Dalla Valle L, Michieli F, Benato F, Skobo T, Alibardi L
Department of Biology, University of Padova, Padova, Italy.
J Exp Zool B Mol Dev Evol. 2013 Nov;320(7):428-41. doi: 10.1002/jez.b.22517. Epub 2013 Jun 21.
The tough corneous layer in the carapace and plastron of hard-shelled turtles derives from the accumulation of keratin-associated beta-proteins (KAbetaPs, formerly called beta-keratins) while these proteins are believed to be absent in soft-shelled turtles. Our bioinformatics and molecular study has instead shown that the epidermis of the soft-shelled turtle Apalone spinifera expresses beta-proteins like or even in higher amount than in the hard-shelled turtle Pseudemys nelsoni. The analysis of a carapace cDNAs library has allowed the identification and characterization of three alpha-keratins of type I and of ten beta-proteins (beta-keratins). The acidic alpha-keratins probably combine with the basic beta-proteins but the high production of beta-proteins in A. spinifera is not prevalent over that of alpha-keratin so that their combination does not determine the formation of hard corneous material. Furthermore the presence of a proline and cisteine in the beta-sheet region of beta-proteins in A. spinifera may be unsuited to form hard masses of corneous material. The higher amount of beta-proteins over alpha-keratins instead occurs in keratinocytes of the hard and inflexible epidermis of P. nelsoni determining the deposition of hard corneous material. The study suggests that the hardness of the corneous layer derives not exclusively from the interactions between alpha-keratins with KAbetaPs but also from the different dynamic of accumulation and loss of corneocytes in the corneous layer of the hard shelled turtles where a prevalent accumulation and piling of corneocytes takes place versus the soft shelled turtle where a rapid turnover of the stratum corneum occurs.
硬壳龟的背甲和腹甲中的坚韧角质层源自与角蛋白相关的β-蛋白（KAbetaPs，以前称为β-角蛋白）的积累，而软壳龟被认为不存在这些蛋白质。相反，我们的生物信息学和分子研究表明，软壳龟光滑鳖的表皮表达的β-蛋白与硬壳龟纳尔逊伪龟的相似，甚至含量更高。对背甲cDNA文库的分析使得能够鉴定和表征三种I型α-角蛋白和十种β-蛋白（β-角蛋白）。酸性α-角蛋白可能与碱性β-蛋白结合，但光滑鳖中β-蛋白的高产量并不比α-角蛋白普遍，因此它们的结合并不能决定硬角质材料的形成。此外，光滑鳖β-蛋白的β-折叠区域中脯氨酸和半胱氨酸的存在可能不适合形成硬角质材料块。相反，纳尔逊伪龟坚硬且不易弯曲的表皮角质形成细胞中β-蛋白的含量高于α-角蛋白，这决定了硬角质材料的沉积。该研究表明，角质层的硬度不仅源于α-角蛋白与KAbetaPs之间的相互作用，还源于硬壳龟角质层中角质形成细胞积累和损失的不同动态，硬壳龟角质形成细胞普遍积累和堆积，而软壳龟角质层则发生快速周转。
