PHOTOTRANSDUCTION IN UNICELLS AND INVERTEBRATES - STATE OF THE PROBLEM

This survey reviews the findings of contemporary research on photoreception and phototransduction mechanisms in unicellates (halobacteria, flagellar green algae) and in the microvillous photoreceptors of a number of invertebrates (squid, octopus, horsehoe crab, cuttlefish, fly). So far, no catalytic cascade triggered by sensory rhodospin has been demonstrated in the unicellates. Basic research efforts are being directed to the study of photocycle intermediates (halobacteria) and to the details of the photosensitive rhodopsin activation process (flagellar green algae). The presence of a Schiff base, double bonds in an isoprene chain, and cis-trans isomerization is not essential for rhodopsin activation in the chromophore of green algae. The role of sensory rhodopsin of green algae in the autoinduction of its own biosynthesis has been revealed. The ability of algal sensory rhodopsin to catalyze the photoinduced GTP-dependent activation of phosphodiesterase in a model system is discussed. A catalytic cascade in invertebrate microvillous photoreceptors has been demonstrated. Properties of these light-induced, regulator proteins are discussed: G protein, phospholipase C, and metarhodopsin kinase. Inconsistencies in the findings on the role of cGMP as mediator of cation conductance in microvillous photoreceptors are noted. The roles of G protein in phospholipase C activation and inositol triphosphate in [Ca2+]i regulation have been demonstrated. Mechanisms for the conversion reaction of phosphorylated rhodopsin into its initial form due to dephosphorylation by Ca2+-dependent phosphatase are discussed. Diagrams of the known and proposed light-dependent and dark reactions that take place in the receptors are presented.