Beta-keratin

Structure

Beta-keratins were named due to their composition of stacked beta sheets in the epidermal stratum corneum, distinguishing them from alpha-keratins, which are intermediate-filament proteins rich in alpha helices. Recent studies suggest that the term keratin should be restricted to alpha-keratins. As a result, "beta-keratins" are now often referred to as "corneous beta-proteins" or "keratin-associated beta-proteins."

Function

β-keratins contribute significantly to the rigidity and waterproofing of reptilian skin by being impregnated into the stratum corneum. This provides protection against desiccation and enhances durability in terrestrial environments. In birds, β-keratin is found in the scales, beaks, claws, and feathers. Phylogenetic studies indicate that feather β-keratins evolved from scale β-keratins. The scale β-keratins form the basal group in avians, followed by claw β-keratin genes, with further recombination leading to new feather and feather-like β-keratin genes. These evolutionary patterns correlate with genomic loci.

Evolution in birds

Changes in β-keratins may have influenced the evolution of powered flight. Molecular dating suggests that avian β-keratin genes began diverging from their crocodilian relatives approximately 216 million years ago. However, feather β-keratins did not begin differentiating until around 125 million years ago, coinciding with the adaptive radiation of birds during the Cretaceous. Modern feather β-keratins exhibit increased elasticity, which may have contributed to the development of flight.

Feathered non-avian dinosaurs, such as Anchiornis and Archaeopteryx, likely possessed avian β-keratins but lacked the specialized feather β-keratins, raising questions about their flight capabilities.

Fossil evidence

The small alvarezsaurid dinosaur Shuvuuia deserti was once believed to have a feather-like skin covering composed of β-keratin. An immunohistochemical analysis by Schweitzer et al. (1999) initially supported this conclusion. However, subsequent research by Saitta et al. (2018) refuted this claim, demonstrating that the fibers analyzed consisted of inorganic calcium phosphate rather than β-keratin.

References

References

1. (July 2007). "Beta-keratins of differentiating epidermis of snake comprise glycine-proline-serine-rich proteins with an avian-like gene organization". Developmental Dynamics.
2. (February 2007). "Cloning and characterization of scale beta-keratins in the differentiating epidermis of geckoes show they are glycine-proline-serine-rich proteins with a central motif homologous to avian beta-keratins". Developmental Dynamics.
3. (September 2013). "Immunohistochemical demonstration of keratins in the epidermal layers of the Malayan pangolin (Manis javanica), with remarks on the evolution of the integumental scale armour". European Journal of Histochemistry.
4. (June 2016). "The molecular organization of the beta-sheet region in Corneous beta-proteins (beta-keratins) of sauropsids explains its stability and polymerization into filaments". Journal of Structural Biology.
5. (February 2013). "Cornification in reptilian epidermis occurs through the deposition of keratin-associated beta-proteins (beta-keratins) onto a scaffold of intermediate filament keratins". Journal of Morphology.
6. (May 2010). "Genomic organization and molecular phylogenies of the beta (beta) keratin multigene family in the chicken (Gallus gallus) and zebra finch (Taeniopygia guttata): implications for feather evolution". BMC Evolutionary Biology.
7. (December 2011). "Linking the molecular evolution of avian beta (β) keratins to the evolution of feathers". Journal of Experimental Zoology. Part B, Molecular and Developmental Evolution.
8. (May 2010). "Narrow primary feather rachises in Confuciusornis and Archaeopteryx suggest poor flight ability". Science.
9. (August 1999). "Beta-keratin specific immunological reactivity in feather-like structures of the cretaceous alvarezsaurid, ''Shuvuuia deserti''". The Journal of Experimental Zoology.
10. (November 2018). "Preservation of feather fibers from the Late Cretaceous dinosaur ''Shuvuuia deserti'' raises concern about immunohistochemical analyses on fossils.". Organic Geochemistry.