Abstract
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.
Highlights
Vertebrate photoreceptor cells are neurosensory cells of unique morphology and specialized function
Numerous articles in the last decades have summarized the basic features of the phototransduction process (e.g., Stryer, 1991; Kaupp and Koch, 1992; Koch, 1994; Pugh and Lamb, 2000; Luo et al, 2008; Wensel, 2008; Arshavsky and Burns, 2012; Korenbrot, 2012; Palczewski, 2012): coupling of visual pigments to the heterotrimeric G protein transducin, activation of the effector phosphodiesterase PDE6 by the G protein and efficient hydrolysis of the second messenger cyclic nucleotide guanosine 3,5 cyclic monophosphate with high turnover rates, regulation of the cyclic nucleotide-gated
Signaling in photoreceptor cells (CNG)-channel by cyclic guanosine monophosphate (cGMP) and re-synthesis of cGMP by a guanylate cyclase (GC) complex that is controlled by a powerful Ca2+-dependent feedback loop (Dizhoor et al, 2010; Koch et al, 2010)
Summary
Edited by: Jean-Marc Taymans, UMR1172, Jean-Pierre Aubert Research Center, France. Reviewed by: Alexander Dizhoor, Salus University, USA Johann Helmut Brandstätter, FAU Erlangen-Nuremberg, Germany. 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+. 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). 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.
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