Genes for hair and avian keratins

This paper briefly reviews the present level of understanding of the genes that code for hair keratins and the keratins of avian feather and scale. The emphasis to date has been on the structure of the genes, the derivation of amino acid sequences for several of the proteins from the coding sequences, and the organization of the different gene families within genomic DNA. Genomic sequences for the proteins of the three main gene families for wool keratin have been isolated from sheep genomic libraries and their detailed analysis by DNA sequencing is proceeding. Already, representative sequences for the genes of the microfibril (alpha-filament or IF) proteins and for the cysteine-rich and HGT matrix proteins are available. With these sequences as sources of probes, it is now possible to move from organizational and structural studies and undertake the study of the control of the expression of these genes either in wool (hair) follicles in vivo or after transfection with specific keratin genes of suitable epidermal cells in culture. Parallel with these studies is the investigation of the fate of trichohyalin droplets in the inner root sheath during its maturation, which is coordinated with keratinization in the hair itself. These unusual proteins and the changes they undergo suggest a highly specific function. The discovery of that function might come from the use of molecular cloning techniques especially for the synthesis of quantities of protein adequate for study. The avian keratins of feather and scale are coded for by a large family of evolutionarily related genes as shown in recent investigations of the DNA sequences. Again, derived protein sequences have provided a view of the feather and scale structures previously unavailable. Information on the fine structure of the genes is enabling the examination of their expression in vivo during embryonic development and the possible significance in this development of the histidine-rich ("fast") protein. The lack of any detectable transcripts from characterized feather keratin genes injected into Xenopus oocytes is presenting the possibility of testing for tissue-specific protein factors that might be responsible for the specific activation of keratin genes.