Many protein architectures exhibit evidence of internal rotational symmetry postulated to be the result of gene duplication/fusion events involving a primordial polypeptide motif. A common feature of such structures is a domain-swapped arrangement at the interface of the N- and C-termini motifs and postulated to provide cooperative interactions that promote folding and stability. De novo designed symmetric protein architectures have demonstrated an ability to accommodate circular permutation of the N- and C-termini in the overall architecture; however, the folding requirement of the primordial motif is poorly understood, and tolerance to circular permutation is essentially unknown. The β-trefoil protein fold is a threefold-symmetric architecture where the repeating ~42-mer "trefoil-fold" motif assembles via a domain-swapped arrangement. The trefoil-fold structure in isolation exposes considerable hydrophobic area that is otherwise buried in the intact β-trefoil trimeric assembly. The trefoil-fold sequence is not predicted to adopt the trefoil-fold architecture in ab initio folding studies; rather, the predicted fold is closely related to a compact "blade" motif from the β-propeller architecture. Expression of a trefoil-fold sequence and circular permutants shows that only the wild-type N-terminal motif definition yields an intact β-trefoil trimeric assembly, while permutants yield monomers. The results elucidate the folding requirements of the primordial trefoil-fold motif, and also suggest that this motif may sample a compact conformation that limits hydrophobic residue exposure, contains key trefoil-fold structural features, but is more structurally homologous to a β-propeller blade motif.

中文翻译：

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三叶形折叠主题的从头开始折叠揭示了与β螺旋桨叶片主题的结构相似性。

许多蛋白质结构显示出内部旋转对称性的证据，推测其是涉及原始多肽基序的基因复制/融合事件的结果。这种结构的共同特征是在N和C末端基序界面处的域交换排列，并假定提供促进折叠和稳定性的协同相互作用。从头设计的对称蛋白质结构已经证明能够在整个结构中适应N和C末端的圆形排列。然而，对原始基序的折叠要求了解得很少，并且对圆形排列的耐受性基本上是未知的。β-三叶蛋白折叠是三重对称的结构，其中重复的〜42-mer“三叶折叠”基元通过结构域交换排列组装。孤立的三叶折叠结构暴露出相当大的疏水区域，否则该区域将被掩埋在完整的β-三叶三聚体组件中。从头开始折叠研究，预计三叶折叠序列不会采用三叶折叠结构。相反，预测的折叠与β螺旋桨结构中的紧凑“叶片”图案紧密相关。三叶折叠序列和圆形置换子的表达表明，只有野生型的N末端基序定义产生完整的β-三叶三聚体装配体，而置换产生单体。结果阐明了原始三叶折叠基序的折叠要求，并且还表明该基序可以采样一个紧凑的构象，该构象限制了疏水性残基的暴露，包含关键的三叶折叠结构特征，