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 cyclase1 (ROS-GC1) is the original mem ber of the membrane guanylate cyclase subfamily whose distinctive feature i s 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 activ ity. The phototransduction- and the synapse-linked domains are separate, an d they are located in the intracellular region of ROS-GC1. These domains se nse Ca2+ signals via Ca2+-binding proteins. These proteins are ROS-GC activ ating proteins, GCAPs. GCAPs control ROS-GC1 activity through two opposing regulatory modes. In one mode, at nanomolar concentrations of Ca2+, the GCA Ps activate the cyclase and as the Ca2+ concentrations rise, the cyclase is progressively inhibited. This mode operates in phototransduction via two G CAPs: 1 and 2. The second mode occurs at micromolar concentrations of Ca2via S100 beta. Here, the rise of Ca2+ concentrations progressively stimulat es the enzyme. This mode is linked with the retinal synaptic activity. In b oth modes, the final step in Ca2+ signal transduction involves ROS-GC dimer ization, which causes the cyclase activation. The identity of the dimerizat ion domain is not known. A heterozygous, triple mutation -E786D, R787C, T78 8M- in ROS-GC1 has been connected with autosomal cone-rod dystrophy in a Br itish 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 si gnificantly raises the GCAP1- and GCAP2-dependent maximal velocity of the c yclase, but this compensation, however, is not sufficient to override the b asal cyclase activity. (3) It converts the cyclase into a form that only ma rginally responds to S100 beta. The mutant produces insufficient amounts of the cyclic GMP needed to drive the machinery of phototransduction and of t he retinal synapse at an optimum level. The underlying cause of the breakdo wn of both types of machinery is that, in contrast to the native ROS-GC1, t he mutant cyclase is unable to change from its monomeric to the dimeric for m, 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 reg ion that is critical in its dimer formation, and, thus, discloses a single unifying mechanistic theme underlying the complex pathology of the disease.