Immunocytochemistry indicates that glycine-rich beta-proteins are present in the beta-layer, while cysteine-rich beta-proteins are present in beta- and alpha-layers of snake epidermis

Evidence of a molecular boundary lubricant at snakeskin surfaces

During slithering locomotion the ventral scales at a snake's belly are in direct mechanical interaction with the environment, while the dorsal scales provide optical camouflage and thermoregulation. Recent work has demonstrated that compared to dorsal scales, ventral scales provide improved lubrication and wear protection. While biomechanic adaption of snake motion is of growing interest in the fields of material science and robotics, the mechanism for how ventral scales influence the friction between the snake and substrate, at the molecular level, is unknown. In this study, we characterize the outermost surface of snake scales using sum frequency generation (SFG) spectra and near-edge X-ray absorption fine structure (NEXAFS) images collected from recently shed California kingsnake (Lampropeltis californiae) epidermis. SFG's nonlinear optical selection rules provide information about the outermost surface of materials; NEXAFS takes advantage of the shallow escape depth of the electrons to probe the molecular structure of surfaces. Our analysis of the data revealed the existence of a previously unknown lipid coating on both the ventral and dorsal scales. Additionally, the molecular structure of this lipid coating closely aligns to the biological function: lipids on ventral scales form a highly ordered layer which provides both lubrication and wear protection at the snake's ventral surface.

Presence of a glycine-cysteine-rich beta-protein in the oberhautchen layer of snake epidermis marks the formation of the shedding layer

The complex differentiation of snake epidermis largely depends on the variation in the production of glycine-cysteine-rich versus glycine-rich beta-proteins (beta-keratins) that are deposited on a framework of alpha-keratins. The knowledge of the amino acid sequences of beta-proteins in the snake Pantherophis guttatus has allowed the localization of a glycine-cysteine-rich beta-protein in the spinulated oberhautchen layer of the differentiating shedding complex before molting takes place. This protein decreases in the beta-layer and disappears in mesos and alpha-layers. Conversely, while the mRNA for a glycine-rich beta-protein is highly expressed in differentiating beta-cells, the immunolocalization for this protein is low in these cells. This discrepancy between expression and localization suggests that the epitope in glycine-rich beta-proteins is cleaved or modified by posttranslational processes that take place during the differentiation and maturation of the beta-layer. The present study suggests that among the numerous beta-proteins coded in the snake genome to produce epidermal layers with different textures, the glycine-cysteine-rich beta-protein marks the shedding complex formed between alpha- and beta-layers that allows for molting while its disappearance between the beta- and alpha-layers (mesos region for scale growth) is connected to the formation of the alpha-layers.

Immunolocalization of alpha-keratins and associated beta-proteins in lizard epidermis shows that acidic keratins mix with basic keratin-associated beta-proteins

The differentiation of the corneous layers of lizard epidermis has been analyzed by ultrastructural immunocytochemistry using specific antibodies against alpha-keratins and keratin associated beta-proteins (KAbetaPs, formerly indicated as beta-keratins). Both beta-cells and alpha-cells of the corneous layer derive from the same germinal layer. An acidic type I alpha-keratin is present in basal and suprabasal layers, early differentiating clear, oberhautchen, and beta-cells. Type I keratin apparently disappears in differentiated beta- and alpha-layers of the mature corneous layers. Conversely, a basic type II alpha-keratin rich in glycine is absent or very scarce in basal and suprabasal layers and this keratin likely does not pair with type I keratin to form intermediate filaments but is weakly detected in the pre-corneous and corneous alpha-layer. Single and double labeling experiments show that in differentiating beta-cells, basic KAbetaPs are added and replace type-I keratin to form the hard beta-layer. Epidermal alpha-keratins contain scarce cysteine (0.2-1.4 %) that instead represents 4-19 % of amino acids present in KAbetaPs. Possible chemical bonds formed between alpha-keratins and KAbetaPs may derive from electrostatic interactions in addition to cross-linking through disulphide bonds. Both the high content in glycine of keratins and KAbetaPs may also contribute to increase the hydrophobicy of the beta- and alpha-layers and the resistance of the corneous layer. The increase of gly-rich KAbetaPs amount and the bonds to the framework of alpha-keratins give rise to the inflexible beta-layer while the cys-rich KAbetaPs produce a pliable alpha-layer.