The β-keratin chain with four 34-residue repeats that is conserved across the lepidosaurs (lizards, snakes and tuatara) contains three linker regions as well as a short, conserved N-terminal domain and a longer, more variable C-terminal domain. Earlier modelling had shown that only six classes of structure involving the four 34-residue repeats were possible. In three of these the 34-residue repeats were confined to a single filament (Classes 1, 2 and 3) whereas in the remaining three classes the repeats lay in two, three or four filaments, with some of the linkers forming interfilament connections (Classes 4, 5 and 6). In this work the members of each class of structure (a total of 20 arrangements) have been described and a comparison has been made of the topologies of each of the linker regions. This provides new constraints on the structure of the chain as a whole. Also, analysis of the sequences of the three linker regions has revealed that the central linker (and only the central linker) contains four short regions displaying a distinctive dipeptide repeat of the form (S-X)_2,3 separated by short regions containing proline and cysteine residues. By analogy with silk fibroin proteins this has the capability of forming a β-sheet-like conformation. Using the topology and sequence data the evidence suggests that the four 34-residue repeat chain adopts a Class 4a structure with a β-sandwich in filament 1 connected through the central linker to a β-sandwich in filament 2.

在鳞龙类（蜥蜴、蛇和大蜥蜴）中保守的具有四个 34 个残基重复序列的 β-角蛋白链包含三个接头区域以及一个短的、保守的 N 端结构域和一个更长、更可变的 C 端结构域。早期的建模表明只有六类结构涉及四个 34 个残基重复序列是可能的。在其中三个中，34 个残基的重复被限制在一个单丝上（第 1、2 和 3 类），而在其余三个类中，重复位于两个、三个或四个长丝中，其中一些接头形成了丝间连接（第 1 类、第 2 类和第 3 类） 4、5 和 6）。在这项工作中，已经描述了每一类结构的成员（总共 20 种排列），并对每个接头区域的拓扑结构进行了比较。这对整个链的结构提供了新的约束。此外，对三个接头区域的序列分析表明，中央接头（并且只有中央接头）包含四个短区域，显示出独特的二肽重复形式（SX）_2,3由含有脯氨酸和半胱氨酸残基的短区域隔开。通过与丝素蛋白类比，它具有形成β-片状构象的能力。使用拓扑和序列数据，证据表明四个 34 个残基的重复链采用 4a 类结构，丝 1 中的 β-夹心通过中心接头连接到细丝 2 中的 β-夹心。