Regulatory modes of rod outer segment membrane guanylate cyclase differ in catalytic efficiency and Ca2+-sensitivity

In rod phototransduction, cyclic GMP synthesis by membrane bound guanylate cyclase ROS-GC1 is under Ca2+-dependent negative feedback control mediated by guanylate cyclase-activating proteins, GCAP-1 and GCAP-2. The cellular concentration of GCAP-1 and GCAP-2 approximately sums to the cellular concentration of a functional ROS-GC1 dimer. Both GCAPs increase the catalytic efficiency (k(cat)/K-m) 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. Catalytic efficiency of ROS-GC1 increases 25-fold when it is reconstituted with myristoylated GCAP-1, but only by a factor of 3.4 with nonmyristoylated GCAP-1. In contrast to GCAP1, myristoylation of GCAP-2 has only a minor effect on k(cat)/K-m. The increase with both myristoylated and non-myristoylated GCAP-2 is 10 to 13-fold. GCAPs also confer different Ca2+-sensitivities to ROS-GC1. Activation of the cyclase by GCAP-1 is half-maximal at 707 nM free [Ca2+], while that by GCAP-2 is at 100 nM. The findings show that differences in catalytic efficiency and Ca2+-sensitivity of ROS-GC1 are conferred by GCAP-1 and GCAP-2. The results further indicate the concerted operation of two 'GCAP modes' that would extend the dynamic range of cyclase regulation within the physiological range of free cytoplasmic Ca2+ in photoreceptor cells.