Chondrule formation in particle-rich nebular regions at least hundreds of kilometres across

Chondrules are millimetre-sized spherules (mostly silicate) that dominate the texture of primitive meteorites. Their formation mechanism is debated, but their sheer abundance suggests that the mechanism was both energetic and ubiquitous in the early inner Solar System. The processes suggested--such as shock waves, solar flares or nebula lightning--operate on different length scales that have been hard to relate directly to chondrule properties. Chondrules are depleted in volatile elements, but surprisingly they show little evidence for the associated loss of lighter isotopes one would expect. Here we report a model in which molten chondrules come to equilibrium with the gas that was evaporated from other chondrules, and which explains the observations in a natural way. The regions within which the chondrules formed must have been larger than 150-6,000 km in radius, and must have had a precursor number density of at least 10 m(-3). These constraints probably exclude nebula lightning, and also make formation far from the nebula midplane problematic. The wide range of chondrule compositions may be the result of different combinations of the local concentrations of precursors and the local abundance of water ice or vapour.     

Palindromic assembly of the giant muscle protein titin in the sarcomeric Z-disk

The Z-disk of striated and cardiac muscle sarcomeres is one of the most densely packed cellular structures in eukaryotic cells. It provides the architectural framework for assembling and anchoring the largest known muscle filament systems by an extensive network of protein-protein interactions, requiring an extraordinary level of mechanical stability. Here we show, using X-ray crystallography, how the amino terminus of the longest filament component, the giant muscle protein titin, is assembled into an antiparallel (2:1) sandwich complex by the Z-disk ligand telethonin. The pseudosymmetric structure of telethonin mediates a unique palindromic arrangement of two titin filaments, a type of molecular assembly previously found only in protein-DNA complexes. We have confirmed its unique architecture in vivo by protein complementation assays, and in vitro by experiments using fluorescence resonance energy transfer. The model proposed may provide a molecular paradigm of how major sarcomeric filaments are crosslinked, anchored and aligned within complex cytoskeletal networks.     

Rad54 protein promotes branch migration of Holliday junctions

Homologous recombination has a crucial function in the repair of DNA double-strand breaks and in faithful chromosome segregation. The mechanism of homologous recombination involves the search for homology and invasion of the ends of a broken DNA molecule into homologous duplex DNA to form a cross-stranded structure, a Holliday junction (HJ). A HJ is able to undergo branch migration along DNA, generating increasing or decreasing lengths of heteroduplex. In both prokaryotes and eukaryotes, the physical evidence for HJs, the key intermediate in homologous recombination, was provided by electron microscopy. In bacteria there are specialized enzymes that promote branch migration of HJs. However, in eukaryotes the identity of homologous recombination branch-migration protein(s) has remained elusive. Here we show that Rad54, a Swi2/Snf2 protein, binds HJ-like structures with high specificity and promotes their bidirectional branch migration in an ATPase-dependent manner. The activity seemed to be conserved in human and yeast Rad54 orthologues. In vitro, Rad54 has been shown to stimulate DNA pairing of Rad51, a key homologous recombination protein. However, genetic data indicate that Rad54 protein might also act at later stages of homologous recombination, after Rad51 (ref. 13). Novel DNA branch-migration activity is fully consistent with this late homologous recombination function of Rad54 protein.