Guanylate cyclase activating protein 1 (GCAP1) is a neuronal Ca(2+) sensor (NCS) that regulates the activation of rod outer segment guanylate cyclases (ROS-GCs) in photoreceptors. In this study, we investigated the Ca(2+)-induced effects on the conformation and the thermal stability of four GCAP1 variants associated with hereditary human cone dystrophies. Ca(2+) binding stabilized the conformation of all the GCAP1 variants independent of myristoylation. The myristoylated wild-type GCAP1 was found to have the highest Ca(2+) affinity and thermal stability, whereas all the mutants showed decreased Ca(2+) affinity and significantly lower thermal stability in both apo and Ca(2+)-loaded forms. No apparent cooperativity of Ca(2+) binding was detected for any variant. Finally, the non-myristoylated mutants were still capable of activating ROS-GC1, but the measured cyclase activity was shifted toward high, nonphysiological Ca(2+) concentrations. Thus, we conclude that distorted Ca(2+)-sensor properties could lead to cone dysfunction.

ABSTRACT:The GUCA1A gene encodes the guanylate cyclase activating protein 1 (GCAP1) of mammalian rod and cone photoreceptor cells, which is involved in the Ca 2+ -dependent negative feedback regulation of membrane bound guanylate cyclases in the retina. Mutations in the GUCA1A gene have been associated with different forms of cone dystrophies leading to impaired cone vision and retinal degeneration. Here we report the identification of three novel and one previously detected GUCA1A mutations: c.265G>A (p.Glu89Lys), c.300T>A (p.Asp100Glu), c.476G>T (p.Gly159Val) and c.451C>T (p.Leu151Phe). The clinical data of the patients carrying these mutations were compared with the functional consequences of the mutant GCAP1 forms. For this purpose we purified the heterologously expressed GCAP1 forms and investigated whether the mutations affected the Ca 2+ -triggered conformational changes and the apparent interaction affinity with the membrane bound guanylate cyclase. Furthermore, we analyzed Ca 2+ -dependent regulatory modes of wildtype and mutant GCAP1 forms. Although all novel mutants were able to act as a Ca 2+ -sensor protein, they differed in their Ca 2+ -dependent activation profiles leading to a persistent stimulation of guanylate cyclase activities at physiological intracellular Ca 2+ concentration.

SummaryThe autosomal dominant form of retinitis pigmentosa (adRP) is a blindness-causing conformational disease largely linked to mutations of rhodopsin. Molecular simulations coupled to the graph-based protein structure network (PSN) analysis and in vitro experiments were conducted to determine the effects of 33 adRP rhodopsin mutations on the structure and routing of the opsin protein. The integration of atomic and subcellular levels of analysis was accomplished by the linear correlation between indices of mutational impairment in structure network and in routing. The graph-based index of structural perturbation served also to divide the mutants in four clusters, consistent with their differences in subcellular localization and responses to 9-cis retinal. The stability core of opsin inferred from PSN analysis was targeted by virtual screening of over 300,000 anionic compounds leading to the discovery of a reversible orthosteric inhibitor of retinal binding more effective than retinal in improving routing of three adRP mutants.

Fundamental components of signaling pathways are switch modes in key proteins that control start, duration, and ending of diverse signal transduction events. A large group of switch proteins are Ca(2+) sensors, which undergo conformational changes in response to oscillating intracellular Ca(2+) concentrations. Here we use dynamic light scattering and a recently developed approach based on surface plasmon resonance to compare the protein dynamics of a diverse set of prototypical Ca(2+)-binding proteins including calmodulin, troponin C, recoverin, and guanylate cyclase-activating protein. Surface plasmon resonance biosensor technology allows monitoring conformational changes under molecular crowding conditions, yielding for each Ca(2+)-sensor protein a fingerprint profile that reflects different hydrodynamic properties under changing Ca(2+) conditions and is extremely sensitive to even fine alterations induced by point mutations. We see, for example, a correlation between surface plasmon resonance, dynamic light scattering, and size-exclusion chromatography data. Thus, changes in protein conformation correlate not only with the hydrodynamic size, but also with a rearrangement of the protein hydration shell and a change of the dielectric constant of water or of the protein-water interface. Our study provides insight into how rather small signaling proteins that have very similar three-dimensional folding patterns differ in their Ca(2+)-occupied functional state under crowding conditions.

Progressive visual impairment leading to blindness is often associated with inherited retinal dystrophies. These disorders correlate in most cases with mutations in genes that code for proteins of the visual transduction system in rod and cone photoreceptor cells. Recent progress has highlighted the involvement of a neuronal calcium sensor protein that is specifically expressed in rod and cone cells and operates as a guanylate cyclase-activating protein (GCAP). A group of patients suffering from cone or cone-rod dystrophies carry mutations in the GCAP1 gene, and biochemical analysis of GCAP1 function revealed that for most of these mutations GCAP1 exhibits a disturbance in its Ca(2+)-sensing and its guanylate cyclase-activating properties. Cellular consequences of different GCAP1 mutations are compared and discussed.