Brief Communications

Topoisomerases alter DNA topology, an essential activity in all organisms. Bacterial type II topoisomerases are targets for antimicrobials such as quinolones, whose binding mode was unclear. Now the crystal structures of pneumococcal topoisomerase IV in a DNA cleavage complex bound to moxifloxacin or clinafloxacin provide insight into how these drugs work and how bacteria can acquire resistance.

WAVE proteins in the WASP family are controlled by incorporation into the WAVE regulatory complex (WRC), which transmits information from the Rac GTPase to the actin cytoskeleton. By reconstituting human and fly WRCs, the native complex is shown to be inactive. Rac activates the WRC, but does not cause subunit dissociation. These results reconcile previous work and reveal common regulatory principles for the WASP family.

The nuclear transport receptor CRM1 mediates protein export from the nucleus through recognition of leucine-rich nuclear export signals on substrates. Structural analysis, based in part on the recent structure of a CRM1-SNUPN complex, reveal determinants for substrate binding and suggest a mechanism for binding partner-assisted dissociation of SNUPN in the cytoplasm.

BMAL1 has a central role in the mammalian circadian clock, acting as a transcriptional activator. The activity of BMAL1 is controlled by post-translational modifications such as acetylation and SUMOylation. Now the kinase CK2a is shown to phosphorylate BMAL1 at Ser90, and this is essential for BMAL1's function in the circadian clock.

Methylation at particular residues on histone tails has been associated with various functions and, in the case of dimethylated histone H3 arginine 2 (H3R2), cross-talk with methylation of a nearby lysine has been shown to be linked to transcriptional repression. Budding yeast monomethylated H3R2 is now shown to be associated with active loci and involved in activation of meiotic genes upon induction of sporulation.

Riboswitches are RNA domains that alter gene expression in response to ligand binding. The structure of the Bacillus subtilis preQ1 ribsoswitch, which recognizes the conserved modified nucleobase preQ1, in complex with its ligand indicates how an RNA of only 34 nucleotides recognizes its ligand.

Expanded CGG repeats cause chromosomal fragility and hereditary neurological disorders in humans. These repeats adopt unusual structures that block DNA replication in vitro and in unicellular organisms. Mirkin and co-workers asked whether the same holds true in mammalian cells. They find that CGG repeats stall replication forks in a length-dependent, but orientation-independent, manner and do not seem to trigger an intra–S phase checkpoint response. They suggest that fragile sites arise because the cell cycle continues before replication is complete and the under-replicated areas would convert into constrictions and/or double-stranded breaks.

Sortilin is a neuronal receptor involved in sorting and signal transduction. The crystal structure of the mature Sortilin ectodomain bound to one of its ligands, neurotensin, reveals a binding tunnel formed by the Sortilin β-propeller domain. Combined with binding and mutagenesis studies, the findings suggest that Sortilin substrates compete for access to the tunnel so that only one ligand binds at a time.

Type I restriction-modification enzymes recognize a target sequence and translocate DNA from both sides while remaining stationary, creating supercoiled loops and cleaving at nonspecific sites several kilobases away. The crystal structure of the motor subunit of EcoR124I is now solved, providing insight into these complex machines.

Plants contain atypical RNA polymerases that have been implicated in RNA silencing. An analysis of RNA polymerase V composition now reveals that it unexpectedly shares some, but not all, subunits found in RNA polymerase II, indicating that it may be a derived version of this polymerase complex. Additional subunits are also identified and implicated in RNA-mediated silencing.

TIPE2 is involved in immune homeostasis, and it has been assumed that it contained a death effector domain (DED). Now the crystal structure of TIPE2 reveals that it does not possess a DED, but instead has a previously uncharacterized fold, with a large central cavity that might accommodate a ligand.

Group II introns are retroelements that have invaded the genomes of many prokaryotes and eukaryotes. The structure of a self-spliced group IIC intron cocrystallized with ligated exons (the target substrate) reveals the metal ions that have a role in catalysis and the intron sequences that are important in exon recognition in group II introns.

Kinesins are molecular motors that slide along microtubules. A quantum dot is now attached to a microtubule, allowing the visualization of its rotation as it is moved by kinesins. The rotational pitch provides information about the motor, revealing the low processivity of human mitotic kinesin Eg5.

Histone methylation has important consequences for chromatin activity. Now, histones with methyllysine analogs are used to reconstitute nucleosomes: the crystal structures show no global changes in nucleosomes with H3K79me2 and H4K20me3, but the latter modification enhances compaction of nucleosomal arrays.

Repeat-addition processivity (RAP), that is, generating multiple DNA repeats from a single template without primer dissociation, is a key property of telomerase. In the Tetrahymena reverse-transcriptase component of telomerase, a single amino acid mutation causes a profound and specific defect in RAP without altering enzymatic activity.

The NMR structure of the H2A.Z-H2B histone chaperone, Chz1, reveals electrostatic interactions between Chz1 and the histone pair via a long, irregular chain with two capping helices, and, based on a model, the possibility that Chz1 has a more active role in histone replacement is suggested.

PPARγ is a nuclear receptor that regulates metabolic homeostasis. It is activated by nitrated and oxidized fatty acids. The crystal structure of the ligand binding domain of PPARγ in complex with a physiological ligand, nitrated linoleic acid, is now described, showing differences with synthetic agonists that may have physiological relevance.

Toll-like receptor 3 (TLR3) recognizes double-stranded RNA (dsRNA) molecules produced by many viruses and activates an inflammatory response. Synthetic dsRNAs such as small interfering RNAs have been shown to activate TLR3. Now the TLR3 ectodomain is found to contain two dsRNA binding sites, and the implications for dsRNA recognition and selectivity and downstream signaling are discussed.

Cytoplasmic O-GlcNac modification of proteins is thought to have dynamic interplay with phosphorylation and thus be involved in regulation of signaling processes. The complete structure of an OGT homolog is now presented, suggesting how diverse ligands can be presented to the active site of the enzyme.