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Beta-keratin (β-keratin) is a member of a structural protein family found in the epidermis of reptiles and birds.[1][2] Beta-keratins were named so because they are components of epidermal stratum corneum rich in stacked beta sheets, in contrast to alpha-keratins, intermediate-filament proteins also found in stratum corneum and rich in alpha helices.[3] Because the accurate use of the term keratin is limited to the alpha-keratins, the term "beta-keratins" in recent works is replaced by "corneous beta-proteins"[3] or "keratin-associated beta-proteins."[4]
β-keratins add much more rigidity to reptilian skin than alpha-keratins alone do to mammalian skin. β-keratins are impregnated into the stratum corneum of the reptilian skin, providing waterproofing and the prevention of desiccation.
The scales, beaks, claws and feathers of birds contain β-keratin of the avian family. Phylogenetic studies of β-keratin sequences show that feather β-keratins evolved from scale β-keratins.[5] The scale β-keratins form the basal group in avians. Duplication and divergence events then led to claw β-keratin genes, and further recombination resulted in new feather and feather-like avian β-keratin genes. Evidence for these duplication events comes from the correlation of feather β-keratin clade structure with their genomic loci.[6]
Changes in β-keratins may have also influenced the development of powered flight. A recent study using molecular dating methods to link the evolution of avian β-keratin genes in general to that of feathers specifically reveals that the avian β-keratin family began diverging from the crocodile family about 216 million years ago.[6] It also found that the feather β-keratin family did not begin diverging until 125 million years ago, a date consistent with the adaptive radiation of birds during the Cretaceous. β-keratins found in modern feathers have increased elasticity, a factor that may have contributed to their role in flight.[6] Thus, feathered relatives of birds such as Anchiornis and Archaeopteryx, whose flight capabilities have been questioned,[7] would have had avian, but not feather, β-keratins.
The small alvarezsaurid dinosaur Shuvuuia deserti shows evidence of a featherlike skin covering. Analysis by Schweitzer et al. (1999) showed that these featherlike structures consisted of beta-keratin.[8] This has since been refuted by Saitta et al., finding that the fibers analyzed instead consisted of inorganic calcium phosphate as evidenced by fluorescence under cross polarised light.[9] Signals from immunohistochemical analyses on fossil samples are prone to false positives and must be used with caution when dealing with geological samples.
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