Article

Nucleoporin Mediated Nuclear Positioning and Silencing of HMR

Ludwig-Maximilians-Universität München, Germany
PLoS ONE (Impact Factor: 3.53). 07/2011; 6(7):e21923. DOI: 10.1371/journal.pone.0021923
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ABSTRACT The organization of chromatin domains in the nucleus is an important factor in gene regulation. In eukaryotic nuclei, transcriptionally silenced chromatin clusters at the nuclear periphery while transcriptionally poised chromatin resides in the nuclear interior. Recent studies suggest that nuclear pore proteins (NUPs) recruit loci to nuclear pores to aid in insulation of genes from silencing and during gene activation. We investigated the role of NUPs at a native yeast insulator and show that while NUPs localize to the native tDNA insulator adjacent to the silenced HMR domain, loss of pore proteins does not compromise insulation. Surprisingly we find that NUPs contribute to silencing at HMR and are able to restore silencing to a silencing-defective HMR allele when tethered to the locus. We show that the perinuclear positioning of heterochromatin is important for the NUP-mediated silencing effect and find that loss of NUPs result in decreased localization of HMR to the nuclear periphery. We also show that loss of telomeric tethering pathways does not eliminate NUP localization to HMR, suggesting that NUPs may mediate an independent pathway for HMR association with the nuclear periphery. We propose that localization of NUPs to the tDNA insulator at HMR helps maintain the intranuclear position of the silent locus, which in turn contributes to the fidelity of silencing at HMR.

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Available from: Rohinton T Kamakaka, Jul 31, 2015
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    • "The 3D clustering of tDNAs at the nucleolus is also dependent on condensins [70] but the loss of condensin mediated tDNA clustering within the nucleus does not appear to lead to a change in tDNA transcription suggesting that clustering is separable from tDNA transcription. Interestingly, the tRNA gene insulator adjacent to the silenced HMR locus does not localize with the nucleolus or centromere but associates with the nuclear pore [64] [72] though pore localization is also not necessary for insulation . Whether this tDNA recruits condensins is not known but this tDNA insulator does recruit cohesins [66], and mutations in the cohesins affect insulation [9], though the exact mechanism by which cohesins contribute to insulator activity is also unknown. "
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    • "Indeed, it was shown that, despite the fact that nuclear pore proteins can bind the native tDNA insulator adjacent to HMR, the loss of their binding did not compromise insulator activity, even though they contributed to silencing at this locus (Ruben et al. 2011). To reconcile the apparently conflicting role between NPCs and repressive telomeric foci, one might propose that the impact of NPCs on gene expression is not simply pore or position dependent, but depends on the binding of transactivators or repressors to cis-acting elements (Ruben et al. 2011). Consistent with this notion, increasing the association of the HXK1 subtelomeric gene with the nuclear periphery through a neutral anchor improved both its repression of glucose medium and its activation in the absence of glucose (Taddei et al. 2006). "
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