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Contrary to conventional wisdom, mineralization is not the only strategy evolved for the formation of hard, stiff materials. Indeed, the sclerotized mouthparts of marine invertebrates exhibit Young's modulus and hardness approaching 10 and 1 GPa, respectively, with little to no help from mineralization. Based on biochemical analyses, three of these mouthparts, the jaws of glycerid and nereid polychaetes and a squid beak, reveal a largely organic composition dominated by glycine- and histidine-rich proteins. Despite the well-known metal ion binding by the imidazole side-chain of histidine and the suggestion that this interaction provides mechanical support in nereid jaws, there is at present no universal molecular explanation for the relationship of histidine to mechanical properties in these sclerotized structures.

Rad51 is a core component of the eukaryotic homologous recombination machinery and is responsible for key mechanistic steps during strand invasion. Higher order oligomers of Rad51 display a remarkable degree of structural variation, forming rings, compressed filaments, and elongated filaments. It is unclear whether Rad51 can transition directly between these different oligomeric structures without disassembling first into monomers. We have used single-molecule microscopy to investigate the behavior of human Rad51 assembled on double-stranded DNA. Our results show that human Rad51 can form elongated nucleoprotein filaments on DNA, but ATP hydrolysis causes a decrease in their length without concomitant dissociation of protein. Compressed Rad51 filaments can re-elongate when presented with either ATP or the non-hydrolyzable analog AMP-PNP, and these cycles of elongation and compression are reversible. A Rad51 mutant deficient in ATP hydrolysis is locked into an extended conformation that is incapable of transitioning to a compressed filament. Similarly, wild-type Rad51 bound to DNA in the presence of AMP-PNP was trapped in the elongated state. Proteins incapable of transitioning to the compressed state were also highly resistant to dissociation from the DNA. Taken together, our results indicate that nucleotide hydrolysis by human Rad51 triggers a reversible structural transition leading to filaments with reduced helical pitch.DNA curtain ͉ homologous recombination ͉ single molecule imaging D ouble-stranded DNA breaks (DSBs) are 1 of the most deleterious forms of DNA damage and can lead to cell death or oncogenic transformation. Homologous recombination (HR) is an evolutionarily conserved pathway used to repair DSBs, and is essential for maintaining genomic stability (1, 2). When a DSB occurs, the 5Ј ends of the DNA are resected, yielding long 3Ј single-stranded DNA (ssDNA) overhangs, which are the loading site for a DNA recombinase. The recombinase aligns the ssDNA with a homologous double stranded DNA (dsDNA) and then invades the duplex to form a D-loop. The invading end can then serve as a primer for the replication machinery, which uses the homologous duplex as a template, and the resulting products are resolved to restore the continuity of the chromosomes.The DNA transactions that take place during HR are mediated by members of the RAD52 epistasis group of proteins (1-3). This includes the recombinase Rad51, which plays a central role in HR and assembles into a nucleoprotein filament on the ssDNA overhangs generated at the DSB (4, 5). This filament is responsible for catalyzing the pairing, alignment, and strand invasion steps during recombination (6). Rad51 is sufficient to catalyze these reactions in vitro (7), however, numerous accessory factors are required in vivo and their functions range from facilitating Rad51 loading at the outset of the reaction to promoting the disassembly of Rad51 upon completion of strand invasion (1,3,8,9).Human Rad51 has a flexible N-terminal domain and a central ATP-binding cor...

The tooth-like mouthparts of some animals consist of biomacromolecular scaffolds with few mineral components, making them intriguing paradigms of biostructural materials. In this study, the abrasion resistance of the jaws of one such animal, the bloodworm Glycera dibranchiata, has been evaluated by nanoindentation, nanoscratching, and wear testing. The hardest, stiffest, and most abrasion-resistant materials are found within a thin (<3 m) surface layer near the jaw tip and a thicker (10 -20 m) subsurface layer, both rich in unmineralized Cu. These results are consistent with the supposition that Cu ions are involved in the formation of intermolecular coordination complexes between proteins, creating a highly cross-linked molecular network. The intervening layer contains aligned atacamite [Cu2(OH)3Cl] fibers and exhibits hardness and stiffness (transverse to the alignment direction) that are only slightly higher than those of the bulk material but lower than those of the two Cu-rich layers. Furthermore, the atacamitecontaining layer is the least abrasion-resistant, by a factor of Ϸ3, even relative to the bulk material. These observations are broadly consistent with the behavior of engineering polymer composites with hard fiber or particulate reinforcements. The alignment of fibers parallel to the jaw surface, and the fiber proximity to the surface, are both suggestive of a natural adaptation to enhance bending stiffness and strength rather than to endow the surface regions with enhanced abrasion resistance. glycera jaw ͉ nanoindentation ͉ scratching ͉ wear