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B. Battelle, N.–A. Robinson, K.E. Kempler; A Circadian Clock and Light Influence the Distribution of Myosin III in Limulus Lateral Eye Photoreceptors . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5526.
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Our goal is to understand the molecular mechanisms underlying circadian changes in visual function. Limulus eyes, our model, undergo dramatic circadian changes in structure and function such that their sensitivity and responsiveness to light increases dramatically at night in response to signals from a circadian clock. We speculate that myosin III (myoIII), a quantitatively major protein in Limulus photoreceptors, underlies some of these changes as it is a major target for phosphorylation by clock input to the eyes. Limulus myoIII is an actin binding protein and a kinase. During the day in the light, myoIII is distributed throughout the photoreceptor and concentrated over the rays of the photosensitive, actin–rich rhabdom. Here we tested whether clock input to the eyes at night alters the distribution of myoIII in the photoreceptors. MyoIII is also phosphorylated in response to light. Therefore we also examined the impact of light on the distribution of myoIII.
Frozen sections of lateral eyes fixed in methanolic formaldehyde were immunostained for myoIII and actin. The locations of the antigens were visualized with fluorescent secondary antibodies and examined with a confocal microscope. Several comparisons were made between eyes of the same animal. 1. Eyes with or without clock input were fixed at night in the dark. 2. Eyes with clock input were fixed at night, or during the early morning, in the dark or following a 30 min exposure to light. 3. Eyes without clock input were fixed in the light following 6 days of constant light or in the dark following 6 days of constant darkness. In all experiments, clock input to a lateral eye was blocked by cutting its optic nerve.
At night in the dark, myoIII was less concentrated over the rays of the rhabdom of eyes that received clock input compared with eyes to which clock input had been blocked. In eyes that received clock input at night, a 30 min exposure to light increased the concentration of myoIII over the rhabdom. However, long–term exposure to constant light led to a decrease in myoIII over the rhabdom compared to eyes maintained in constant darkness.
The concentration of myoIII at the actin–rich rhabdom is influenced by signals from a circadian clock and by light. The effects of light are complex. We speculate the changes in myoIII distribution are regulated by myoIII phosphorylation which may change myoIII's affinity for actin. Changes in the concentration of myoIII at the rhabdom may influence the stability of actin in rhabdomeral microvilli and play a role in regulating rhabdom shedding.
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