PreNUPtials

The nuclear export of mRNA requires that the nucleic acid associate with specific export factors in a ribonucleoprotein complex (mRNP). As the mRNP translocates through the nuclear pore complex (NPC), it sheds some of the export factors to make room for the binding of cytoplasmic proteins required for mRNA localization and translation. This mRNP remodeling is thought to be facilitated by components of the NPC, nucleoporins, and by Dbp5, a member of a large family of DEAD/DExD box helicases. In yeast, Dbp5 seems to interact with mRNA during transcription, shuttles between the nucleus and cytoplasm, and is essential for mRNA export. Dbp5 also binds to nucleoporin 159 (Nup159). Nup159 is only found at the cytoplasmic face of the NPC, and may provide positional information to Dbp5. Weis and colleagues show that the N-terminal domain of Nup159, required for mRNA export, forms a seven-bladed β-propeller structure. Mutagenesis studies indicate that residues within the Nup159 surface loops are required for its interactions with Dbp5. These interactions are critical for the steady-state localization of the helicase to the cytoplasmic side of the NPC and for mRNA export. These data support a model in which Nup159 specifically tethers and concentrates Dbp5 at the nuclear periphery where it can remodel the mRNP as it exits the nucleus. An alternative but not mutually exclusive model is that the interactions with Nup159 facilitate efficient release of the helicase from mRNPs exiting the nucleus and modulate the import of Dbp5 back into the nucleus. (Mol. Cell 16, 749–760, 2004) EJ

Energy for life

Restricting energy consumption is a strategy used by many organisms to extend lifespan. However, little is known about how energy levels and aging are linked. In humans, the AMP:ATP ratio is higher in aged fibroblasts than in young fibroblasts and is a sensitive measure of energy levels. AMP-activated protein kinase (AMPK) acts as an energy sensor to coordinate the cell's response to conditions where energy is limited, such as hypoxia and during exercise. It is allosterically activated by AMP and inhibited by ATP, and is therefore active when the AMP:ATP ratio is high. Apfeld and colleagues have found that Caenorhabditis elegans can actively sense changes in energy levels and adjust its lifespan accordingly. They identified a C. elegans AMPKα subunit homolog, AAK-2, that possesses AMPK catalytic activity that can be increased by AMP. C. elegans extends its lifetime when challenged early in life with sublethal doses of a stressor, like high temperature or oxidative stress. The authors show that the AMP:ATP ratios change in response to these conditions and that aak-2 activity is required for the lifetime extension. Mutational analysis also implicated aak-2 in an insulin-like signaling pathway that regulates lifetime. So it appears that, like AMPK, AAK-2 is a sensor that links lifespan to information about energy levels as well as insulin-like signals. Identification of AAK-2 phosphorylation targets will help reveal its role in prolonging lifespan. (Genes Dev. 18, 3004–3009, 2004) MM

hATs off

Transposons are ubiquitous mobile genetic elements found in most genomes examined. In fact, a large part of some genomes, including that of man, is composed of transposable elements. The first transposon identified, the maize Activator element, belongs to the hAT family. hAT transposons have been identified in most eukaryotic lineages, including plants, fungi, animals and man but little is known about their mechanism of transposition. Using purified Hermes transposase from the house fly, Craig and colleagues have determined the mechanism of movement of this element. They show that the transposase binds to the inverted repeat sequences at the transposon ends and that binding is followed by a double-strand break reaction that proceeds through formation of a hairpin intermediate on the flanking DNA. They further show that the 3′ OH end of the Hermes DNA covalently joins to the target DNA. This same mechanism is used in V(D)J recombination reactions to produce diverse immunoglobulin and T-cell receptor genes. They also identified an acidic DDE motif that mutagenesis showed to be important for the DNA breakage and joining steps of the reaction. This same motif is important for the catalytic activity of members of the retroviral integrase superfamily, the RAG recombinase and RNaseH. The finding of an RNaseH-like fold with conserved amino acids was surprising since the hAT family was thought to be distinct from the retroviral integrase superfamily. Finally, the mechanistic similarity between the hAT and RAG reactions supports the notion that V(D)J recombination could have evolved from transposable elements. (Nature, 432, 995–1001, 2004) BK

Bundle up with Polycomb

Polycomb group (PcG) proteins are transcription repressors that mediate epigenetic silencing in multicellular organisms. These proteins are often found in large complexes that also contain chromatin modifying factors, such as histone methylating enzymes. It has been hypothesized that PcG-containing complexes can induce the formation of chromatin structure that represses transcription. To test this hypothesis, Kingston and colleagues visualize the effect of the core complex of Drosophila Polycomb Repressive Complex 1 (PRC1) on oligonucleosomal arrays by electron microscopy. They showed that the PRC1 core complex induces condensation of nucleosome arrays from an extended, beads-on-a-string conformation to a compact structure. The condensation reaction seems to require one PRC1 core complex for every three or four nucleosomes and involves interactions between the PRC1 core complex and the nucleosomes, not the linker DNA. In addition, the histone tails are dispensable for the observed effect. The authors showed that the C-terminal region of a subunit of the PRC1 core complex called Posterior Sex Combs (PSC) is crucial for condensation of the nucleosomal array. This same region of PSC is also important for gene silencing in vivo. Taken together, these data suggest that the condensed nucleosomal array observed in this study might underlie the gene-silencing activities of the PcG proteins. (Science, 306, 1574–1577, 2004) HPF

Research highlights written by Hwa-ping Feng, Evelyn Jabri, Boyana Konforti and Michelle Montoya.