Impairment of the rod outer segment membrane guanylate cyclase dimerization in a cone - Rod dystrophy results in defective calcium signaling

Rod outer segment membrane guanylate cyclasel (ROS-GC1) is the original member of the membrane guanylate cyclase subfamily whose distinctive feature is that it transduces diverse intracellularly generated Ca²⁺ signals in the sensory neurons. In the vertebrate retinal neurons, ROS-GC1 is pivotal for the operations of phototransduction and, most likely, of the synaptic activity. The phototransduction- and the synapse-linked domains are separate, and they are located in the intracellular region of ROS-GC1. These domains sense Ca²⁺ signals via Ca²⁺-binding proteins. These proteins are ROS-GC activating proteins, GCAPs. GCAPs control ROS-GC1 activity through two opposing regulatory modes. In one mode, at nanomolar concentrations of Ca²⁺, the GCAPs activate the cyclase and as the Ca²⁺ concentrations rise, the cyclase is progressively inhibited. This mode operates in phototransduction via two GCAPs: 1 and 2. The second mode occurs at micromolar concentrations of Ca²⁺ via S 100β. Here, the rise of Ca²⁺ concentrations progressively stimulates the enzyme. This mode is linked with the retinal synaptic activity. In both modes, the final step in Ca²⁺ signal transduction involves ROS-GC dimerization, which causes the cyclase activation. The identity of the dimerization domain is not known. A heterozygous, triple mutation - E786D, R787C, T788M - in ROS-GC1 has been connected with autosomal cone - rod dystrophy in a British family. The present study shows the biochemical consequences of this mutation on the phototransduction- and the synapse-linked components of the cyclase. (1) It severely damages the intrinsic cyclase activity. (2) It significantly raises the GCAP1- and GCAP2-dependent maximal velocity of the cyclase, but this compensation, however, is not sufficient to override the basal cyclase activity. (3) It converts the cyclase into a form that only marginally responds to S 100β. The mutant produces insufficient amounts of the cyclic GMP needed to drive the machinery of phototransduction and of the retinal synapse at an optimum level. The underlying cause of the breakdown of both types of machinery is that, in contrast to the native ROS-GC1, the mutant cyclase is unable to change from its monomeric to the dimeric form, the form required for the functional integrity of the enzyme. The study defines the CORD in molecular terms, at a most basic level identifies a region that is critical in its dimer formation, and, thus, discloses a single unifying mechanistic theme underlying the complex pathology of the disease.