Hard (Beta-)keratins in the epidermis of reptiles: composition, sequence, and molecular organization.
Beta-keratins form the hard corneous material of reptilian scales. In the present review, the distribution and molecular characteristics of beta-keratins in reptiles are presented. In lepidosaurians immunoreactive, protein bands at 12-18 kDa are generally present with less frequent proteins at higher molecular weight. In chelonians, bands at 13-18 and 22-24 kDa are detected. In crocodilians, bands at 14-20 kDa and weaker bands at 30-32 kDa are seen. Protein bands above 25 kDa are probably polymerized beta-keratins or aggregates. Two-dimensional gel electrophoresis shows that beta-keratins are mainly basic and that acidic-neutral keratins may derive from post-translational modifications. Beta-keratins comprise glycine-proline-rich and cystein-proline-rich proteins of 13-19 kDa. Beta-keratin genes may or may not contain introns and are present in multiple copies with a linear organization as in avian beta-keratin genes. Despite amino acid differences toward N- and C-terminals all beta-keratins share high homology in their central, beta-folded region of 20 amino acids, indicated as core-box. This region is implicated in the formation of beta-keratin filaments of scales, claws, and feathers. The homology of the core-box suggests that these proteins evolved from a progenitor sequence present in the stem of reptiles. Beta-keratins have diversified in their amino acid sequences producing secondary (and tertiary) conformations that suited them for their mechanical role in scales. In birds, a small beta-keratin has allowed the formation of feathers. It is suggested that beta-keratins represent the reptilian counterpart of keratin associated or matrix proteins present in mammalian hairs, claws, and horns.
Toni
Mattia
M
Dipartimento di Biologia evoluzionistica sperimentale, University of Bologna, Italy, and Dipartimento di Biologia, University of Padova, Italy.
Dalla Valle
Luisa
L
Alibardi
Lorenzo
L
D000595:Amino Acid Sequence; D000818:Animals; D015180:Electrophoresis, Gel, Two-Dimensional; D004817:Epidermis / Q000378:metabolism*; D007633:Keratins / Q000378:metabolism; D008953:Models, Anatomic; D008954:Models, Biological; D008969:Molecular Sequence Data; D011499:Protein Processing, Post-Translational; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D040901:Proteomics / Q000379:methods; D012104:Reptiles; D017386:Sequence Homology, Amino Acid; D053800:beta-Keratins / Q000378:metabolism*
D053800:beta-Keratins; D007633:Keratins