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A myosin III from Limulus eyes is a clock-regulated phosphoprotein
B A Battelle1, A W Andrews, B G Calman
1Whitney Laboratory and Department of Neuroscience, University of Florida, St. Augustine, Florida 32086, USA.
Summary
Horseshoe crab eyes exhibit daily changes controlled by circadian rhythms. A novel myosin III protein is phosphorylated, likely mediating these light-adaptive structural and functional shifts in photoreceptors.
Area of Science:
- * Neuroscience
- * Molecular Biology
- * Chronobiology
Background:
- * The lateral eyes of horseshoe crabs (Limulus polyphemus) show daily structural and functional adaptations for nocturnal vision.
- * Circadian neural input regulates photoreceptor shape, pigment migration, and phototransduction.
- * Biochemical pathways linking circadian input to these changes remain largely unknown.
Purpose of the Study:
- * To identify the biochemical mechanisms underlying circadian regulation of Limulus eyes.
- * To characterize a key 122 kDa phosphoprotein regulated by the circadian clock.
Main Methods:
- * Cloning and sequencing of cDNA encoding the 122 kDa phosphoprotein.
- * Biochemical analysis of the protein's properties, including calmodulin binding and phosphorylation sites.
- * Immunolocalization of the protein within photoreceptor cells.
Main Results:
- * The 122 kDa phosphoprotein is identified as a novel member of the myosin III family (Limulus myosin III).
- * Limulus myosin III binds calmodulin independently of calcium and is phosphorylated in its globular head, likely by cAMP-dependent protein kinase.
- * The protein is localized throughout the photoreceptor, including the rhabdom.
Conclusions:
- * Circadian clock input to Limulus eyes leads to increased cAMP and phosphorylation of Limulus myosin III.
- * Phosphorylation of Limulus myosin III is proposed to play a role in mediating the structural and functional adaptations of the eye.
- * This study reveals a novel role for myosin III in circadian-regulated visual processes.