Volume 16 Issue 12, December 2009

Chromosome end protection is accomplished by telomeres. How cells cope with spontaneously unprotected telomeres while avoiding cell cycle arrest or cell death is a fascinating question.

Flu viruses package essential functions into a small integral membrane protein known as M2. Such small membrane proteins represent major challenges for structural biology. A new study presented in this issue details the structure and functions of the influenza B M2 protein through the use of functional domain–specific solution NMR spectroscopy.

Ubiquitin chains have critical roles in activating the NF-κB pathway and mediating immune responses. Recent structural work on distinct ubiquitin chains in complexes with selective ubiquitin-binding domains provides an explanation for directionality and specificity in the NF-κB pathway.

c-di-GMP is a bacterial second messenger implicated in processes such as biofilm formation and switches between motile and sedentary lifestyles. The structure of the c-di-GMP–binding GEMM riboswitch is now presented with ligand and the large conformational changes between ligand-bound and unbound forms analyzed by small-angle X-ray scattering.

The GEMM riboswitch is conserved in diverse bacteria and recognizes the second messenger c-di-GMP which mediates many processes, such as the transition between sedentary and motile behavior. The structure of the GEMM riboswitch with ligand now elucidates ligand recognition and specificity.

Some proteins move along DNA, searching for a specific target. Now these proteins are shown to follow a helical path, i.e., they rotate while sliding, hence maintaining a specific orientation to the DNA helix. This is accomplished by tracking single molecules of labeled human oxoguanine DNA glycosylase 1, alone or bound to a bulky streptavidin moiety, and calculating their diffusion constants.

PYL-PYR proteins were recently described as receptors for the plant hormone abscisic acid (ABA) and as inhibitors of the phosphatases ABI1 and ABI2 in the presence of ABA. The crystal structures of PYL2 in its apo and ABA-bound forms and of the ternary complex PYL1–ABA–ABI1 have now been solved, providing insight into ABA sensing and signaling.

Pre-mRNA splicing is catalyzed by the spliceosome in a two-step reaction. Both catalytic steps have now been reconstituted using purified, defined components. This system identifies a role for Cwc25 in the first step of splicing and allows future detailed mechanistic analyses of splicing.

Telomeric DNA is protected by the shelterin complex, whose disruption triggers DNA-damage responses, checkpoint activation and chromosomal fusions. Now analysis of human cell lines reveals a spontaneously occurring intermediate state in which the DNA-damage response is activated at the telomeres without cell cycle arrest or chromosomal fusions, and with TRF2 playing a central role in determining such a state.

Inward-rectifier K⁺ channels respond to voltage via blockage by intracellular polyamines. How these blockers work is not entirely clear. Now a crystal structure of the cytoplasmic portion of Kir3.1 reveals five ion sites, and functional analyses indicate these ions are displaced by spermine binding.

Argonaute family members are effectors in small RNA-mediated silencing. In vitro analysis of the four human Argonautes demonstrates that AGO1 and AGO2 mediate multiple rounds of microRNA strand dissociation, mediate siRNA passenger-strand cleavage activity, and indicate distinct mechanisms from mRNA endonuclease activity.

Influenza B virus is responsible for about half of all the seasonal flu cases. The integral protein BM2 oligomerizes and forms a pH-activated proton channel that is essential for viral entry into host cells. The solution structures of the membrane-embedded chain domain and the C-terminal cytoplasmic domain reveal significant differences from the AM2 protein of influenza A virus and explain antiviral drug resistance.

On specific DNA sequences in vitro, a nucleosome is a polar barrier to RNA polymerase II (Pol II). Further analyses of the sequences underlying this barrier effect now indicate the formation of a loop that would preserve the position of the nucleosome on the DNA, while allowing passage of Pol II.

The eukaryotic group II chaperonin TRiC can block polyQ tract aggregation, present in proteins such as Htt. Here the TRiC-Htt interaction is examined using in vitro and in vivo experiments, revealing that TRiC does not physically block the polyQ tract, but rather sequesters a short N-terminal sequence that promotes the amyloidogenic conformation.

H2A.Z is implicated in genome stability across species. Acetylation of this histone variant in S. pombe is now found to be involved in maintaining condensed chromosomes during mitosis, with premature dissociation of condensin occurring in its absence.

The crucial ion-binding events that drive H⁺/Cl⁻ exchange in the bacterial transporter CLC-ec1 are now probed by isothermal calorimetry and detergent-solubilized proteins. The results indicate that transport via CLC channels have an inherent directionality, rather than being driven by an electrochemical gradient.

Condensins are protein complexes essential for eukaryotic mitosis and whose chromosome association is regulated by phosphorylation and dephosphorylation events. Now protein phosphatase PP2A is important for association of condensin II to mitotic chromosomes, but its catalytic activity is not required.

The bacterial transcriptional termination factor Rho is a hexameric helicase that tracks along RNA and dissociates DNA-RNA hybrids. Here the activity of Rho is examined using nucleotide analog interference mapping, revealing that the helicase takes large, 7-nt steps, triggered by contacts with 2′OH in the tracked RNA substrate.

How potassium channels select intracellular K⁺ over Na⁺ and Li⁺ is investigated, using experimental and theoretical approaches. The results indicate that selectivity is not due to higher affinity for K⁺ binding, but rather to an energy barrier blocking Na⁺ and Li⁺ entry in the presence of K⁺.

De novo methylation of DNA can affect the function of underlying genes and transposons in plants. Using a genetic screen, two factors required for de novo demethylation in Arabidopsis thaliana are identified and analyzed.

The TAK1 kinase binds K63-linked ubiquitin specifically via its TAB2 subunit. The structure of the TAB2 NZF domain in complex with K63-linked ubiquitins now indicates that this domain interacts with neighboring ubiquitins through distinct sites, explaining the basis of specific recognition.