In rod phototransduction, cyclic GMP synthesis by mem-brane bound guanylate cyclase ROS-GC1 is under Ca 2+ -dependent negative feedback control mediated by guanylate cyclase-activating proteins, GCAP-1 and GCAP-2. The cellular concentration of GCAP-1 and GCAP-2 approxi-mately sums to the cellular concentration of a functional ROS-GC1 dimer. Both GCAPs increase the catalytic effi-ciency (kcat /Km) of ROS-GC1. However, the presence of a myristoyl group in GCAP-1 has a strong impact on the regulation of ROS-GC1, this is in contrast to GCAP-2
