The Keratin Chains of Avian Scale Tissue

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Abstract
The three major proteins of chick scale keratin were isolated as their S‐carboxymethylated derivatives and shown to be similar or identical in molecular weight by gel filtration but to he distinct by amino acid analysis and gel electrophoresis. The major amino‐terminal sequence of scale keratin chains was determined and shown to be highly homologous to the corresponding region of feather keratin chains. The carboxyl‐terminal peptides of the three scale keratin fractions differed in sequence but were all homologous to the carboxyl‐terminal segment of feather keratin chains. The pronounced concentration of cysteine residues at the amino‐terminal and carboxyl‐terminal segments suggested a similar role for these regions in both scale and feather keratin chains, namely to provide a disulphidelinked matrix to maintain the organisation of fibrils which arise from the internal hydrophobic segments of both types of chain. Analysis of a large hydrophobic segment from each of the three isolated protein fractions revealed that each was composed largely of repeating tripeptide units of the type Gly‐Gly‐X (where X = Phe, Leu or Tyr). At a few positions in each hydrophobic peptide, microheterogeneity was apparent in the sequences indicating that each isolated protein fraction was composed of at least three different chains each encoded by a different gene. A minimum of nine keratin genes are therefore expressed in scale tissue.