Immunocytochemistry and protein analysis suggest that reptilian claws contain small high cysteine-glycine proteins.

The present study analyzes the structure and the main proteins of reptilian claws. Mature claws are formed by two to four layers of keratinocytes, a transitional layer of spindle-shaped cells and a thick corneous layer. Transitional cells elongate and merge into a compact corneous layer that is immunoreactive for beta-keratins, now indicated as sauropsid keratin-associated proteins (sKAPs). Most proteins extracted from claws in representative reptiles have a molecular weight of 13-20kDa, an acidic to basic isoelectric point, and are identified from the positive immunoreactivity to beta-keratin antibodies. The comparative analysis between lizard and avian claw beta-keratins shows the presence of an internal region of 20 amino acids with the highest identity, indicated as core-box, within an extended 32-amino acid region with a prevalent beta-sheet secondary conformation. This region is structurally equivalent to a 32-amino acid region present in scale beta-keratins of most reptiles. Both reptilian and avian keratins contain glycine-rich regions for stabilization of the beta-keratin polymer. The N- and C-regions contain most cysteine for disulphide-bonds formation. Claw proteins contain higher amount of cysteine and glycine than other scale proteins, suggesting that claw proteins are specialized cysteine-glycine-rich proteins suited to produce a very hard corneous material.