Sauropsids Cornification is Based on Corneous Beta‐Proteins, a Special Type of Keratin‐Associated Corneous Proteins of the Epidermis

Abstract

The evolution of the process of cornification in amniote epidermis from the general process of keratinization present in simple epithelia of anamniotes took place through the evolution of specialized intermediate filament (α) keratins, keratin-associated proteins (KAPs) and corneous proteins (CPs). The scanty information on the three-dimensional conformation of known KAPs and CPs indicate these proteins contain α-helix, random coiled, or beta sheets with different lengths and organizations. CP genes originated in a chromosome locus indicated as epidermal differentiation complex (EDC), and transformed the epidermal keratinization of anamniotes into the cornified epidermis and skin appendages of amniotes (claws, beaks, and feathers). In particular, peculiar genes encoding for small proteins with a central region of 34 amino acids conformed as beta sheets were originated in the EDC of sauropsids (reptiles and birds). These proteins were traditionally indicated as beta-keratins because they form filaments of 3-4 nm in diameter and show an X-ray beta pattern. Different from other proteins of the EDC, dimers of these corneous beta-proteins associate into long polymers of filamentous proteins utilized in sauropsids skin appendages, such as scales and feathers. Future challenges in this area of research will be the study on gene regulation and expression for these proteins, their origin and evolution in different lineages of sauropsids, and their role in determining the material properties of sauropsid scales and other skin appendages.

Identification and comparative analysis of the epidermal differentiation complex in snakes

“…SEDC genes form a continuous cluster in snakes and include a sub-cluster of genes that encode corneous beta-proteins (CBPs), also known as beta-keratins14. These proteins are characterized by a conserved core domain that is predicted to form a beta-sheet38.…”

“…In all amniotes, keratinocytes proliferate in the basal layer and differentiate in the suprabasal layers of the epidermis1314. Signaling between the epidermis and the underlying dermis controls the patterning of the epidermis and the formation of skin appendages15161718.…”

Alpha‐keratin and corneous beta protein in the parakeratinized epithelium of the tongue in the domestic goose (Anser anser f. domestica)

“…The tyrosine and glycine‐rich regions are present in the C‐terminal domains of the CBP in turtles and squamates (Fraser& Parry, ; Gregg & Rogers, ). It is believed that the physico‐chemical properties of glycine and tyrosine in CBP are responsible for limited extensibility, microbiological resistance, and hydrophobicity, and has a protective function (Alibardi, ; Alibardi, Toni, & Valle, ; Calvaresi et al, ; Fraser & Parry, ).…”

Immunolocalization of a Histidine-Rich Epidermal Differentiation Protein in the Chicken Supports the Hypothesis of an Evolutionary Developmental Link between the Embryonic Subperiderm and Feather Barbs and Barbules

“…The cornified skin barrier of amniotes and cornified skin appendages such as claws, hair, and feathers are formed by epidermal keratinocytes that differentiate by inducing the expression of specific sets of genes [1–4]. The stratification of the epidermis begins in the embryo and involves the establishment of the periderm as the superficial layer in all amniotes and the formation of a subperiderm in archosaurs [5–10].…”