Nup98 recruits the Wdr82-Set1A/COMPASS complex to promoters to regulate H3K4 trimethylation in hematopoietic progenitor cells.

Recent studies have shown that a subset of nucleoporins (Nups) can detach from the nuclear pore complex and move into the nuclear interior to regulate transcription. One such dynamic Nup, called Nup98, has been implicated in gene activation in healthy cells and has been shown to drive leukemogenesis when mutated in patients with acute myeloid leukemia (AML). Here we show that in hematopoietic cells, Nup98 binds predominantly to transcription start sites to recruit the Wdr82-Set1A/COMPASS (complex of proteins associated with Set1) complex, which is required for deposition of the histone 3 Lys4 trimethyl (H3K4me3)-activating mark. Depletion of Nup98 or Wdr82 abolishes Set1A recruitment to chromatin and subsequently ablates H3K4me3 at adjacent promoters. Furthermore, expression of a Nup98 fusion protein implicated in aggressive AML causes mislocalization of H3K4me3 at abnormal regions and up-regulation of associated genes. Our findings establish a function of Nup98 in hematopoietic gene activation and provide mechanistic insight into which Nup98 leukemic fusion proteins promote AML.

Hyperphosphorylation amplifies UPF1 activity to resolve stalls in nonsense-mediated mRNA decay.

Many gene expression factors contain repetitive phosphorylation sites for single kinases, but the functional significance is poorly understood. Here we present evidence for hyperphosphorylation as a mechanism allowing UPF1, the central factor in nonsense-mediated decay (NMD), to increasingly attract downstream machinery with time of residence on target mRNAs. Indeed, slowing NMD by inhibiting late-acting factors triggers UPF1 hyperphosphorylation, which in turn enhances affinity for factors linking UPF1 to decay machinery. Mutational analyses reveal multiple phosphorylation sites contributing to different extents to UPF1 activity with no single site being essential. Moreover, the ability of UPF1 to undergo hyperphosphorylation becomes increasingly important for NMD when downstream factors are depleted. This hyperphosphorylation-dependent feedback mechanism may serve as a molecular clock ensuring timely degradation of target mRNAs while preventing degradation of non-targets, which, given the prevalence of repetitive phosphorylation among central gene regulatory factors, may represent an important general principle in gene expression.

Evolution of a transcriptional regulator from a transmembrane nucleoporin.

Nuclear pore complexes (NPCs) emerged as nuclear transport channels in eukaryotic cells ∼1.5 billion years ago. While the primary role of NPCs is to regulate nucleo-cytoplasmic transport, recent research suggests that certain NPC proteins have additionally acquired the role of affecting gene expression at the nuclear periphery and in the nucleoplasm in metazoans. Here we identify a widely expressed variant of the transmembrane nucleoporin (Nup) Pom121 (named sPom121, for "soluble Pom121") that arose by genomic rearrangement before the divergence of hominoids. sPom121 lacks the nuclear membrane-anchoring domain and thus does not localize to the NPC. Instead, sPom121 colocalizes and interacts with nucleoplasmic Nup98, a previously identified transcriptional regulator, at gene promoters to control transcription of its target genes in human cells. Interestingly, sPom121 transcripts appear independently in several mammalian species, suggesting convergent innovation of Nup-mediated transcription regulation during mammalian evolution. Our findings implicate alternate transcription initiation as a mechanism to increase the functional diversity of NPC components.

Dynamic association of NUP98 with the human genome.

Faithful execution of developmental gene expression programs occurs at multiple levels and involves many different components such as transcription factors, histone-modification enzymes, and mRNA processing proteins. Recent evidence suggests that nucleoporins, well known components that control nucleo-cytoplasmic trafficking, have wide-ranging functions in developmental gene regulation that potentially extend beyond their role in nuclear transport. Whether the unexpected role of nuclear pore proteins in transcription regulation, which initially has been described in fungi and flies, also applies to human cells is unknown. Here we show at a genome-wide level that the nuclear pore protein NUP98 associates with developmentally regulated genes active during human embryonic stem cell differentiation. Overexpression of a dominant negative fragment of NUP98 levels decreases expression levels of NUP98-bound genes. In addition, we identify two modes of developmental gene regulation by NUP98 that are differentiated by the spatial localization of NUP98 target genes. Genes in the initial stage of developmental induction can associate with NUP98 that is embedded in the nuclear pores at the nuclear periphery. Alternatively, genes that are highly induced can interact with NUP98 in the nuclear interior, away from the nuclear pores. This work demonstrates for the first time that NUP98 dynamically associates with the human genome during differentiation, revealing a role of a nuclear pore protein in regulating developmental gene expression programs.

The role of Nup98 in transcription regulation in healthy and diseased cells.

Nuclear pore complex (NPC) proteins are known for their critical roles in regulating nucleocytoplasmic traffic of macromolecules across the nuclear envelope. However, recent findings suggest that some nucleoporins (Nups), including Nup98, have additional functions in developmental gene regulation. Nup98, which exhibits transcription-dependent mobility at the NPC but can also bind chromatin away from the nuclear envelope, is frequently involved in chromosomal translocations in a subset of patients suffering from acute myeloid leukemia (AML). A common paradigm suggests that Nup98 translocations cause aberrant transcription when they are recuited to aberrant genomic loci. Importantly, this model fails to account for the potential loss of wild type (WT) Nup98 function in the presence of Nup98 translocation mutants. Here we examine how the cell might regulate Nup98 nucleoplasmic protein levels to control transcription in healthy cells. In addition, we discuss the possibility that dominant negative Nup98 fusion proteins disrupt the transcriptional activity of WT Nup98 in the nucleoplasm to drive AML.

Upf1 ATPase-dependent mRNP disassembly is required for completion of nonsense- mediated mRNA decay.

Cellular mRNAs exist in messenger ribonucleoprotein (mRNP) complexes, which undergo transitions during the lifetime of the mRNAs and direct posttranscriptional gene regulation. A final posttranscriptional step in gene expression is the turnover of the mRNP, which involves degradation of the mRNA and recycling of associated proteins. How tightly associated protein components are released from degrading mRNPs is unknown. Here, we demonstrate that the ATPase activity of the RNA helicase Upf1 allows disassembly of mRNPs undergoing nonsense-mediated mRNA decay (NMD). In the absence of Upf1 ATPase activity, partially degraded NMD mRNA intermediates accumulate in complex with NMD factors and concentrate in processing bodies. Thus, disassembly and completion of turnover of mRNPs undergoing NMD requires ATP hydrolysis by Upf1. This uncovers a previously unappreciated and potentially regulated step in mRNA decay and raises the question of how other mRNA decay pathways release protein components of substrate mRNPs.

The control of mRNA decapping and P-body formation.

mRNA decapping is a critical step in eukaryotic cytoplasmic mRNA turnover. Cytoplasmic mRNA decapping is catalyzed by Dcp2 in conjunction with its coactivator Dcp1 and is stimulated by decapping enhancer proteins. mRNAs associated with the decapping machinery can assemble into cytoplasmic mRNP granules called processing bodies (PBs). Evidence suggests that PB-associated mRNPs are translationally repressed and can be degraded or stored for subsequent translation. However, whether mRNP assembly into a PB is important for translational repression, decapping, or decay has remained controversial. Here, we discuss the regulation of decapping machinery recruitment to specific mRNPs and how their assembly into PBs is governed by the relative rates of translational repression, mRNP multimerization, and mRNA decay.

TTP and BRF proteins nucleate processing body formation to silence mRNAs with AU-rich elements.

In mammalian cells, mRNAs with AU-rich elements (AREs) are targeted for translational silencing and rapid degradation. Here we present evidence that in human cells the proteins Tristetraprolin (TTP) and BRF-1 deliver ARE-mRNAs to processing bodies (PBs), cytoplasmic assemblies of mRNAs, and associated factors that promote translational silencing and mRNA decay. First, depletion of endogenous TTP and BRF proteins, or overexpression of dominant-negative mutant TTP proteins, impairs the localization of reporter ARE-mRNAs in PBs. Second, TTP and BRF-1 localize tethered mRNAs to PBs. Third, TTP can nucleate PB formation on untranslated mRNAs even when other mRNAs are trapped in polysomes by cycloheximide treatment. ARE-mRNA localization in PBs is mediated by the TTP N- and C-terminal domains and occurs downstream from mRNA polysome release, which in itself is not sufficient for mRNA PB localization. The accumulation of ARE-mRNAs in PBs is strongly enhanced when the mRNA decay machinery is rendered limiting by mRNA decay enzyme depletion or TTP/BRF-1 overexpression. Based on these observations, we propose that the PB functions as a reservoir that sequesters ARE-mRNAs from polysomes, thereby silencing ARE-mRNA function even when mRNA decay is delayed. This function of the PB can likely be extended to other mRNA silencing pathways, such as those mediated by microRNAs, premature termination codons, and mRNA deadenylation.