Brief Communications in 2010

Myotonic dystrophy type 1 (DM1), a multisystem disorder affecting skeletal and smooth muscle and other essential systems, belongs to a group of RNA gain-of-function disorders caused by the expansion of a CUG trinucleotide repeat (CUG^exp) in the 3' untranslated region of the DM protein kinase (DMPK) gene. Furling and coworkers have developed an optimized human U7-snRNA containing a poly-CAG antisense sequence to target the CUG^exp RNAs. These constructs cause specific degradation of pathogenic but not wild-type DMPK RNA products. By abolishing the RNA gain-of-function toxicity responsible for pathogenesis, these hU7-snRNAs could affect gene silencing in other RNA-dominant disorders expressing expanded repeats.

Nascent peptides with signal sequences must be recognized in order to be correctly targeted within the cell. The cryo-EM structure of the E. coli signal recognition particle in complex with its receptor and the translating ribosome is now presented, indicating conformational changes that lead to receptor recruitment.

Despite recognizing an epitope on HIV-1 gp41 that partially overlaps with those from broadly neutralizing antibodies, mAb 13H11 is non-neutralizing. Now the crystal structure and binding studies of 13H11 Fab with a gp41 peptide reveal why: the antigen assumes a helical structure consistent with the post-fusion conformation of gp41.

The 2009 pandemic flu influenza A H1N1 strain has caused great public health concern. Now the structure of H1N1 neuraminidase (NA) reveals that it lacks the characteristic additional cavity at its active site, known as the 150-cavity, found in all other known group 1 NAs.

The exon junction complex (EJC) is found on spliced mRNAs and influences post-transcriptional regulation. It is now shown that in Drosophila melanogaster, the EJC is bound to some but not all spliced junctions, suggesting that its assembly by the splicing machinery is a regulated process.

The structure of Moxifloxacin, a quinolone antibacterial, in complex with Acinetobacter baumannii topoisomerase IV and DNA now shows how the drug stacks between base pairs at the DNA cleavage site. Moxifloxacin contacts the protein through a non-catalytic Mg²⁺, and the structure gives insight into the mode of inhibition and possible basis of drug resistance.

BRCA2 is a tumor suppressor that interacts with RAD51 and functions in homologous recombination, but understanding its precise functions has been hampered by difficulties in purifying such a large protein. Now purified full-length human BRCA2 is shown to bind ∼6 molecules of RAD51 and to promote RAD51 binding to RPA-covered ssDNA in a manner stimulated by DSS1.

Tumor suppressor protein BRCA2 interacts with RAD51 and functions in homologous recombination, but understanding its precise functions has been hampered by difficulties in purifying such a large protein. Now purified full-length human BRCA2 is shown to bind selectively to ssDNA, to promote RAD51 binding to ssDNA while reducing its association with dsDNA, and to stimulate RAD51-mediated DNA strand exchange.

The MSL complex is conserved and in Drosophila melanogaster is involved in spreading of gene activation on the male X chromosome. Structural and functional analyses of the MSL3 chromodomain now indicate that DNA binding promotes methylated histone tail binding, suggesting coordinated binding that is here proposed to promote cross-nucleosome interaction in an array.

Previously, the molecular chaperone Hsp90 has been shown to copurify with RISC (RNA-induced silencing complex), involved in small RNA-mediated silencing. It is now shown that Hsp90 is needed for AGO2 to be loaded with a siRNA duplex by the RISC-loading complex suggesting a model where it is involved in modifying AGO2 conformation.

mTERF is a mitochondrial protein involved in transcription and replication pausing. The crystal structure of human mTERF bound to DNA now reveals the presence of nine repeats that form a left-handed solenoid, named the Zurdo domain.

The crystal structure of human DNA pol beta with DNA template and an incoming 8-oxo-dGTP forming a mutagenic pair with template base A is now presented, revealing the structural constraints within the polymerase active site that result in this mutagenic incorporation.

The PWWP domain has been identified in a number of nuclear proteins that interact with histones including BRPF1. Structural and functional analysis of this domain from BRPF1 now argue that it specifically recognizes methylated histone H3 Lys36, identifying this widely conserved domain as a reader of this mark.

Many actin-binding proteins contain calponin homology (CH) domains, but the manner in which these domains interact with F-actin has been controversial. Electron microscopy analyses show that the tandem CH domains of α-actinin bind to F-actin in an open conformation and that opening of these domains might be a key regulatory mechanism for proteins with tandem CH domains.

Nonsense-mediated mRNA decay (NMD) recognizes and degrades mRNAs with premature termination codons. In yeast, this occurs by decapping followed by 5' to 3' exonucleolytic digestion. New work shows that substrates of NMD pathway are targeted for decapping while the mRNA is associated with polyribosomes. These findings are in contrast to previous work which suggested that NMD occurs in ribosome-free P bodies but are consistent with recent work showing that 'normal' mRNAs are decapped co-translationally.

The solution structures of the two PAR-binding zinc finger (PBZ) modules from APLF, a human protein putatively involved in DNA damage response, are now presented. Together with binding studies with PAR fragments and mutagenesis, the work sheds light on PAR recognition by PBZ modules.

Respiratory syncytial virus (RSV) is a highly contagious illness in young children. The structure of antibody drug motavizumab in complex with a 24-residue peptide corresponding to its epitope on RSV-fusion glycoprotein suggests why it is more potent than its predecessor, palivizumab (Synagis).

GW182 has been implicated in the effector steps of microRNA-mediated repression and recently shown to interact with the Poly(A) binding protein C-terminal domain (PABPC1). The structure of PABPC1 in complex with a human GW182 paralog peptide, now gives insight into the interactions needed to elicit target deadenylation.