore.

The Caenorhabditis elegans LSD1 H3K4me2 demethylase SPR-5 reprograms epigenetic transcriptional memory during passage through the germ line. Here we show that mutants in the H3K9me2 methyltransferase, met-2, result in transgenerational epigenetic effects that parallel spr-5 mutants. In addition, we find that spr-5;met-2 double mutants have a synergistic effect on sterility, H3K4me2, and spermatogenesis expression. These results implicate MET-2 as a second histone-modifying enzyme in germ-line reprogramming and suggest a model in which SPR-5 and MET-2 function cooperatively to reestablish an epigenetic ground state required for the continued immortality of the C. elegans germ line. Without SPR-5 and MET-2, we find that the ability to express spermatogenesis genes is transgenerationally passed on to the somatic cells of the subsequent generation. This indicates that H3K4me2 may act in the maintenance of cell fate. Finally, we demonstrate that reducing H3K4me2 causes a large increase in H3K9me2 added by the SPR-5;MET-2 reprogramming mechanism. This finding suggests a novel histone code interaction in which the input chromatin environment dictates the output chromatin state. Taken together, our results provide evidence for a broader reprogramming mechanism in which multiple enzymes coordinately regulate histone information during passage through the germ line. Since the discovery of histone modifications and the initial proposal of the histone code (1), a number of experiments have implicated histone modifications in the maintenance of transcriptional memory (2-8). Despite this implicit association, an understanding of how histone modifications are regulated developmentally, as well as the exact relationship between certain histone modifications, remains elusive. For example, at fertilization, the highly differentiated transcriptional program of the germ line may be reprogrammed by maternal factors in the oocyte to reestablish the epigenetic ground state of the zygote. This reprogramming capacity of the oocyte is implied by the ability of oocyte factors to reprogram a differentiated cell during somatic cell nuclear transfer (SCNT) (9, 10). However, the low efficiency observed in SCNT and in the similar induction of pluripotent stem cells highlights the complexity involved in reprogramming differentiated fates back to a ground pluripotent state (11,12). Moreover, embryos derived from SCNT have been shown to continue to express genes from their previous differentiated state (13). These results indicate that germ-line reprogramming requires the resetting of an epigenetic transcriptional memory, but the mechanism of this reprogramming is not well understood.One emerging player in transcriptional memory is dimethylation of histone 3 at lysine 4 (H3K4me2). During transcription, H3K4 methyltransferases, such as mixed-lineage leukemia (MLL), associate with a core COMPASS complex consisting of WDR5, RbBP5, and ASH2L (14). This complex promotes H3K4 methyltransferase activity and may enable the methyltransferase to interact w...

BackgroundThe Ccr4-Not complex is a key eukaryotic regulator of gene transcription and cytoplasmic mRNA degradation. Whether this complex also affects aspects of post-transcriptional gene regulation, such as mRNA export, remains largely unexplored. Human Caf1 (hCaf1), a Ccr4-Not complex member, interacts with and regulates the arginine methyltransferase PRMT1, whose targets include RNA binding proteins involved in mRNA export. However, the functional significance of this regulation is poorly understood.Methodology/Principal FindingsHere we demonstrate using co-immunoprecipitation approaches that Ccr4-Not subunits interact with Hmt1, the budding yeast ortholog of PRMT1. Furthermore, using genetic and biochemical approaches, we demonstrate that Ccr4-Not physically and functionally interacts with the heterogenous nuclear ribonucleoproteins (hnRNPs) Nab2 and Hrp1, and that the physical association depends on Hmt1 methyltransferase activity. Using mass spectrometry, co-immunoprecipitation and genetic approaches, we also uncover physical and functional interactions between Ccr4-Not subunits and components of the nuclear pore complex (NPC) and we provide evidence that these interactions impact mRNA export.Conclusions/SignificanceTaken together, our findings suggest that Ccr4-Not has previously unrealized functional connections to the mRNA processing/export pathway that are likely important for its role in gene expression. These results shed further insight into the biological functions of Ccr4-Not and suggest that this complex is involved in all aspects of mRNA biogenesis, from the regulation of transcription to mRNA export and turnover.

In this issue of Molecular Cell, Brickner and colleagues (Light et al., 2010) identify a DNA sequence that mediates transcriptional memory and retention of recently active INO1 at the nuclear pore complex.

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