Cell biology of adhesive setae in gecko lizards.

Adhesive devices of digital pads of gecko lizards are formed by microscopic hair-like structures termed setae that derive from the interaction between the oberhautchen and the clear layer of the epidermis. The two layers form the shedding complex and permit skin shedding in lizards. Setae consist of a resistant but flexible corneous material largely made of keratin-associated beta-proteins (KA beta Ps, formerly called beta-keratins) of 8-22 kDa and of alpha-keratins of 45-60 kDa. In Gekko gecko, 19 sauropsid keratin-associated beta-proteins (sKAbetaPs) and at least two larger alpha-keratins are expressed in the setae. Some sKA beta Ps are rich in cysteine (111-114 amino acids), while others are rich in glycine (169-219 amino acids). In the entire genome of Anolis carolinensis 40 Ka beta Ps are present and participate in the formation of all types of scales, pad lamellae and claws. Nineteen sKA beta Ps comprise cysteine-rich 9.2-14.4 kDa proteins of 89-142 amino acids, and 19 are glycine-rich 16.5-22.0 kDa proteins containing 162-225 amino acids, and only two types of sKA beta Ps are cysteine- and glycine-poor proteins. Genes coding for these proteins contain an intron in the 5'-non-coding region, a typical characteristic of most sauropsid Ka beta Ps. Gecko KA beta Ps show a central amino acid region of high homology and a beta-pleated conformation that is likely responsible for the polymerization of Ka beta Ps into long and resistant filaments. The association of numerous filaments, probably over a framework of alpha-keratins, permits the formation of bundles of corneous material for the elongation of setae, which may be over 100 microm long. The terminals branching off each seta may derive from the organization of the cytoskeleton and from the mechanical separation of keratin bundles located at the terminal apex of setae.