Vertebrate photoreceptor cells are exquisite light detectors operating under very dim and bright illumination. The photoexcitation and adaptation machinery in photoreceptor cells consists of protein complexes that can form highly ordered supramolecular structures and control the homeostasis and mutual dependence of the secondary messengers cyclic guanosine monophosphate (cGMP) and Ca2+. The visual pigment in rod photoreceptors, the G protein-coupled receptor rhodopsin is organized in tracks of dimers thereby providing a signaling platform for the dynamic scaffolding of the G protein transducin. Illuminated rhodopsin is turned off by phosphorylation catalyzed by rhodopsin kinase (GRK1) under control of Ca2+-recoverin. The GRK1 protein complex partly assembles in lipid raft structures, where shutting off rhodopsin seems to be more effective. Re-synthesis of cGMP is another crucial step in the recovery of the photoresponse after illumination. It is catalyzed by membrane bound sensory guanylate cyclases (GCs) and is regulated by specific neuronal Ca2+-sensor proteins called guanylate cyclase-activating proteins (GCAPs). At least one GC (ROS-GC1) was shown to be part of a multiprotein complex having strong interactions with the cytoskeleton and being controlled in a multimodal Ca2+-dependent fashion. The final target of the cGMP signaling cascade is a cyclic nucleotide-gated (CNG) channel that is a hetero-oligomeric protein located in the plasma membrane and interacting with accessory proteins in highly organized microdomains. We summarize results and interpretations of findings related to the inhomogeneous organization of signaling units in photoreceptor outer segments.

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INTRODUCTIONThis review examines the roles of guanosine 3' ,5' -cyclic monophosphate (cGMP) and Ca2+ ions in signal transduction of vertebrate photoreceptors. Rod and cone photoreceptor cells transduce the absorption of light into a brief voltage pulse by closing cation channels in the plasma membrane. In recent years there have been many striking advances in understanding the physico chemical basis of photo-electrical excitation in photoreceptor cells. An in tracellular messenger, cGMP, carries the message of the absorption event to ion channels in the surface membrane of the photoreceptor outer segment. Light decreases the cGMP concentration by activating an enzyme cascade that hydrolytic ally destroys the intracellular messenger. A negative feedback loop involving Ca2+ and a guanylyl cyclase (GC) controls the recovery of the photoreceptor cell from the light response. A similar transduction scheme with cAMP as the intracellular messenger exists in sensory neurons of the olfactory epithelium (73). Light also initiates a sequence of events that changes the sensitivity and response kinetics of photoreceptors. These cellular processes-collectively called adaptation-enable the photoreceptor cell to adjust its sensitivity to the ambient illumination. Although we know much less about adaptation, recent experiments suggest that Ca2+ ions play an important role by controlling the formation of cGMP. 153 0066-4278/92/0315-0153$02.00 Annu. Rev. Physiol. 1992.54:153-176. Downloaded from www.annualreviews.org by University of Sussex on 10/08/12. For personal use only. Quick links to online content Further ANNUAL REVIEWS 154KAUPP & KOCH cGMP is continuously synthesized and degraded by an enzymatic cycle that involves several proteins. Similarly, Ca2+ homeostasis of the cell is con trolled by a cycle that involves Ca2+ influx through the cGMP-gated channel and subsequent extrusion of Ca2+ from the cell by a NalCa, K-exchanger. The cGMP-and the Ca2+ -cycle communicate with each other through a network of enzymatic reactions. A change in one cycle also causes a change in the other, and vice versa. This review focuses on the molecular components that constitute the cGMP-and the Ca2+ -cycle and the multiple interactions between these two messenger systems.

Macular dystrophy leads to progressive loss of central vision and shows symptoms similar to age-related macular degeneration. Genetic screening of patients diagnosed with macular dystrophy disclosed a novel mutation in the GUCA1A gene, namely a c.526C > T substitution leading to the amino acid substitution p.L176F in the guanylate cyclase-activating protein 1 (GCAP1). The same variant was found in three families showing an autosomal dominant mode of inheritance. For a full functional characterization of the L176F mutant we expressed and purified the mutant protein and measured key parameters of its activating properties, its Ca2+/Mg2+-binding, and its Ca2+-induced conformational changes in comparison to the wildtype protein. The mutant was less sensitive to changes in free Ca2+, resulting in a constitutively active form under physiological Ca2+-concentration, showed significantly higher activation rates than the wildtype (90-fold versus 20-fold) and interacted with an higher apparent affinity with its target guanylate cyclase. However, direct Ca2+-binding of the mutant was nearly similar to the wildtype; binding of Mg2+ occurred with higher affinity. We performed molecular dynamics simulations for comparing the Ca2+-saturated inhibiting state of GCAP1 with the Mg2+-bound activating states. The L176F mutant exhibited significantly lower flexibility, when three Ca2+ or two Mg2+ were bound forming probably the structural basis for the modified GCAP1 function.

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