Immunocytochemical localization of opsin, visual arrestin, myosin III, andcalmodulin in Limulus lateral eye retinular cells and ventral photoreceptors

The photoreceptors of the horseshoe crab Limulus polyphemus are classical p reparations for studies of the photoresponse and its modulation by circadia n clocks. An extensive literature details their physiology and ultrastructu re, but relatively little is known about their biochemical organization lar gely because of a lack of antibodies specific for Limulus photoreceptor pro teins. We developed antibodies directed against Limulus opsin, visual arres tin, and myosin III, and we have used them to examine the distributions of these proteins in the Limulus visual system. We also used a commercial anti body to examine the distribution of calmodulin in Limulus photoreceptors. F ixed frozen sections of lateral eye were examined with conventional fluores cence microscopy; ventral photoreceptors were studied with confocal microsc opy. Opsin, visual arrestin, myosin III, and calmodulin are all concentrate d at the photosensitive rhabdomeral membrane, which is consistent with thei r participation in the photoresponse. Opsin and visual arrestin, but not my osin III or calmodulin, are also concentrated in extra-rhabdomeral vesicles thought to contain internalized rhabdomeral membrane. In addition, visual arrestin and myosin III were found widely distributed in the cytosol of pho toreceptors, suggesting that they have functions in addition to their roles in phototransduction. Our results both clarify and raise new questions abo ut the functions of opsin, visual arrestin, myosin III, and calmodulin in p hotoreceptors and set the stage for future studies of the impact of light a nd clock signals on the structure and function of photoreceptors. (C) 2001 Wiley-Liss, Inc.