Beta-keratin (β-keratin) is a structural

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

found in the epidermis of

reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

s, and pangolins.

Structure

Beta-keratins were named due to their composition of stacked

beta sheet The beta sheet (β-sheet, also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a gene ...

s in the epidermal

stratum corneum The stratum corneum (Latin language, Latin for 'horny layer') is the outermost layer of the epidermis (skin), epidermis. Consisting of dead tissue, it protects underlying tissue from infection, dehydration, chemicals and mechanical stress. It is ...

, distinguishing them from

alpha-keratin Alpha-keratin, or α-keratin, is a type of keratin found in mammalian vertebrates. This protein is the primary component in hairs, horn (anatomy), horns, claws, Nail (anatomy), nails and the Epidermis, epidermis layer of the skin. α-keratin is a S ...

s, which are intermediate-filament proteins rich in alpha helices. Recent studies suggest that the term ''

keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. It is the key structural material making up Scale (anatomy), scales, hair, Nail (anatomy), nails, feathers, horn (anatomy), horns, claws, Hoof, hoove ...

'' 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

. This provides protection against

desiccation Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container. The ...

and enhances durability in terrestrial environments. In birds, β-keratin is found in the

scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory), an object defined on a set of points * Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original * Scale factor, a number ...

, beaks, claws, and

feathers Feathers are epidermis (zoology), epidermal growths that form a distinctive outer covering, or plumage, on both Bird, avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in ...

. 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

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 The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

. Modern feather β-keratins exhibit increased elasticity, which may have contributed to the development of flight. Feathered non-avian dinosaurs, such as ''

Anchiornis ''Anchiornis'' is a genus of small, four-winged Paraves, paravian dinosaurs, with only one known species, the type species ''Anchiornis huxleyi'', named for its similarity to modern birds. The Latin name ''Anchiornis'' derives from a Greek word m ...

'' and ''

Archaeopteryx ''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird'') is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaîos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...

'', likely possessed avian β-keratins but lacked the specialized feather β-keratins, raising questions about their flight capabilities.

Fossil evidence

The small alvarezsaurid

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

'' 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. This highlights the potential for false positives in immunohistochemical analyses of fossilized remains.

References

External links