Introduction

The general acceptance of "rhodopsin" as a generic designation for a visual pigment is a consequence of the rapidly increasing progress in the cloning and sequencing of rhodopsins and the application of genetical, gene technological and molecular physiological methods. These studies indicate that a rhodopsin serves as the primary light receptor protein in the visual systems of all animals investigated so far, independent of the structural and functional complexity, of the optical apparatus, or of the neuronal networks which animals have developed to analyse and process the information encoded in a light signal. The reason why visual perception is generally mediated by a rhodopsin is seen in a common origin of all visual systems, which depends on the the action of two key genes. These genes, which are proposed to have already interacted in a prototypic photoreceptor are, first, a gene coding for a rhodopsin (opsin), which has been aquired to allow photons to be absorbed, and, second, a gene that directs rhodopsin expression. From this gene equipment of a photoreceptor prototype the existing diversity of visual systems may have evolved by intercalary evolution. Pax 6 genes have been shown to operate as master control genes for eye morphogenesis and divergent rhodopsin genes are expressed in a terminal step of photoreceptor differentiation. The latter specify which wavelengths of light will be absorbed [1-4].

The structure of rhodopsins is remarkably conserved throughout evolution: animal rhodopsins consist of an apoprotein, designated as opsin, and a chomophore (11-cw-retinaldehyde or a closely related form of retinal). A key feature of rhodopsin is the folding of its single amino acid chain into a secondary structure with seven transmembrane a helices (Figure 1). Rhodopsins are therefore classified with 7 TM receptors. The chromophore is, without any known exception, covalently linked to the side chain of a lysine located in transmembrane helix seven.

In view of the structural conservation of rhodopsins from simple invertebrate organisms to man, one may ask whether it makes sense to differentiate between an invertebrate and a vertebrate subgroup of visual pigments. Since a taxon called "invertebrates" does not exist, whereas the vertebrates constitute a well-defined group within the phylum of chordata, there might be primarily practical reasons to individually deal with the rhodopsins expressed in invertebrate photoreceptors. At present, information is available on the amino acid sequences of more than 60 vertebrate rhodopsins and of a similar number of rhodopsins from invertebrates. This situation is likely to shift rapidly in favour of the invertebrate rhodopsins. From the number of species described to date, one may estimate that about 107 different invertebrate rhodopsins exist, with a single species harbouring as many as 16 rhodopsins [5]. In relation to about 10s different rhodopsins of vertebrates, more than 99% of the existing rhodopsin genes are expected to be expressed in the photosensitive cells of invertebrates. This abundance is likely to have led to a considerably higher divergence in the structure and function of rhodopsin than is indicated by the current level of information, which mainly stems from vertebrate rhodopsins. Thus, only a close look at the invertebrates will provide the information on what is general and what is special in the function of rhodopsins.

chromophore binding site

disulfide bridge

Figure 1. Secondary structure model of invertebrate rhodopsins. The amino acid chain is folded into seven transmembrane a helices. The C-terminus is located intracellularly, the N-terminus extracellularly. Functionally important domains, as discussed in the text, are highlighted.

extracellular space disulfide bridge glycosylation sites cytosol plasma membrane

Figure 1. Secondary structure model of invertebrate rhodopsins. The amino acid chain is folded into seven transmembrane a helices. The C-terminus is located intracellularly, the N-terminus extracellularly. Functionally important domains, as discussed in the text, are highlighted.

The current state of knowledge about invertebrate rhodopsins is determined primarily by work directed to study rhodopsin-related events in phototrans-duction in a limited number of model systems. One such system is the compound eye of the fruitfly Drosophila, which provides a model system that can be dissected by a combination of genetical, molecular biological and physiological methods in a way unmatched by other visual systems [6-13]. Visual systems like the eyes of cephalopods (octopus, squid) provide the opportunity to study rhodopsin functions with sophisticated biophysical and biochemical methods, due to the availability of large amounts of rhodopsin-containing photoreceptor membranes.

Comparative studies of different invertebrate visual systems show that evolutionary modifications have occurred to optimize eyes to particular types of visual input. These modifications concern the photochemistry of rhodopsin, the mobility of rhodopsin, its spectral tuning by sensitizing pigments and the type of G-proteins activated upon a light stimulus. Furthermore, non-phototransducing functions of the visual pigment are concerned, e.g. membrane targeting and endocytosis of rhodopsin or its role in triggering apoptosis.

The main focus of this review are aspects of invertebrate rhodopsin structure and function as well as rhodopsin-related events in the activation and control of phototransduction. Some of these topics have also been reviewed recently elsewhere [13-18].

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