Department of Chemistry and Biochemistry, The Ohio State University, 484 W 12th Avenue, Columbus, Ohio 43210, USA.
Nat Commun. 2016 Nov 18;7:13458. doi: 10.1038/ncomms13458.
Tip link filaments convey force and gate inner-ear hair-cell transduction channels to mediate perception of sound and head movements. Cadherin-23 and protocadherin-15 form tip links through a calcium-dependent interaction of their extracellular domains made of multiple extracellular cadherin (EC) repeats. These repeats are structurally similar, but not identical in sequence, often featuring linkers with conserved calcium-binding sites that confer mechanical strength to them. Here we present the X-ray crystal structures of human protocadherin-15 EC8-EC10 and mouse EC9-EC10, which show an EC8-9 canonical-like calcium-binding linker, and an EC9-10 calcium-free linker that alters the linear arrangement of EC repeats. Molecular dynamics simulations and small-angle X-ray scattering experiments support this non-linear conformation. Simulations also suggest that unbending of EC9-10 confers some elasticity to otherwise rigid tip links. The new structure provides a first view of protocadherin-15's non-canonical EC linkers and suggests how they may function in inner-ear mechanotransduction, with implications for other cadherins.
尖端连接丝传递力，并开启内耳毛细胞转导通道，从而介导声音和头部运动的感知。钙黏蛋白-23 和原钙黏蛋白-15 通过其细胞外结构域的钙离子依赖性相互作用形成尖端连接丝，这些结构域由多个细胞外钙黏蛋白（EC）重复组成。这些重复结构相似，但序列并不完全相同，通常具有带有保守钙离子结合位点的连接子，赋予它们机械强度。在这里，我们展示了人原钙黏蛋白-15 EC8-EC10 和鼠 EC9-EC10 的 X 射线晶体结构，它们显示出 EC8-9 典型的钙离子结合连接子，以及改变 EC 重复线性排列的 EC9-10 无钙离子连接子。分子动力学模拟和小角度 X 射线散射实验支持这种非线性构象。模拟还表明，EC9-10 的非弯曲赋予了原本僵硬的尖端连接丝一定的弹性。新结构提供了原钙黏蛋白-15 非典型 EC 连接子的第一个视图，并提示了它们在内耳机械转导中的功能，这对其他钙黏蛋白具有影响。
