title: Four-stranded coiled-coil elastic protein in the byssus of the giant clam, Tridacna maxima.
abstract: An elastic protein with a secondary structure distinct from all well-known load-bearing proteins is found in the byssus of the giant clam,  Tridacna maxima . The byssus consists of a bundle of hundreds of individual threads, each measuring about about 100 μm in diameter, which exhibit a tendon-like mechanical response. The amino acid composition of  Tridacna  byssus, however, is unlike tendon collagen, lacking high glycine, proline, and hydroxyproline. Wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) measurements suggest that the constituent nanofibrils of the byssal threads are distinct from known secondary structure motifs previously reported for elastic proteins including the collagen triple-helix, the β-sheet nanocrystalline domains of silks, or the double-stranded coiled-coil regions of intermediate filaments. Instead, X-ray diffraction data indicate a structural organization in which four coiled-coil α-helices form a stable rope-like structure, which then further pack in a pseudohexagonal lattice to form nanofibrils. Amino acid composition analysis shows unusually high concentrations of acidic as well as basic residues, suggesting that the four-helix structure is stabilized by strong ionic interactions between oppositely charged residues in neighboring strands. The composition also suggests additional stabilization by disulfide cross-linking. On a larger scale, scanning and conventional transmission electron microscope (STEM and TEM) observations indicate that the nanofibrils exhibit an alternating periodicity of about 500 nm along the axial direction. A molecular model that combines the mechanical properties with the structural characteristics of the  Tridacna  byssal threads is proposed.
authors.affiliation: Schools of Materials Science and Engineering and Biological Sciences, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798. ali.miserez@ntu.edu.sg
authors.forename: Ali
authors.initials: A
authors.lastname: Miserez
authors.forename: Youli
authors.initials: Y
authors.lastname: Li
authors.forename: Joel
authors.initials: J
authors.lastname: Cagnon
authors.forename: James C
authors.initials: JC
authors.lastname: Weaver
authors.forename: J Herbert
authors.initials: JH
authors.lastname: Waite
mesh_terms: D000596:Amino Acids / Q000032:analysis*; D000818:Animals; D049872:Bivalvia / Q000737:chemistry / Q000502:physiology*; D018360:Crystallography, X-Ray; D004220:Disulfides / Q000737:chemistry; D004548:Elasticity; D004591:Electrophoresis, Polyacrylamide Gel; D007382:Intermediate Filaments / Q000737:chemistry / Q000648:ultrastructure; D008855:Microscopy, Electron, Scanning; D008958:Models, Molecular; D057139:Nanofibers / Q000737:chemistry* / Q000648:ultrastructure; D009994:Osmolar Concentration; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011506:Proteins / Q000737:chemistry* / Q000302:isolation & purification; D053838:Scattering, Small Angle; D016474:Weight-Bearing; D014961:X-Ray Diffraction
chemical_list: D000596:Amino Acids; D004220:Disulfides; D011506:Proteins