Fraser R D Bruce, Parry David A D
Institute of Fundamental Sciences, Massey University, Private Bag 11-222, Palmerston North, New Zealand.
J Struct Biol. 2008 Apr;162(1):1-13. doi: 10.1016/j.jsb.2008.01.011. Epub 2008 Feb 2.
Avian feathers have a filament-matrix texture and X-ray diffraction studies show that the filament has a helical structure with four repeating units per turn. Each repeating unit consists of a pair of twisted beta-sheets related by a perpendicular diad, and the twist in the sheets is of opposite hand to that of the helix. Each sheet is believed to comprise a 32-residue segment of the feather keratin molecule, which contains around 100 residues, the remainder constituting the matrix. In the present contribution, the sequence of emu feather is mapped to the low-resolution model derived earlier from X-ray studies. This shows that the inner surface of the "beta-sandwich" is densely populated by hydrophobic residues and that the charged residues and cysteine residues lie on the outer surface. In addition, the inner residues in the repeating unit mesh neatly together in layers oriented perpendicular to the filament axis. Amino acid sequences from a range of avian and reptilian keratins were collected and a 32-residue segment corresponding to the filament framework could be identified in every case, supporting the notion that there is a common plan for the filament framework in all of these materials. The hairpin turns in the beta-sheet were also identified and shown to be unusually rich in proline residues and also of variable composition. Two variants of the mapping were found which have complimentary conformations of the hairpin turns and these are illustrated and discussed. Since feather keratin yields a fiber rather than a crystalline X-ray pattern refinement of the model is restricted to trial-and-error methods and the assumptions made in its derivation are critically examined and some possible modifications discussed.
鸟类羽毛具有丝状-基质结构，X射线衍射研究表明，丝状物具有螺旋结构，每圈有四个重复单元。每个重复单元由一对通过垂直二轴对称的扭曲β-折叠片组成，片层中的扭曲方向与螺旋方向相反。据信每个片层由羽毛角蛋白分子的一个32个残基的片段组成，该分子包含约100个残基，其余部分构成基质。在本论文中，鸸鹋羽毛的序列被映射到早期通过X射线研究得到的低分辨率模型上。这表明“β-三明治”的内表面密集地分布着疏水残基，而带电残基和半胱氨酸残基位于外表面。此外，重复单元中的内部残基在垂直于丝状物轴的层中整齐地交织在一起。收集了一系列鸟类和爬行动物角蛋白的氨基酸序列，并且在每种情况下都能鉴定出与丝状物框架相对应的一个32个残基的片段，这支持了所有这些材料中丝状物框架存在共同模式的观点。β-折叠片中的发夹弯也被鉴定出来，并且显示出脯氨酸残基含量异常丰富且组成可变。发现了两种映射变体，它们具有互补的发夹弯构象，并对此进行了说明和讨论。由于羽毛角蛋白产生的是纤维而不是晶体X射线图案，因此模型的细化仅限于试错法，并且对其推导过程中所做的假设进行了严格审查，并讨论了一些可能的修改。
