Beta-keratin
Template:Distinguish Template:Cs1 config Template:Infobox protein family Beta-keratin (β-keratin) is a structural protein found in the epidermis of reptiles, birds, and pangolins.<ref name="pmid17576619">Template:Cite journal</ref><ref name="pmid17191254">Template:Cite journal</ref><ref>Template:Cite journal</ref>
StructureEdit
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.<ref name="pmid26965557">Template:Cite journal</ref> 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"<ref name="pmid26965557" /> or "keratin-associated beta-proteins."<ref name="pmid23065677">Template:Cite journal</ref>
FunctionEdit
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.<ref>Template:Cite journal</ref> 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.<ref name="Greenwold 2011">Template:Cite journal</ref>
EvolutionEdit
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.<ref name="Greenwold 2011" /> Modern feather β-keratins exhibit increased elasticity, which may have contributed to the development of flight.<ref name="Greenwold 2011" />
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.<ref>Template:Cite journal</ref>
Fossil evidenceEdit
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.<ref name="Schweitzeretal1999">Template:Cite journal</ref> 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.<ref name="Saittaetal2018">Template:Cite journal</ref> This highlights the potential for false positives in immunohistochemical analyses of fossilized remains.