Cardasis Helene L, Stevens Stanley M, McClung Scott, Kempler Karen E, Powell David H, Eyler John R, Battelle Barbara-Anne
Department of Chemistry, University of Florida, Gainesville, Florida 32611, USA.
Biochemistry. 2007 Dec 4;46(48):13907-19. doi: 10.1021/bi701409f. Epub 2007 Nov 9.
Class III unconventional myosins are critical for the normal function of auditory hair cells and the function and maintenance of photoreceptors; however, the roles of class III myosins in these sensory cells are unknown. Class III myosins are unique in that they have a kinase domain at their N-terminus; thus, they may have both signaling and motor functions. In the horseshoe crab Limulus polyphemus, enhanced phosphorylation of an abundant, photoreceptor specific class III myosin at night correlates with well-characterized circadian changes in photoreceptor structure and function. Thus, the Limulus visual system may be particularly useful for investigating the properties, modulation, and functions of a class III myosin. Previously, we showed that two sites within the actin interface of full-length Limulus myosin III expressed in baculovirus are substrates for both cyclic AMP-dependent protein kinase and autophosphorylation. In the current study, mass spectrometry was used to show that these same sites are phosphorylated in the endogenous protein extracted from Limulus lateral eye, and that enhanced phosphorylation at these sites occurs in vivo in response to natural circadian clock input to these eyes. These findings demonstrate in vivo changes in myosin III phosphorylation in response to a natural stimulus. This phosphorylation may modulate myosin III-actin interactions.
Ⅲ类非常规肌球蛋白对于听觉毛细胞的正常功能以及光感受器的功能和维持至关重要；然而，Ⅲ类肌球蛋白在这些感觉细胞中的作用尚不清楚。Ⅲ类肌球蛋白的独特之处在于其N端有一个激酶结构域；因此，它们可能同时具有信号传导和运动功能。在鲎（Limulus polyphemus）中，一种丰富的、光感受器特异性Ⅲ类肌球蛋白在夜间的磷酸化增强与光感受器结构和功能中已得到充分表征的昼夜节律变化相关。因此，鲎的视觉系统可能对于研究Ⅲ类肌球蛋白的特性、调节和功能特别有用。此前，我们表明，在杆状病毒中表达的全长鲎肌球蛋白Ⅲ的肌动蛋白界面内的两个位点是环磷酸腺苷依赖性蛋白激酶和自身磷酸化的底物。在当前研究中，质谱分析表明，从鲎侧眼中提取的内源性蛋白中这些相同的位点也被磷酸化，并且在体内，响应于这些眼睛接收到的自然昼夜节律信号输入，这些位点的磷酸化增强。这些发现证明了肌球蛋白Ⅲ磷酸化在体内对自然刺激的响应变化。这种磷酸化可能会调节肌球蛋白Ⅲ与肌动蛋白的相互作用。
