| Abstract
| - Regulation of cAMP and cGMP production is a fundamental step in a broad range of signaltransduction systems, including phototransduction. To identify regions within photoreceptor guanylyl cyclase1 (GC1) that interact with GC-activating proteins (GCAPs), we synthesized the intracellular fragment ofGC1, residues 491−1110, as a set of 15 amino acid long, partially overlapping peptides on the surface ofindividual pins arranged in a microtiter plate format. This pin assay identified 8 peptides derived fromdifferent regions of the GC1 intracellular domain that bind GCAPs. Peptide variants containing thesesequences were synthesized as free peptides and tested for their ability to inhibit GC1 stimulation byGCAPs. A free peptide,968GTFRMRHMPEVPVRIRIG, from the catalytic domain of GC1 was the strongestinhibitor of GCAP1/GCAP2-mediated activation. In native GC1, this polypeptide fragment is likely toform a loop between α-helix 3 and β-strand 4. When this region in GC1 was replaced by the correspondingsequence of GCAP-insensitive GC type A, GCAPs did not stimulate the GC1 mutant. The correspondingloops in related adenylyl cyclase (AC) are involved in the activating and inhibiting interactions with Gsαand Giα, respectively. Thus, despite interacting with different activating proteins, both AC and GC activitymay be modulated through their respective regions within catalytic domains.
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