Identification of a guanylyl cyclase-activating protein-binding site within the catalytic domain of retinal guanylyl cyclase 1.

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Regulation of cAMP and cGMP production is a fundamental step in a broad range of signal transduction systems, including phototransduction. To identify regions within photoreceptor guanylyl cyclase 1 (GC1) that interact with GC-activating proteins (GCAPs), we synthesized the intracellular fragment of GC1, residues 491-1110, as a set of 15 amino acid long, partially overlapping peptides on the surface of individual pins arranged in a microtiter plate format. This pin assay identified 8 peptides derived from different regions of the GC1 intracellular domain that bind GCAPs. Peptide variants containing these sequences were synthesized as free peptides and tested for their ability to inhibit GC1 stimulation by GCAPs. A free peptide,968GTFRMRHMPEVPVRIRIG, from the catalytic domain of GC1 was the strongest inhibitor of GCAP1/GCAP2-mediated activation. In native GC1, this polypeptide fragment is likely to form a loop between alpha-helix 3 and beta-strand 4. When this region in GC1 was replaced by the corresponding sequence of GCAP-insensitive GC type A, GCAPs did not stimulate the GC1 mutant. The corresponding loops in related adenylyl cyclase (AC) are involved in the activating and inhibiting interactions with Gs alpha and Gi alpha, respectively. Thus, despite interacting with different activating proteins, both AC and GC activity may be modulated through their respective regions within catalytic domains.