Meeting Abstract
Reef-building corals lack even basic visual structures, yet display notable photosensitivities. Light influences tentacle movement, regulates circadian clocks, and influences the timing of spawning. Light also influences the behavior of coral larvae. Putative opsins have been identified in corals and implicated in these processes. However, the biochemistry and whether these receptors function as light receptors, remains unknown. The characterization of cnidarian opsins, in general, has proven challenging, and, to date, only one cnidarian opsin (a green-sensitive, Gs-coupled opsin) has been characterized in a jellyfish. Also, previous analyses of predicted cnidarian opsins suggest that anthozoan opsins are more diverse and likely signal via multiple pathways. We have identified six putative opsins in the Indo-Pacific coral, Acropora millepora. Phylogenetic analysis of the predicted proteins indicates they represent three distinct groups of anthozoan opsins identified previously. We heterologously expressed these receptors in mammalian cells and demonstrated photosensitivity of five opsins. Using the Glosensor cAMP luciferase reporter assay we determined the spectral sensitivity for three receptors. All three appear to couple to Gs and have absorption maxima in the blue-green region of the spectrum (471, 473 and 476nm). A fourth opsin activated Gi in a light-dependent manor, and a fifth produced a calcium response upon stimulation with light, consistent with Gq signaling. Our results support that hypothesis that anthozoans opsins signal through multiple G protein signaling pathways. Additional biochemical experiments and immunohistochemistry are in progress.
