A MYOSIN-III FROM LIMULUS EYES IS A CLOCK-REGULATED PHOSPHOPROTEIN

The lateral eyes of the horseshoe crab Limulus polyphemus undergo dram atic daily changes in structure and function that lead to enhanced ret inal sensitivity and responsiveness to light at night. These changes a re controlled by a circadian neural input that alters photoreceptor ac id pigment cell shape, pigment migration, and phototransduction. Clock input to the eyes also regulates photomechanical movements within pho toreceptors, including membrane shedding. The biochemical mechanisms u nderlying 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 phospho protein 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, pr obably by cAMP-dependent protein kinase. The protein is present throug hout the photoreceptor, including the region occupied by the photosens itive rhabdom, We propose that the phosphorylation of Limulus myosin I II is involved in one or more of the structural and functional changes that occur in Limulus eyes in response to clock input.