Fig. 3.

Feather structure and secondary β-keratin structure. (a) Feather structure is shown as barbules/barbs, barbules, rachis; (b) Conserved amino acids of the barbules/barbs (NCBI accession number: NP_001095201.2), barbules (NP_001264699.1), feather rachis (NP_001264688.1) (Jin et al., 2017) with the β-strands in each sequence indicated in red; (c) The monomeric central structure of β-keratin in feathers. The structure is based on the amino compositions of the filament framework segments in avian feather keratin (Fraser and Parry, 2008); (d) The central monomeric structure of β-keratin, where the four strands are usually antiparallel; (e) Dimeric structure (β-sandwich) of β-keratin; (f) The basic repeating structure of the β-filaments, with the four dimers rotated by approx. 45° to form a helix structure, with the fifth dimer at the bottom assuming an inverted sandwich position compared to the first dimer (Calvaresi et al., 2016). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)