The Kinase Homology Domain of Retinal Guanylyl Cyclases 1 and 2 Specifies the Affinity and Cooperativity of Interaction with Guanylyl Cyclase Activating Protein-2

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Abstract
RetGC-1 and RetGC-2 are photoreceptor membrane guanylyl cyclases that are regulated by the Ca2+-binding protein, GCAP-2. We used a protease protection assay to localize regions of the intracellular domains of RetGCs important for the interaction with GCAP-2 and found that GCAP-2 reduces the access of trypsin to a site in the kinase homology domain (KHD) of RetGC-1. The protective effect of GCAP-2 is independent of Ca2+. We also found that RetGC-2 and GCAP-2 interact cooperatively with high affinity, but RetGC-1 and GCAP-2 interact noncooperatively with low affinity. By analyzing RetGC-1/RetGC-2 chimeras we demonstrated that the affinity and cooperativity of the interaction with GCAP-2 is dictated by the structure of the KHD. These findings suggest that GCAP-2 interacts constituitively with the KHDs of RetGC-1 and RetGC-2 and that cGMP synthesis is controlled by Ca2+-dependent conformational changes in the RetGC/GCAP complex.