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Expression level and activity profile of membrane bound guanylate cyclase type 2 in rod outer segments

Rod and cone cells of the mammalian retina harbor two types of a membrane bound guanylate cyclase (GC), rod outer segment guanylate cyclase type 1 (ROS‐GC1) and ROS‐GC2. Both enzymes are regulated by small Ca2+‐binding proteins named GC‐activating proteins that operate as Ca2+ sensors and enable cyclases to respond to changes of intracellular Ca2+after illumination. We determined the expression level of ROS‐GC2 in bovine ROS preparations and compared it with the level of ROS‐GC1 in ROSs. The molar ratio of a ROS‐GC2 dimer to rhodopsin was 1 : 13 200. The amount of ROS‐GC1 was 25‐fold higher than the amount of ROS‐GC2. Heterologously expressed ROS‐GC2 was differentially activated by GC‐activating protein 1 and 2 at low free Ca2+ concentrations. Mutants of GC‐activating protein 2 modulated ROS‐GC2 in a manner different from their action on ROS‐GC1 indicating that the Ca2+ sensitivity of the Ca2+ sensor is controlled by the mode of target–sensor interaction.