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      Structural Origin of the Strain-Hardening of Spider Silk

      , , , ,
      Advanced Functional Materials
      Wiley-Blackwell

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          Reversible unfolding of individual titin immunoglobulin domains by AFM.

          Single-molecule atomic force microscopy (AFM) was used to investigate the mechanical properties of titin, the giant sarcomeric protein of striated muscle. Individual titin molecules were repeatedly stretched, and the applied force was recorded as a function of the elongation. At large extensions, the restoring force exhibited a sawtoothlike pattern, with a periodicity that varied between 25 and 28 nanometers. Measurements of recombinant titin immunoglobulin segments of two different lengths exhibited the same pattern and allowed attribution of the discontinuities to the unfolding of individual immunoglobulin domains. The forces required to unfold individual domains ranged from 150 to 300 piconewtons and depended on the pulling speed. Upon relaxation, refolding of immunoglobulin domains was observed.
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            Determining Beta-Sheet Crystallinity in Fibrous Proteins by Thermal Analysis and Infrared Spectroscopy

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              Self-assembly of spider silk proteins is controlled by a pH-sensitive relay.

              Nature's high-performance polymer, spider silk, consists of specific proteins, spidroins, with repetitive segments flanked by conserved non-repetitive domains. Spidroins are stored as a highly concentrated fluid dope. On silk formation, intermolecular interactions between repeat regions are established that provide strength and elasticity. How spiders manage to avoid premature spidroin aggregation before self-assembly is not yet established. A pH drop to 6.3 along the spider's spinning apparatus, altered salt composition and shear forces are believed to trigger the conversion to solid silk, but no molecular details are known. Miniature spidroins consisting of a few repetitive spidroin segments capped by the carboxy-terminal domain form metre-long silk-like fibres irrespective of pH. We discovered that incorporation of the amino-terminal domain of major ampullate spidroin 1 from the dragline of the nursery web spider Euprosthenops australis (NT) into mini-spidroins enables immediate, charge-dependent self-assembly at pH values around 6.3, but delays aggregation above pH 7. The X-ray structure of NT, determined to 1.7 A resolution, shows a homodimer of dipolar, antiparallel five-helix bundle subunits that lack homologues. The overall dimeric structure and observed charge distribution of NT is expected to be conserved through spider evolution and in all types of spidroins. Our results indicate a relay-like mechanism through which the N-terminal domain regulates spidroin assembly by inhibiting precocious aggregation during storage, and accelerating and directing self-assembly as the pH is lowered along the spider's silk extrusion duct.
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                Author and article information

                Journal
                Advanced Functional Materials
                Adv. Funct. Mater.
                Wiley-Blackwell
                1616301X
                February 22 2011
                February 22 2011
                : 21
                : 4
                : 772-778
                Article
                10.1002/adfm.201001397
                4ea3bf5f-9533-41be-aac7-8699797c07d7
                © 2011

                http://doi.wiley.com/10.1002/tdm_license_1

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