Battelle B A, Andrews A W, Calman B G, Sellers J R, Greenberg R M, Smith W C
Whitney Laboratory and Department of Neuroscience, University of Florida, St. Augustine, Florida 32086, USA.
J Neurosci. 1998 Jun 15;18(12):4548-59. doi: 10.1523/JNEUROSCI.18-12-04548.1998.
The lateral eyes of the horseshoe crab Limulus polyphemus undergo dramatic daily changes in structure and function that lead to enhanced retinal sensitivity and responsiveness to light at night. These changes are controlled by a circadian neural input that alters photoreceptor and pigment cell shape, pigment migration, and phototransduction. Clock input to the eyes also regulates photomechanical movements within photoreceptors, including membrane shedding. The biochemical mechanisms underlying these diverse effects of the clock on the retina are unknown, but a major biochemical consequence of activating clock input to the eyes is a rise in the concentration of cAMP in photoreceptors and the phosphorylation of a 122 kDa visual system-specific protein. We have cloned and sequenced cDNA encoding the clock-regulated 122 kDa phosphoprotein and show here that it is a new member of the myosin III family. We report that Limulus myosin III is similar to other unconventional myosins in that it binds to calmodulin in the absence of Ca2+; it is novel in that it is phosphorylated within its myosin globular head, probably by cAMP-dependent protein kinase. The protein is present throughout the photoreceptor, including the region occupied by the photosensitive rhabdom. We propose that the phosphorylation of Limulus myosin III is involved in one or more of the structural and functional changes that occur in Limulus eyes in response to clock input.
鲎（美洲鲎）的侧眼在结构和功能上每天都会发生显著变化，从而在夜间提高视网膜对光的敏感度和反应性。这些变化受昼夜节律神经输入的控制，该输入会改变光感受器和色素细胞的形状、色素迁移以及光转导。眼睛的生物钟输入还会调节光感受器内的光机械运动，包括膜脱落。生物钟对视网膜产生这些多样影响的生化机制尚不清楚，但激活眼睛生物钟输入的一个主要生化结果是光感受器中cAMP浓度升高以及一种122 kDa视觉系统特异性蛋白的磷酸化。我们已经克隆并测序了编码受生物钟调节的122 kDa磷蛋白的cDNA，在此表明它是肌球蛋白III家族的一个新成员。我们报告称，鲎肌球蛋白III与其他非常规肌球蛋白相似，即在没有Ca2+的情况下它能与钙调蛋白结合；它的新颖之处在于它在肌球蛋白球状头部内被磷酸化，可能是由cAMP依赖性蛋白激酶催化。该蛋白存在于整个光感受器中，包括光敏视杆所占区域。我们推测鲎肌球蛋白III的磷酸化参与了鲎眼因生物钟输入而发生的一种或多种结构和功能变化。
