Feather keratin in Pavo cristatus: A tentative structure

Abstract

Abstract Background F-keratin forms an evolutionary conserved nanocomposite which allows birds to fly. Structural models for F-keratin are all based on the pioneering X-ray diffraction studies on seagull F-keratin, first published in 1932, confirmed for other species and refined over the years. There is, however, no experimental proof because native F-keratin does not form a perfect molecular crystal as required for structure determination. Methods Peacock’s tail feathers were systematically re-investigated by taking diffraction patterns at different rotation angles. Using the recently developed AlphaFold algorithm, a collection of 3D models of arbitrarily truncated and multiplied Pavo cristatus F-keratin sequences was created. The shape, dimensions, density and interfacial exposure of functionally relevant amino acid side chains of the calculated 3D building blocks were used as the initial selection criteria for filamentous F-keratin precursors. Full reproducibility of in silico folding and agreement with previous results from mechanical testing, biochemical analyses and SAXS experiments was mandatory for suggesting the tentative structure for the novel F-keratin repeating unit. Results The filament of the F-keratin polymer is an alternating arrangement of two units called "N-block" and "C-block": Four strands AA 1–52 form a disulfide-stabilized twisted parallelepiped, with 89° internal rotation within eight levels of β-sandwiches. Four strands AA 81–100 form a two-level “β-sandwich” in which aromatic residues provide resilience, like vertebral discs in a spinal column. The pitch of an N+C-block octamer is 10 nm. Solidification may involve "C-blocks" to temporarily mold into "C-wedges" of 18° tilt, which align F-keratin into laterally amorphous fiber-reinforced composites of 9.5 nm axial periodicity. This experimentally significant distance corresponds to the fully stretched AA 53–80 matrix segment. The deformed “spinal column” unwinds under compression when F-keratin filaments perfectly align horizontally into stacked sheets in the solid state. Conclusions At present, the tentative structure presented here is without alternatives.