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 mu m 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 beta-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 alpha-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.