Walker R G, Hudspeth A J
Howard Hughes Medical Institute and Laboratory of Sensory Neuroscience, The Rockefeller University, New York, NY 10021-6399, USA.
Proc Natl Acad Sci U S A. 1996 Mar 5;93(5):2203-7. doi: 10.1073/pnas.93.5.2203.
Deflection of the mechanically sensitive hair bundle atop a hair cell opens transduction channels, some of which subsequently reclose during a Ca2+-dependent adaptation process. Myosin I in the hair bundle is thought to mediate this adaptation; in the bullfrog's hair cell, the relevant isozyme may be the 119-kDa amphibian myosin I beta. Because this molecule resembles other forms of myosin I, we hypothesized that calmodulin, a cytoplasmic receptor for Ca2+, regulates the ATPase activity of myosin. We identified an approximately 120-kDa calmodulin-binding protein that shares with hair-bundle myosin I the properties of being photolabeled by vanadate-trapped uridine nucleotides and immunoreactive with a monoclonal antibody raised against mammalian myosin I beta. To investigate the possibility that calmodulin mediates Ca2+-dependent adaptation, we inhibited calmodulin action and measured the results with two distinct assays. Calmodulin antagonists increased photolabeling of hair-bundle myosin I by nucleotides. In addition, when introduced into hair cells through recording electrodes, calmodulin antagonists abolished adaptation to sustained mechanical stimuli. Our evidence indicates that calmodulin binds to and controls the activity of hair-bundle myosin I, the putative adaptation motor.
毛细胞顶部机械敏感的毛束发生偏转时会打开转导通道，其中一些通道随后会在依赖Ca2+的适应过程中重新关闭。毛束中的肌球蛋白I被认为介导了这种适应；在牛蛙的毛细胞中，相关的同工酶可能是119 kDa的两栖类肌球蛋白Iβ。由于该分子类似于其他形式的肌球蛋白I，我们推测Ca2+的细胞质受体钙调蛋白调节肌球蛋白的ATP酶活性。我们鉴定出一种约120 kDa的钙调蛋白结合蛋白，它与毛束肌球蛋白I具有以下共同特性：被钒捕获的尿苷核苷酸光标记，并且与针对哺乳动物肌球蛋白Iβ产生的单克隆抗体发生免疫反应。为了研究钙调蛋白介导依赖Ca2+的适应的可能性，我们抑制钙调蛋白的作用，并通过两种不同的测定方法测量结果。钙调蛋白拮抗剂增加了核苷酸对毛束肌球蛋白I的光标记。此外，当通过记录电极引入毛细胞时，钙调蛋白拮抗剂消除了对持续机械刺激的适应。我们的证据表明，钙调蛋白与毛束肌球蛋白I（假定的适应马达）结合并控制其活性。
