Putative inositol 1,4,5-trisphosphate receptor localized to endoplasmic reticulum in Limulus photoreceptors

Invertebrate microvillar photoreceptors utilize the phosphoinositide cascade to transduce light stimuli and inositol 1,4,5-trisphosphate is thought to be one of the messengers that triggers the electrical response by mobilizing intracellular stored calcium. To further characterize the role of the phosphoinositide signaling pathway in invertebrate phototransduction, we have examined the distribution of inositol 1,4,5-trisphosphate receptors in Limulus lateral eye and ventral nerve photoreceptors using an immunohistochemical approach combined with confocal microphotolysis of caged inositol 1,4,5-trisphosphate. We have localized the inositol 1,4,5-trisphosphate receptor using an antibody raised against a highly conserved region of the N-terminal of the protein. In lateral eye photoreceptors, the antibody intensely stains cytoplasm directly beneath the photoreceptive microvilli, containing subrhabdomeral cisternae of endoplasmic reticulum. In ventral nerve photoreceptors, the distribution of immunostaining was more homogeneous than within the lateral eye photoreceptors. Simultaneous confocal microphotolysis of caged inositol 1,4,5-trisphosphate and Ca2+ measurements using the fluorescent indicator Calcium Green 5N were performed to estimate inositol 1,4,5-trisphosphate-induced Ca2+ release in functionally distinct areas of the ventral nerve photoreceptors. This is the first direct demonstration of the localization of putative inositol 1,4,5-trisphosphate receptor in invertebrate visual cells. The inositol 1,4,5-trisphosphate receptor appears to be localized predominantly to endoplasmic reticulum and taken in conjunction with earlier physiological data from other workers, our result supports a central role for the phosphoinositide pathway in visual transduction in Limulus photoreceptors.